| ====================== |
| Writing an ALSA Driver |
| ====================== |
| |
| :Author: Takashi Iwai <tiwai@suse.de> |
| |
| Preface |
| ======= |
| |
| This document describes how to write an `ALSA (Advanced Linux Sound |
| Architecture) <http://www.alsa-project.org/>`__ driver. The document |
| focuses mainly on PCI soundcards. In the case of other device types, the |
| API might be different, too. However, at least the ALSA kernel API is |
| consistent, and therefore it would be still a bit help for writing them. |
| |
| This document targets people who already have enough C language skills |
| and have basic linux kernel programming knowledge. This document doesn't |
| explain the general topic of linux kernel coding and doesn't cover |
| low-level driver implementation details. It only describes the standard |
| way to write a PCI sound driver on ALSA. |
| |
| This document is still a draft version. Any feedback and corrections, |
| please!! |
| |
| File Tree Structure |
| =================== |
| |
| General |
| ------- |
| |
| The file tree structure of ALSA driver is depicted below. |
| |
| :: |
| |
| sound |
| /core |
| /oss |
| /seq |
| /oss |
| /include |
| /drivers |
| /mpu401 |
| /opl3 |
| /i2c |
| /synth |
| /emux |
| /pci |
| /(cards) |
| /isa |
| /(cards) |
| /arm |
| /ppc |
| /sparc |
| /usb |
| /pcmcia /(cards) |
| /soc |
| /oss |
| |
| |
| core directory |
| -------------- |
| |
| This directory contains the middle layer which is the heart of ALSA |
| drivers. In this directory, the native ALSA modules are stored. The |
| sub-directories contain different modules and are dependent upon the |
| kernel config. |
| |
| core/oss |
| ~~~~~~~~ |
| |
| The codes for PCM and mixer OSS emulation modules are stored in this |
| directory. The rawmidi OSS emulation is included in the ALSA rawmidi |
| code since it's quite small. The sequencer code is stored in |
| ``core/seq/oss`` directory (see `below <#core-seq-oss>`__). |
| |
| core/seq |
| ~~~~~~~~ |
| |
| This directory and its sub-directories are for the ALSA sequencer. This |
| directory contains the sequencer core and primary sequencer modules such |
| like snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when |
| ``CONFIG_SND_SEQUENCER`` is set in the kernel config. |
| |
| core/seq/oss |
| ~~~~~~~~~~~~ |
| |
| This contains the OSS sequencer emulation codes. |
| |
| include directory |
| ----------------- |
| |
| This is the place for the public header files of ALSA drivers, which are |
| to be exported to user-space, or included by several files at different |
| directories. Basically, the private header files should not be placed in |
| this directory, but you may still find files there, due to historical |
| reasons :) |
| |
| drivers directory |
| ----------------- |
| |
| This directory contains code shared among different drivers on different |
| architectures. They are hence supposed not to be architecture-specific. |
| For example, the dummy pcm driver and the serial MIDI driver are found |
| in this directory. In the sub-directories, there is code for components |
| which are independent from bus and cpu architectures. |
| |
| drivers/mpu401 |
| ~~~~~~~~~~~~~~ |
| |
| The MPU401 and MPU401-UART modules are stored here. |
| |
| drivers/opl3 and opl4 |
| ~~~~~~~~~~~~~~~~~~~~~ |
| |
| The OPL3 and OPL4 FM-synth stuff is found here. |
| |
| i2c directory |
| ------------- |
| |
| This contains the ALSA i2c components. |
| |
| Although there is a standard i2c layer on Linux, ALSA has its own i2c |
| code for some cards, because the soundcard needs only a simple operation |
| and the standard i2c API is too complicated for such a purpose. |
| |
| synth directory |
| --------------- |
| |
| This contains the synth middle-level modules. |
| |
| So far, there is only Emu8000/Emu10k1 synth driver under the |
| ``synth/emux`` sub-directory. |
| |
| pci directory |
| ------------- |
| |
| This directory and its sub-directories hold the top-level card modules |
| for PCI soundcards and the code specific to the PCI BUS. |
| |
| The drivers compiled from a single file are stored directly in the pci |
| directory, while the drivers with several source files are stored on |
| their own sub-directory (e.g. emu10k1, ice1712). |
| |
| isa directory |
| ------------- |
| |
| This directory and its sub-directories hold the top-level card modules |
| for ISA soundcards. |
| |
| arm, ppc, and sparc directories |
| ------------------------------- |
| |
| They are used for top-level card modules which are specific to one of |
| these architectures. |
| |
| usb directory |
| ------------- |
| |
| This directory contains the USB-audio driver. In the latest version, the |
| USB MIDI driver is integrated in the usb-audio driver. |
| |
| pcmcia directory |
| ---------------- |
| |
| The PCMCIA, especially PCCard drivers will go here. CardBus drivers will |
| be in the pci directory, because their API is identical to that of |
| standard PCI cards. |
| |
| soc directory |
| ------------- |
| |
| This directory contains the codes for ASoC (ALSA System on Chip) |
| layer including ASoC core, codec and machine drivers. |
| |
| oss directory |
| ------------- |
| |
| Here contains OSS/Lite codes. |
| All codes have been deprecated except for dmasound on m68k as of |
| writing this. |
| |
| |
| Basic Flow for PCI Drivers |
| ========================== |
| |
| Outline |
| ------- |
| |
| The minimum flow for PCI soundcards is as follows: |
| |
| - define the PCI ID table (see the section `PCI Entries`_). |
| |
| - create ``probe`` callback. |
| |
| - create ``remove`` callback. |
| |
| - create a :c:type:`struct pci_driver <pci_driver>` structure |
| containing the three pointers above. |
| |
| - create an ``init`` function just calling the |
| :c:func:`pci_register_driver()` to register the pci_driver |
| table defined above. |
| |
| - create an ``exit`` function to call the |
| :c:func:`pci_unregister_driver()` function. |
| |
| Full Code Example |
| ----------------- |
| |
| The code example is shown below. Some parts are kept unimplemented at |
| this moment but will be filled in the next sections. The numbers in the |
| comment lines of the :c:func:`snd_mychip_probe()` function refer |
| to details explained in the following section. |
| |
| :: |
| |
| #include <linux/init.h> |
| #include <linux/pci.h> |
| #include <linux/slab.h> |
| #include <sound/core.h> |
| #include <sound/initval.h> |
| |
| /* module parameters (see "Module Parameters") */ |
| /* SNDRV_CARDS: maximum number of cards supported by this module */ |
| static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX; |
| static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR; |
| static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP; |
| |
| /* definition of the chip-specific record */ |
| struct mychip { |
| struct snd_card *card; |
| /* the rest of the implementation will be in section |
| * "PCI Resource Management" |
| */ |
| }; |
| |
| /* chip-specific destructor |
| * (see "PCI Resource Management") |
| */ |
| static int snd_mychip_free(struct mychip *chip) |
| { |
| .... /* will be implemented later... */ |
| } |
| |
| /* component-destructor |
| * (see "Management of Cards and Components") |
| */ |
| static int snd_mychip_dev_free(struct snd_device *device) |
| { |
| return snd_mychip_free(device->device_data); |
| } |
| |
| /* chip-specific constructor |
| * (see "Management of Cards and Components") |
| */ |
| static int snd_mychip_create(struct snd_card *card, |
| struct pci_dev *pci, |
| struct mychip **rchip) |
| { |
| struct mychip *chip; |
| int err; |
| static struct snd_device_ops ops = { |
| .dev_free = snd_mychip_dev_free, |
| }; |
| |
| *rchip = NULL; |
| |
| /* check PCI availability here |
| * (see "PCI Resource Management") |
| */ |
| .... |
| |
| /* allocate a chip-specific data with zero filled */ |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| if (chip == NULL) |
| return -ENOMEM; |
| |
| chip->card = card; |
| |
| /* rest of initialization here; will be implemented |
| * later, see "PCI Resource Management" |
| */ |
| .... |
| |
| err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops); |
| if (err < 0) { |
| snd_mychip_free(chip); |
| return err; |
| } |
| |
| *rchip = chip; |
| return 0; |
| } |
| |
| /* constructor -- see "Driver Constructor" sub-section */ |
| static int snd_mychip_probe(struct pci_dev *pci, |
| const struct pci_device_id *pci_id) |
| { |
| static int dev; |
| struct snd_card *card; |
| struct mychip *chip; |
| int err; |
| |
| /* (1) */ |
| if (dev >= SNDRV_CARDS) |
| return -ENODEV; |
| if (!enable[dev]) { |
| dev++; |
| return -ENOENT; |
| } |
| |
| /* (2) */ |
| err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE, |
| 0, &card); |
| if (err < 0) |
| return err; |
| |
| /* (3) */ |
| err = snd_mychip_create(card, pci, &chip); |
| if (err < 0) |
| goto error; |
| |
| /* (4) */ |
| strcpy(card->driver, "My Chip"); |
| strcpy(card->shortname, "My Own Chip 123"); |
| sprintf(card->longname, "%s at 0x%lx irq %i", |
| card->shortname, chip->port, chip->irq); |
| |
| /* (5) */ |
| .... /* implemented later */ |
| |
| /* (6) */ |
| err = snd_card_register(card); |
| if (err < 0) |
| goto error; |
| |
| /* (7) */ |
| pci_set_drvdata(pci, card); |
| dev++; |
| return 0; |
| |
| error: |
| snd_card_free(card); |
| return err; |
| } |
| |
| /* destructor -- see the "Destructor" sub-section */ |
| static void snd_mychip_remove(struct pci_dev *pci) |
| { |
| snd_card_free(pci_get_drvdata(pci)); |
| } |
| |
| |
| |
| Driver Constructor |
| ------------------ |
| |
| The real constructor of PCI drivers is the ``probe`` callback. The |
| ``probe`` callback and other component-constructors which are called |
| from the ``probe`` callback cannot be used with the ``__init`` prefix |
| because any PCI device could be a hotplug device. |
| |
| In the ``probe`` callback, the following scheme is often used. |
| |
| 1) Check and increment the device index. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int dev; |
| .... |
| if (dev >= SNDRV_CARDS) |
| return -ENODEV; |
| if (!enable[dev]) { |
| dev++; |
| return -ENOENT; |
| } |
| |
| |
| where ``enable[dev]`` is the module option. |
| |
| Each time the ``probe`` callback is called, check the availability of |
| the device. If not available, simply increment the device index and |
| returns. dev will be incremented also later (`step 7 |
| <#set-the-pci-driver-data-and-return-zero>`__). |
| |
| 2) Create a card instance |
| ~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| struct snd_card *card; |
| int err; |
| .... |
| err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE, |
| 0, &card); |
| |
| |
| The details will be explained in the section `Management of Cards and |
| Components`_. |
| |
| 3) Create a main component |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| In this part, the PCI resources are allocated. |
| |
| :: |
| |
| struct mychip *chip; |
| .... |
| err = snd_mychip_create(card, pci, &chip); |
| if (err < 0) |
| goto error; |
| |
| The details will be explained in the section `PCI Resource |
| Management`_. |
| |
| When something goes wrong, the probe function needs to deal with the |
| error. In this example, we have a single error handling path placed |
| at the end of the function. |
| |
| :: |
| |
| error: |
| snd_card_free(card); |
| return err; |
| |
| Since each component can be properly freed, the single |
| :c:func:`snd_card_free()` call should suffice in most cases. |
| |
| |
| 4) Set the driver ID and name strings. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| strcpy(card->driver, "My Chip"); |
| strcpy(card->shortname, "My Own Chip 123"); |
| sprintf(card->longname, "%s at 0x%lx irq %i", |
| card->shortname, chip->port, chip->irq); |
| |
| The driver field holds the minimal ID string of the chip. This is used |
| by alsa-lib's configurator, so keep it simple but unique. Even the |
| same driver can have different driver IDs to distinguish the |
| functionality of each chip type. |
| |
| The shortname field is a string shown as more verbose name. The longname |
| field contains the information shown in ``/proc/asound/cards``. |
| |
| 5) Create other components, such as mixer, MIDI, etc. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| Here you define the basic components such as `PCM <#PCM-Interface>`__, |
| mixer (e.g. `AC97 <#API-for-AC97-Codec>`__), MIDI (e.g. |
| `MPU-401 <#MIDI-MPU401-UART-Interface>`__), and other interfaces. |
| Also, if you want a `proc file <#Proc-Interface>`__, define it here, |
| too. |
| |
| 6) Register the card instance. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| err = snd_card_register(card); |
| if (err < 0) |
| goto error; |
| |
| Will be explained in the section `Management of Cards and |
| Components`_, too. |
| |
| 7) Set the PCI driver data and return zero. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| pci_set_drvdata(pci, card); |
| dev++; |
| return 0; |
| |
| In the above, the card record is stored. This pointer is used in the |
| remove callback and power-management callbacks, too. |
| |
| Destructor |
| ---------- |
| |
| The destructor, remove callback, simply releases the card instance. Then |
| the ALSA middle layer will release all the attached components |
| automatically. |
| |
| It would be typically just :c:func:`calling snd_card_free()`: |
| |
| :: |
| |
| static void snd_mychip_remove(struct pci_dev *pci) |
| { |
| snd_card_free(pci_get_drvdata(pci)); |
| } |
| |
| |
| The above code assumes that the card pointer is set to the PCI driver |
| data. |
| |
| Header Files |
| ------------ |
| |
| For the above example, at least the following include files are |
| necessary. |
| |
| :: |
| |
| #include <linux/init.h> |
| #include <linux/pci.h> |
| #include <linux/slab.h> |
| #include <sound/core.h> |
| #include <sound/initval.h> |
| |
| where the last one is necessary only when module options are defined |
| in the source file. If the code is split into several files, the files |
| without module options don't need them. |
| |
| In addition to these headers, you'll need ``<linux/interrupt.h>`` for |
| interrupt handling, and ``<linux/io.h>`` for I/O access. If you use the |
| :c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need |
| to include ``<linux/delay.h>`` too. |
| |
| The ALSA interfaces like the PCM and control APIs are defined in other |
| ``<sound/xxx.h>`` header files. They have to be included after |
| ``<sound/core.h>``. |
| |
| Management of Cards and Components |
| ================================== |
| |
| Card Instance |
| ------------- |
| |
| For each soundcard, a “card” record must be allocated. |
| |
| A card record is the headquarters of the soundcard. It manages the whole |
| list of devices (components) on the soundcard, such as PCM, mixers, |
| MIDI, synthesizer, and so on. Also, the card record holds the ID and the |
| name strings of the card, manages the root of proc files, and controls |
| the power-management states and hotplug disconnections. The component |
| list on the card record is used to manage the correct release of |
| resources at destruction. |
| |
| As mentioned above, to create a card instance, call |
| :c:func:`snd_card_new()`. |
| |
| :: |
| |
| struct snd_card *card; |
| int err; |
| err = snd_card_new(&pci->dev, index, id, module, extra_size, &card); |
| |
| |
| The function takes six arguments: the parent device pointer, the |
| card-index number, the id string, the module pointer (usually |
| ``THIS_MODULE``), the size of extra-data space, and the pointer to |
| return the card instance. The extra_size argument is used to allocate |
| card->private_data for the chip-specific data. Note that these data are |
| allocated by :c:func:`snd_card_new()`. |
| |
| The first argument, the pointer of struct :c:type:`struct device |
| <device>`, specifies the parent device. For PCI devices, typically |
| ``&pci->`` is passed there. |
| |
| Components |
| ---------- |
| |
| After the card is created, you can attach the components (devices) to |
| the card instance. In an ALSA driver, a component is represented as a |
| :c:type:`struct snd_device <snd_device>` object. A component |
| can be a PCM instance, a control interface, a raw MIDI interface, etc. |
| Each such instance has one component entry. |
| |
| A component can be created via :c:func:`snd_device_new()` |
| function. |
| |
| :: |
| |
| snd_device_new(card, SNDRV_DEV_XXX, chip, &ops); |
| |
| This takes the card pointer, the device-level (``SNDRV_DEV_XXX``), the |
| data pointer, and the callback pointers (``&ops``). The device-level |
| defines the type of components and the order of registration and |
| de-registration. For most components, the device-level is already |
| defined. For a user-defined component, you can use |
| ``SNDRV_DEV_LOWLEVEL``. |
| |
| This function itself doesn't allocate the data space. The data must be |
| allocated manually beforehand, and its pointer is passed as the |
| argument. This pointer (``chip`` in the above example) is used as the |
| identifier for the instance. |
| |
| Each pre-defined ALSA component such as ac97 and pcm calls |
| :c:func:`snd_device_new()` inside its constructor. The destructor |
| for each component is defined in the callback pointers. Hence, you don't |
| need to take care of calling a destructor for such a component. |
| |
| If you wish to create your own component, you need to set the destructor |
| function to the dev_free callback in the ``ops``, so that it can be |
| released automatically via :c:func:`snd_card_free()`. The next |
| example will show an implementation of chip-specific data. |
| |
| Chip-Specific Data |
| ------------------ |
| |
| Chip-specific information, e.g. the I/O port address, its resource |
| pointer, or the irq number, is stored in the chip-specific record. |
| |
| :: |
| |
| struct mychip { |
| .... |
| }; |
| |
| |
| In general, there are two ways of allocating the chip record. |
| |
| 1. Allocating via :c:func:`snd_card_new()`. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| As mentioned above, you can pass the extra-data-length to the 5th |
| argument of :c:func:`snd_card_new()`, i.e. |
| |
| :: |
| |
| err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE, |
| sizeof(struct mychip), &card); |
| |
| :c:type:`struct mychip <mychip>` is the type of the chip record. |
| |
| In return, the allocated record can be accessed as |
| |
| :: |
| |
| struct mychip *chip = card->private_data; |
| |
| With this method, you don't have to allocate twice. The record is |
| released together with the card instance. |
| |
| 2. Allocating an extra device. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| After allocating a card instance via :c:func:`snd_card_new()` |
| (with ``0`` on the 4th arg), call :c:func:`kzalloc()`. |
| |
| :: |
| |
| struct snd_card *card; |
| struct mychip *chip; |
| err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE, |
| 0, &card); |
| ..... |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| |
| The chip record should have the field to hold the card pointer at least, |
| |
| :: |
| |
| struct mychip { |
| struct snd_card *card; |
| .... |
| }; |
| |
| |
| Then, set the card pointer in the returned chip instance. |
| |
| :: |
| |
| chip->card = card; |
| |
| Next, initialize the fields, and register this chip record as a |
| low-level device with a specified ``ops``, |
| |
| :: |
| |
| static struct snd_device_ops ops = { |
| .dev_free = snd_mychip_dev_free, |
| }; |
| .... |
| snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops); |
| |
| :c:func:`snd_mychip_dev_free()` is the device-destructor |
| function, which will call the real destructor. |
| |
| :: |
| |
| static int snd_mychip_dev_free(struct snd_device *device) |
| { |
| return snd_mychip_free(device->device_data); |
| } |
| |
| where :c:func:`snd_mychip_free()` is the real destructor. |
| |
| The demerit of this method is the obviously more amount of codes. |
| The merit is, however, you can trigger the own callback at registering |
| and disconnecting the card via setting in snd_device_ops. |
| About the registering and disconnecting the card, see the subsections |
| below. |
| |
| |
| Registration and Release |
| ------------------------ |
| |
| After all components are assigned, register the card instance by calling |
| :c:func:`snd_card_register()`. Access to the device files is |
| enabled at this point. That is, before |
| :c:func:`snd_card_register()` is called, the components are safely |
| inaccessible from external side. If this call fails, exit the probe |
| function after releasing the card via :c:func:`snd_card_free()`. |
| |
| For releasing the card instance, you can call simply |
| :c:func:`snd_card_free()`. As mentioned earlier, all components |
| are released automatically by this call. |
| |
| For a device which allows hotplugging, you can use |
| :c:func:`snd_card_free_when_closed()`. This one will postpone |
| the destruction until all devices are closed. |
| |
| PCI Resource Management |
| ======================= |
| |
| Full Code Example |
| ----------------- |
| |
| In this section, we'll complete the chip-specific constructor, |
| destructor and PCI entries. Example code is shown first, below. |
| |
| :: |
| |
| struct mychip { |
| struct snd_card *card; |
| struct pci_dev *pci; |
| |
| unsigned long port; |
| int irq; |
| }; |
| |
| static int snd_mychip_free(struct mychip *chip) |
| { |
| /* disable hardware here if any */ |
| .... /* (not implemented in this document) */ |
| |
| /* release the irq */ |
| if (chip->irq >= 0) |
| free_irq(chip->irq, chip); |
| /* release the I/O ports & memory */ |
| pci_release_regions(chip->pci); |
| /* disable the PCI entry */ |
| pci_disable_device(chip->pci); |
| /* release the data */ |
| kfree(chip); |
| return 0; |
| } |
| |
| /* chip-specific constructor */ |
| static int snd_mychip_create(struct snd_card *card, |
| struct pci_dev *pci, |
| struct mychip **rchip) |
| { |
| struct mychip *chip; |
| int err; |
| static struct snd_device_ops ops = { |
| .dev_free = snd_mychip_dev_free, |
| }; |
| |
| *rchip = NULL; |
| |
| /* initialize the PCI entry */ |
| err = pci_enable_device(pci); |
| if (err < 0) |
| return err; |
| /* check PCI availability (28bit DMA) */ |
| if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 || |
| pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) { |
| printk(KERN_ERR "error to set 28bit mask DMA\n"); |
| pci_disable_device(pci); |
| return -ENXIO; |
| } |
| |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| if (chip == NULL) { |
| pci_disable_device(pci); |
| return -ENOMEM; |
| } |
| |
| /* initialize the stuff */ |
| chip->card = card; |
| chip->pci = pci; |
| chip->irq = -1; |
| |
| /* (1) PCI resource allocation */ |
| err = pci_request_regions(pci, "My Chip"); |
| if (err < 0) { |
| kfree(chip); |
| pci_disable_device(pci); |
| return err; |
| } |
| chip->port = pci_resource_start(pci, 0); |
| if (request_irq(pci->irq, snd_mychip_interrupt, |
| IRQF_SHARED, KBUILD_MODNAME, chip)) { |
| printk(KERN_ERR "cannot grab irq %d\n", pci->irq); |
| snd_mychip_free(chip); |
| return -EBUSY; |
| } |
| chip->irq = pci->irq; |
| card->sync_irq = chip->irq; |
| |
| /* (2) initialization of the chip hardware */ |
| .... /* (not implemented in this document) */ |
| |
| err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops); |
| if (err < 0) { |
| snd_mychip_free(chip); |
| return err; |
| } |
| |
| *rchip = chip; |
| return 0; |
| } |
| |
| /* PCI IDs */ |
| static struct pci_device_id snd_mychip_ids[] = { |
| { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR, |
| PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, }, |
| .... |
| { 0, } |
| }; |
| MODULE_DEVICE_TABLE(pci, snd_mychip_ids); |
| |
| /* pci_driver definition */ |
| static struct pci_driver driver = { |
| .name = KBUILD_MODNAME, |
| .id_table = snd_mychip_ids, |
| .probe = snd_mychip_probe, |
| .remove = snd_mychip_remove, |
| }; |
| |
| /* module initialization */ |
| static int __init alsa_card_mychip_init(void) |
| { |
| return pci_register_driver(&driver); |
| } |
| |
| /* module clean up */ |
| static void __exit alsa_card_mychip_exit(void) |
| { |
| pci_unregister_driver(&driver); |
| } |
| |
| module_init(alsa_card_mychip_init) |
| module_exit(alsa_card_mychip_exit) |
| |
| EXPORT_NO_SYMBOLS; /* for old kernels only */ |
| |
| Some Hafta's |
| ------------ |
| |
| The allocation of PCI resources is done in the ``probe`` function, and |
| usually an extra :c:func:`xxx_create()` function is written for this |
| purpose. |
| |
| In the case of PCI devices, you first have to call the |
| :c:func:`pci_enable_device()` function before allocating |
| resources. Also, you need to set the proper PCI DMA mask to limit the |
| accessed I/O range. In some cases, you might need to call |
| :c:func:`pci_set_master()` function, too. |
| |
| Suppose the 28bit mask, and the code to be added would be like: |
| |
| :: |
| |
| err = pci_enable_device(pci); |
| if (err < 0) |
| return err; |
| if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 || |
| pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) { |
| printk(KERN_ERR "error to set 28bit mask DMA\n"); |
| pci_disable_device(pci); |
| return -ENXIO; |
| } |
| |
| |
| Resource Allocation |
| ------------------- |
| |
| The allocation of I/O ports and irqs is done via standard kernel |
| functions. These resources must be released in the destructor |
| function (see below). |
| |
| Now assume that the PCI device has an I/O port with 8 bytes and an |
| interrupt. Then :c:type:`struct mychip <mychip>` will have the |
| following fields: |
| |
| :: |
| |
| struct mychip { |
| struct snd_card *card; |
| |
| unsigned long port; |
| int irq; |
| }; |
| |
| |
| For an I/O port (and also a memory region), you need to have the |
| resource pointer for the standard resource management. For an irq, you |
| have to keep only the irq number (integer). But you need to initialize |
| this number as -1 before actual allocation, since irq 0 is valid. The |
| port address and its resource pointer can be initialized as null by |
| :c:func:`kzalloc()` automatically, so you don't have to take care of |
| resetting them. |
| |
| The allocation of an I/O port is done like this: |
| |
| :: |
| |
| err = pci_request_regions(pci, "My Chip"); |
| if (err < 0) { |
| kfree(chip); |
| pci_disable_device(pci); |
| return err; |
| } |
| chip->port = pci_resource_start(pci, 0); |
| |
| It will reserve the I/O port region of 8 bytes of the given PCI device. |
| The returned value, ``chip->res_port``, is allocated via |
| :c:func:`kmalloc()` by :c:func:`request_region()`. The pointer |
| must be released via :c:func:`kfree()`, but there is a problem with |
| this. This issue will be explained later. |
| |
| The allocation of an interrupt source is done like this: |
| |
| :: |
| |
| if (request_irq(pci->irq, snd_mychip_interrupt, |
| IRQF_SHARED, KBUILD_MODNAME, chip)) { |
| printk(KERN_ERR "cannot grab irq %d\n", pci->irq); |
| snd_mychip_free(chip); |
| return -EBUSY; |
| } |
| chip->irq = pci->irq; |
| |
| where :c:func:`snd_mychip_interrupt()` is the interrupt handler |
| defined `later <#pcm-interface-interrupt-handler>`__. Note that |
| ``chip->irq`` should be defined only when :c:func:`request_irq()` |
| succeeded. |
| |
| On the PCI bus, interrupts can be shared. Thus, ``IRQF_SHARED`` is used |
| as the interrupt flag of :c:func:`request_irq()`. |
| |
| The last argument of :c:func:`request_irq()` is the data pointer |
| passed to the interrupt handler. Usually, the chip-specific record is |
| used for that, but you can use what you like, too. |
| |
| I won't give details about the interrupt handler at this point, but at |
| least its appearance can be explained now. The interrupt handler looks |
| usually like the following: |
| |
| :: |
| |
| static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id) |
| { |
| struct mychip *chip = dev_id; |
| .... |
| return IRQ_HANDLED; |
| } |
| |
| After requesting the IRQ, you can passed it to ``card->sync_irq`` |
| field: |
| :: |
| |
| card->irq = chip->irq; |
| |
| This allows PCM core automatically performing |
| :c:func:`synchronize_irq()` at the necessary timing like ``hw_free``. |
| See the later section `sync_stop callback`_ for details. |
| |
| Now let's write the corresponding destructor for the resources above. |
| The role of destructor is simple: disable the hardware (if already |
| activated) and release the resources. So far, we have no hardware part, |
| so the disabling code is not written here. |
| |
| To release the resources, the “check-and-release” method is a safer way. |
| For the interrupt, do like this: |
| |
| :: |
| |
| if (chip->irq >= 0) |
| free_irq(chip->irq, chip); |
| |
| Since the irq number can start from 0, you should initialize |
| ``chip->irq`` with a negative value (e.g. -1), so that you can check |
| the validity of the irq number as above. |
| |
| When you requested I/O ports or memory regions via |
| :c:func:`pci_request_region()` or |
| :c:func:`pci_request_regions()` like in this example, release the |
| resource(s) using the corresponding function, |
| :c:func:`pci_release_region()` or |
| :c:func:`pci_release_regions()`. |
| |
| :: |
| |
| pci_release_regions(chip->pci); |
| |
| When you requested manually via :c:func:`request_region()` or |
| :c:func:`request_mem_region()`, you can release it via |
| :c:func:`release_resource()`. Suppose that you keep the resource |
| pointer returned from :c:func:`request_region()` in |
| chip->res_port, the release procedure looks like: |
| |
| :: |
| |
| release_and_free_resource(chip->res_port); |
| |
| Don't forget to call :c:func:`pci_disable_device()` before the |
| end. |
| |
| And finally, release the chip-specific record. |
| |
| :: |
| |
| kfree(chip); |
| |
| We didn't implement the hardware disabling part in the above. If you |
| need to do this, please note that the destructor may be called even |
| before the initialization of the chip is completed. It would be better |
| to have a flag to skip hardware disabling if the hardware was not |
| initialized yet. |
| |
| When the chip-data is assigned to the card using |
| :c:func:`snd_device_new()` with ``SNDRV_DEV_LOWLELVEL`` , its |
| destructor is called at the last. That is, it is assured that all other |
| components like PCMs and controls have already been released. You don't |
| have to stop PCMs, etc. explicitly, but just call low-level hardware |
| stopping. |
| |
| The management of a memory-mapped region is almost as same as the |
| management of an I/O port. You'll need three fields like the |
| following: |
| |
| :: |
| |
| struct mychip { |
| .... |
| unsigned long iobase_phys; |
| void __iomem *iobase_virt; |
| }; |
| |
| and the allocation would be like below: |
| |
| :: |
| |
| err = pci_request_regions(pci, "My Chip"); |
| if (err < 0) { |
| kfree(chip); |
| return err; |
| } |
| chip->iobase_phys = pci_resource_start(pci, 0); |
| chip->iobase_virt = ioremap(chip->iobase_phys, |
| pci_resource_len(pci, 0)); |
| |
| and the corresponding destructor would be: |
| |
| :: |
| |
| static int snd_mychip_free(struct mychip *chip) |
| { |
| .... |
| if (chip->iobase_virt) |
| iounmap(chip->iobase_virt); |
| .... |
| pci_release_regions(chip->pci); |
| .... |
| } |
| |
| Of course, a modern way with :c:func:`pci_iomap()` will make things a |
| bit easier, too. |
| |
| :: |
| |
| err = pci_request_regions(pci, "My Chip"); |
| if (err < 0) { |
| kfree(chip); |
| return err; |
| } |
| chip->iobase_virt = pci_iomap(pci, 0, 0); |
| |
| which is paired with :c:func:`pci_iounmap()` at destructor. |
| |
| |
| PCI Entries |
| ----------- |
| |
| So far, so good. Let's finish the missing PCI stuff. At first, we need a |
| :c:type:`struct pci_device_id <pci_device_id>` table for |
| this chipset. It's a table of PCI vendor/device ID number, and some |
| masks. |
| |
| For example, |
| |
| :: |
| |
| static struct pci_device_id snd_mychip_ids[] = { |
| { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR, |
| PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, }, |
| .... |
| { 0, } |
| }; |
| MODULE_DEVICE_TABLE(pci, snd_mychip_ids); |
| |
| The first and second fields of the :c:type:`struct pci_device_id |
| <pci_device_id>` structure are the vendor and device IDs. If you |
| have no reason to filter the matching devices, you can leave the |
| remaining fields as above. The last field of the :c:type:`struct |
| pci_device_id <pci_device_id>` struct contains private data |
| for this entry. You can specify any value here, for example, to define |
| specific operations for supported device IDs. Such an example is found |
| in the intel8x0 driver. |
| |
| The last entry of this list is the terminator. You must specify this |
| all-zero entry. |
| |
| Then, prepare the :c:type:`struct pci_driver <pci_driver>` |
| record: |
| |
| :: |
| |
| static struct pci_driver driver = { |
| .name = KBUILD_MODNAME, |
| .id_table = snd_mychip_ids, |
| .probe = snd_mychip_probe, |
| .remove = snd_mychip_remove, |
| }; |
| |
| The ``probe`` and ``remove`` functions have already been defined in |
| the previous sections. The ``name`` field is the name string of this |
| device. Note that you must not use a slash “/” in this string. |
| |
| And at last, the module entries: |
| |
| :: |
| |
| static int __init alsa_card_mychip_init(void) |
| { |
| return pci_register_driver(&driver); |
| } |
| |
| static void __exit alsa_card_mychip_exit(void) |
| { |
| pci_unregister_driver(&driver); |
| } |
| |
| module_init(alsa_card_mychip_init) |
| module_exit(alsa_card_mychip_exit) |
| |
| Note that these module entries are tagged with ``__init`` and ``__exit`` |
| prefixes. |
| |
| That's all! |
| |
| PCM Interface |
| ============= |
| |
| General |
| ------- |
| |
| The PCM middle layer of ALSA is quite powerful and it is only necessary |
| for each driver to implement the low-level functions to access its |
| hardware. |
| |
| For accessing to the PCM layer, you need to include ``<sound/pcm.h>`` |
| first. In addition, ``<sound/pcm_params.h>`` might be needed if you |
| access to some functions related with hw_param. |
| |
| Each card device can have up to four pcm instances. A pcm instance |
| corresponds to a pcm device file. The limitation of number of instances |
| comes only from the available bit size of the Linux's device numbers. |
| Once when 64bit device number is used, we'll have more pcm instances |
| available. |
| |
| A pcm instance consists of pcm playback and capture streams, and each |
| pcm stream consists of one or more pcm substreams. Some soundcards |
| support multiple playback functions. For example, emu10k1 has a PCM |
| playback of 32 stereo substreams. In this case, at each open, a free |
| substream is (usually) automatically chosen and opened. Meanwhile, when |
| only one substream exists and it was already opened, the successful open |
| will either block or error with ``EAGAIN`` according to the file open |
| mode. But you don't have to care about such details in your driver. The |
| PCM middle layer will take care of such work. |
| |
| Full Code Example |
| ----------------- |
| |
| The example code below does not include any hardware access routines but |
| shows only the skeleton, how to build up the PCM interfaces. |
| |
| :: |
| |
| #include <sound/pcm.h> |
| .... |
| |
| /* hardware definition */ |
| static struct snd_pcm_hardware snd_mychip_playback_hw = { |
| .info = (SNDRV_PCM_INFO_MMAP | |
| SNDRV_PCM_INFO_INTERLEAVED | |
| SNDRV_PCM_INFO_BLOCK_TRANSFER | |
| SNDRV_PCM_INFO_MMAP_VALID), |
| .formats = SNDRV_PCM_FMTBIT_S16_LE, |
| .rates = SNDRV_PCM_RATE_8000_48000, |
| .rate_min = 8000, |
| .rate_max = 48000, |
| .channels_min = 2, |
| .channels_max = 2, |
| .buffer_bytes_max = 32768, |
| .period_bytes_min = 4096, |
| .period_bytes_max = 32768, |
| .periods_min = 1, |
| .periods_max = 1024, |
| }; |
| |
| /* hardware definition */ |
| static struct snd_pcm_hardware snd_mychip_capture_hw = { |
| .info = (SNDRV_PCM_INFO_MMAP | |
| SNDRV_PCM_INFO_INTERLEAVED | |
| SNDRV_PCM_INFO_BLOCK_TRANSFER | |
| SNDRV_PCM_INFO_MMAP_VALID), |
| .formats = SNDRV_PCM_FMTBIT_S16_LE, |
| .rates = SNDRV_PCM_RATE_8000_48000, |
| .rate_min = 8000, |
| .rate_max = 48000, |
| .channels_min = 2, |
| .channels_max = 2, |
| .buffer_bytes_max = 32768, |
| .period_bytes_min = 4096, |
| .period_bytes_max = 32768, |
| .periods_min = 1, |
| .periods_max = 1024, |
| }; |
| |
| /* open callback */ |
| static int snd_mychip_playback_open(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| runtime->hw = snd_mychip_playback_hw; |
| /* more hardware-initialization will be done here */ |
| .... |
| return 0; |
| } |
| |
| /* close callback */ |
| static int snd_mychip_playback_close(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| /* the hardware-specific codes will be here */ |
| .... |
| return 0; |
| |
| } |
| |
| /* open callback */ |
| static int snd_mychip_capture_open(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| runtime->hw = snd_mychip_capture_hw; |
| /* more hardware-initialization will be done here */ |
| .... |
| return 0; |
| } |
| |
| /* close callback */ |
| static int snd_mychip_capture_close(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| /* the hardware-specific codes will be here */ |
| .... |
| return 0; |
| } |
| |
| /* hw_params callback */ |
| static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream, |
| struct snd_pcm_hw_params *hw_params) |
| { |
| /* the hardware-specific codes will be here */ |
| .... |
| return 0; |
| } |
| |
| /* hw_free callback */ |
| static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream) |
| { |
| /* the hardware-specific codes will be here */ |
| .... |
| return 0; |
| } |
| |
| /* prepare callback */ |
| static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| /* set up the hardware with the current configuration |
| * for example... |
| */ |
| mychip_set_sample_format(chip, runtime->format); |
| mychip_set_sample_rate(chip, runtime->rate); |
| mychip_set_channels(chip, runtime->channels); |
| mychip_set_dma_setup(chip, runtime->dma_addr, |
| chip->buffer_size, |
| chip->period_size); |
| return 0; |
| } |
| |
| /* trigger callback */ |
| static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream, |
| int cmd) |
| { |
| switch (cmd) { |
| case SNDRV_PCM_TRIGGER_START: |
| /* do something to start the PCM engine */ |
| .... |
| break; |
| case SNDRV_PCM_TRIGGER_STOP: |
| /* do something to stop the PCM engine */ |
| .... |
| break; |
| default: |
| return -EINVAL; |
| } |
| } |
| |
| /* pointer callback */ |
| static snd_pcm_uframes_t |
| snd_mychip_pcm_pointer(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| unsigned int current_ptr; |
| |
| /* get the current hardware pointer */ |
| current_ptr = mychip_get_hw_pointer(chip); |
| return current_ptr; |
| } |
| |
| /* operators */ |
| static struct snd_pcm_ops snd_mychip_playback_ops = { |
| .open = snd_mychip_playback_open, |
| .close = snd_mychip_playback_close, |
| .hw_params = snd_mychip_pcm_hw_params, |
| .hw_free = snd_mychip_pcm_hw_free, |
| .prepare = snd_mychip_pcm_prepare, |
| .trigger = snd_mychip_pcm_trigger, |
| .pointer = snd_mychip_pcm_pointer, |
| }; |
| |
| /* operators */ |
| static struct snd_pcm_ops snd_mychip_capture_ops = { |
| .open = snd_mychip_capture_open, |
| .close = snd_mychip_capture_close, |
| .hw_params = snd_mychip_pcm_hw_params, |
| .hw_free = snd_mychip_pcm_hw_free, |
| .prepare = snd_mychip_pcm_prepare, |
| .trigger = snd_mychip_pcm_trigger, |
| .pointer = snd_mychip_pcm_pointer, |
| }; |
| |
| /* |
| * definitions of capture are omitted here... |
| */ |
| |
| /* create a pcm device */ |
| static int snd_mychip_new_pcm(struct mychip *chip) |
| { |
| struct snd_pcm *pcm; |
| int err; |
| |
| err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm); |
| if (err < 0) |
| return err; |
| pcm->private_data = chip; |
| strcpy(pcm->name, "My Chip"); |
| chip->pcm = pcm; |
| /* set operators */ |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, |
| &snd_mychip_playback_ops); |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, |
| &snd_mychip_capture_ops); |
| /* pre-allocation of buffers */ |
| /* NOTE: this may fail */ |
| snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV, |
| &chip->pci->dev, |
| 64*1024, 64*1024); |
| return 0; |
| } |
| |
| |
| PCM Constructor |
| --------------- |
| |
| A pcm instance is allocated by the :c:func:`snd_pcm_new()` |
| function. It would be better to create a constructor for pcm, namely, |
| |
| :: |
| |
| static int snd_mychip_new_pcm(struct mychip *chip) |
| { |
| struct snd_pcm *pcm; |
| int err; |
| |
| err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm); |
| if (err < 0) |
| return err; |
| pcm->private_data = chip; |
| strcpy(pcm->name, "My Chip"); |
| chip->pcm = pcm; |
| .... |
| return 0; |
| } |
| |
| The :c:func:`snd_pcm_new()` function takes four arguments. The |
| first argument is the card pointer to which this pcm is assigned, and |
| the second is the ID string. |
| |
| The third argument (``index``, 0 in the above) is the index of this new |
| pcm. It begins from zero. If you create more than one pcm instances, |
| specify the different numbers in this argument. For example, ``index = |
| 1`` for the second PCM device. |
| |
| The fourth and fifth arguments are the number of substreams for playback |
| and capture, respectively. Here 1 is used for both arguments. When no |
| playback or capture substreams are available, pass 0 to the |
| corresponding argument. |
| |
| If a chip supports multiple playbacks or captures, you can specify more |
| numbers, but they must be handled properly in open/close, etc. |
| callbacks. When you need to know which substream you are referring to, |
| then it can be obtained from :c:type:`struct snd_pcm_substream |
| <snd_pcm_substream>` data passed to each callback as follows: |
| |
| :: |
| |
| struct snd_pcm_substream *substream; |
| int index = substream->number; |
| |
| |
| After the pcm is created, you need to set operators for each pcm stream. |
| |
| :: |
| |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, |
| &snd_mychip_playback_ops); |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, |
| &snd_mychip_capture_ops); |
| |
| The operators are defined typically like this: |
| |
| :: |
| |
| static struct snd_pcm_ops snd_mychip_playback_ops = { |
| .open = snd_mychip_pcm_open, |
| .close = snd_mychip_pcm_close, |
| .hw_params = snd_mychip_pcm_hw_params, |
| .hw_free = snd_mychip_pcm_hw_free, |
| .prepare = snd_mychip_pcm_prepare, |
| .trigger = snd_mychip_pcm_trigger, |
| .pointer = snd_mychip_pcm_pointer, |
| }; |
| |
| All the callbacks are described in the Operators_ subsection. |
| |
| After setting the operators, you probably will want to pre-allocate the |
| buffer and set up the managed allocation mode. |
| For that, simply call the following: |
| |
| :: |
| |
| snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV, |
| &chip->pci->dev, |
| 64*1024, 64*1024); |
| |
| It will allocate a buffer up to 64kB as default. Buffer management |
| details will be described in the later section `Buffer and Memory |
| Management`_. |
| |
| Additionally, you can set some extra information for this pcm in |
| ``pcm->info_flags``. The available values are defined as |
| ``SNDRV_PCM_INFO_XXX`` in ``<sound/asound.h>``, which is used for the |
| hardware definition (described later). When your soundchip supports only |
| half-duplex, specify like this: |
| |
| :: |
| |
| pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX; |
| |
| |
| ... And the Destructor? |
| ----------------------- |
| |
| The destructor for a pcm instance is not always necessary. Since the pcm |
| device will be released by the middle layer code automatically, you |
| don't have to call the destructor explicitly. |
| |
| The destructor would be necessary if you created special records |
| internally and needed to release them. In such a case, set the |
| destructor function to ``pcm->private_free``: |
| |
| :: |
| |
| static void mychip_pcm_free(struct snd_pcm *pcm) |
| { |
| struct mychip *chip = snd_pcm_chip(pcm); |
| /* free your own data */ |
| kfree(chip->my_private_pcm_data); |
| /* do what you like else */ |
| .... |
| } |
| |
| static int snd_mychip_new_pcm(struct mychip *chip) |
| { |
| struct snd_pcm *pcm; |
| .... |
| /* allocate your own data */ |
| chip->my_private_pcm_data = kmalloc(...); |
| /* set the destructor */ |
| pcm->private_data = chip; |
| pcm->private_free = mychip_pcm_free; |
| .... |
| } |
| |
| |
| |
| Runtime Pointer - The Chest of PCM Information |
| ---------------------------------------------- |
| |
| When the PCM substream is opened, a PCM runtime instance is allocated |
| and assigned to the substream. This pointer is accessible via |
| ``substream->runtime``. This runtime pointer holds most information you |
| need to control the PCM: the copy of hw_params and sw_params |
| configurations, the buffer pointers, mmap records, spinlocks, etc. |
| |
| The definition of runtime instance is found in ``<sound/pcm.h>``. Here |
| are the contents of this file: |
| |
| :: |
| |
| struct _snd_pcm_runtime { |
| /* -- Status -- */ |
| struct snd_pcm_substream *trigger_master; |
| snd_timestamp_t trigger_tstamp; /* trigger timestamp */ |
| int overrange; |
| snd_pcm_uframes_t avail_max; |
| snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */ |
| snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/ |
| |
| /* -- HW params -- */ |
| snd_pcm_access_t access; /* access mode */ |
| snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */ |
| snd_pcm_subformat_t subformat; /* subformat */ |
| unsigned int rate; /* rate in Hz */ |
| unsigned int channels; /* channels */ |
| snd_pcm_uframes_t period_size; /* period size */ |
| unsigned int periods; /* periods */ |
| snd_pcm_uframes_t buffer_size; /* buffer size */ |
| unsigned int tick_time; /* tick time */ |
| snd_pcm_uframes_t min_align; /* Min alignment for the format */ |
| size_t byte_align; |
| unsigned int frame_bits; |
| unsigned int sample_bits; |
| unsigned int info; |
| unsigned int rate_num; |
| unsigned int rate_den; |
| |
| /* -- SW params -- */ |
| struct timespec tstamp_mode; /* mmap timestamp is updated */ |
| unsigned int period_step; |
| unsigned int sleep_min; /* min ticks to sleep */ |
| snd_pcm_uframes_t start_threshold; |
| snd_pcm_uframes_t stop_threshold; |
| snd_pcm_uframes_t silence_threshold; /* Silence filling happens when |
| noise is nearest than this */ |
| snd_pcm_uframes_t silence_size; /* Silence filling size */ |
| snd_pcm_uframes_t boundary; /* pointers wrap point */ |
| |
| snd_pcm_uframes_t silenced_start; |
| snd_pcm_uframes_t silenced_size; |
| |
| snd_pcm_sync_id_t sync; /* hardware synchronization ID */ |
| |
| /* -- mmap -- */ |
| volatile struct snd_pcm_mmap_status *status; |
| volatile struct snd_pcm_mmap_control *control; |
| atomic_t mmap_count; |
| |
| /* -- locking / scheduling -- */ |
| spinlock_t lock; |
| wait_queue_head_t sleep; |
| struct timer_list tick_timer; |
| struct fasync_struct *fasync; |
| |
| /* -- private section -- */ |
| void *private_data; |
| void (*private_free)(struct snd_pcm_runtime *runtime); |
| |
| /* -- hardware description -- */ |
| struct snd_pcm_hardware hw; |
| struct snd_pcm_hw_constraints hw_constraints; |
| |
| /* -- timer -- */ |
| unsigned int timer_resolution; /* timer resolution */ |
| |
| /* -- DMA -- */ |
| unsigned char *dma_area; /* DMA area */ |
| dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */ |
| size_t dma_bytes; /* size of DMA area */ |
| |
| struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */ |
| |
| #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE) |
| /* -- OSS things -- */ |
| struct snd_pcm_oss_runtime oss; |
| #endif |
| }; |
| |
| |
| For the operators (callbacks) of each sound driver, most of these |
| records are supposed to be read-only. Only the PCM middle-layer changes |
| / updates them. The exceptions are the hardware description (hw) DMA |
| buffer information and the private data. Besides, if you use the |
| standard managed buffer allocation mode, you don't need to set the |
| DMA buffer information by yourself. |
| |
| In the sections below, important records are explained. |
| |
| Hardware Description |
| ~~~~~~~~~~~~~~~~~~~~ |
| |
| The hardware descriptor (:c:type:`struct snd_pcm_hardware |
| <snd_pcm_hardware>`) contains the definitions of the fundamental |
| hardware configuration. Above all, you'll need to define this in the |
| `PCM open callback`_. Note that the runtime instance holds the copy of |
| the descriptor, not the pointer to the existing descriptor. That is, |
| in the open callback, you can modify the copied descriptor |
| (``runtime->hw``) as you need. For example, if the maximum number of |
| channels is 1 only on some chip models, you can still use the same |
| hardware descriptor and change the channels_max later: |
| |
| :: |
| |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| ... |
| runtime->hw = snd_mychip_playback_hw; /* common definition */ |
| if (chip->model == VERY_OLD_ONE) |
| runtime->hw.channels_max = 1; |
| |
| Typically, you'll have a hardware descriptor as below: |
| |
| :: |
| |
| static struct snd_pcm_hardware snd_mychip_playback_hw = { |
| .info = (SNDRV_PCM_INFO_MMAP | |
| SNDRV_PCM_INFO_INTERLEAVED | |
| SNDRV_PCM_INFO_BLOCK_TRANSFER | |
| SNDRV_PCM_INFO_MMAP_VALID), |
| .formats = SNDRV_PCM_FMTBIT_S16_LE, |
| .rates = SNDRV_PCM_RATE_8000_48000, |
| .rate_min = 8000, |
| .rate_max = 48000, |
| .channels_min = 2, |
| .channels_max = 2, |
| .buffer_bytes_max = 32768, |
| .period_bytes_min = 4096, |
| .period_bytes_max = 32768, |
| .periods_min = 1, |
| .periods_max = 1024, |
| }; |
| |
| - The ``info`` field contains the type and capabilities of this |
| pcm. The bit flags are defined in ``<sound/asound.h>`` as |
| ``SNDRV_PCM_INFO_XXX``. Here, at least, you have to specify whether |
| the mmap is supported and which interleaved format is |
| supported. When the hardware supports mmap, add the |
| ``SNDRV_PCM_INFO_MMAP`` flag here. When the hardware supports the |
| interleaved or the non-interleaved formats, |
| ``SNDRV_PCM_INFO_INTERLEAVED`` or ``SNDRV_PCM_INFO_NONINTERLEAVED`` |
| flag must be set, respectively. If both are supported, you can set |
| both, too. |
| |
| In the above example, ``MMAP_VALID`` and ``BLOCK_TRANSFER`` are |
| specified for the OSS mmap mode. Usually both are set. Of course, |
| ``MMAP_VALID`` is set only if the mmap is really supported. |
| |
| The other possible flags are ``SNDRV_PCM_INFO_PAUSE`` and |
| ``SNDRV_PCM_INFO_RESUME``. The ``PAUSE`` bit means that the pcm |
| supports the “pause” operation, while the ``RESUME`` bit means that |
| the pcm supports the full “suspend/resume” operation. If the |
| ``PAUSE`` flag is set, the ``trigger`` callback below must handle |
| the corresponding (pause push/release) commands. The suspend/resume |
| trigger commands can be defined even without the ``RESUME`` |
| flag. See `Power Management`_ section for details. |
| |
| When the PCM substreams can be synchronized (typically, |
| synchronized start/stop of a playback and a capture streams), you |
| can give ``SNDRV_PCM_INFO_SYNC_START``, too. In this case, you'll |
| need to check the linked-list of PCM substreams in the trigger |
| callback. This will be described in the later section. |
| |
| - ``formats`` field contains the bit-flags of supported formats |
| (``SNDRV_PCM_FMTBIT_XXX``). If the hardware supports more than one |
| format, give all or'ed bits. In the example above, the signed 16bit |
| little-endian format is specified. |
| |
| - ``rates`` field contains the bit-flags of supported rates |
| (``SNDRV_PCM_RATE_XXX``). When the chip supports continuous rates, |
| pass ``CONTINUOUS`` bit additionally. The pre-defined rate bits are |
| provided only for typical rates. If your chip supports |
| unconventional rates, you need to add the ``KNOT`` bit and set up |
| the hardware constraint manually (explained later). |
| |
| - ``rate_min`` and ``rate_max`` define the minimum and maximum sample |
| rate. This should correspond somehow to ``rates`` bits. |
| |
| - ``channel_min`` and ``channel_max`` define, as you might already |
| expected, the minimum and maximum number of channels. |
| |
| - ``buffer_bytes_max`` defines the maximum buffer size in |
| bytes. There is no ``buffer_bytes_min`` field, since it can be |
| calculated from the minimum period size and the minimum number of |
| periods. Meanwhile, ``period_bytes_min`` and define the minimum and |
| maximum size of the period in bytes. ``periods_max`` and |
| ``periods_min`` define the maximum and minimum number of periods in |
| the buffer. |
| |
| The “period” is a term that corresponds to a fragment in the OSS |
| world. The period defines the size at which a PCM interrupt is |
| generated. This size strongly depends on the hardware. Generally, |
| the smaller period size will give you more interrupts, that is, |
| more controls. In the case of capture, this size defines the input |
| latency. On the other hand, the whole buffer size defines the |
| output latency for the playback direction. |
| |
| - There is also a field ``fifo_size``. This specifies the size of the |
| hardware FIFO, but currently it is neither used in the driver nor |
| in the alsa-lib. So, you can ignore this field. |
| |
| PCM Configurations |
| ~~~~~~~~~~~~~~~~~~ |
| |
| Ok, let's go back again to the PCM runtime records. The most |
| frequently referred records in the runtime instance are the PCM |
| configurations. The PCM configurations are stored in the runtime |
| instance after the application sends ``hw_params`` data via |
| alsa-lib. There are many fields copied from hw_params and sw_params |
| structs. For example, ``format`` holds the format type chosen by the |
| application. This field contains the enum value |
| ``SNDRV_PCM_FORMAT_XXX``. |
| |
| One thing to be noted is that the configured buffer and period sizes |
| are stored in “frames” in the runtime. In the ALSA world, ``1 frame = |
| channels \* samples-size``. For conversion between frames and bytes, |
| you can use the :c:func:`frames_to_bytes()` and |
| :c:func:`bytes_to_frames()` helper functions. |
| |
| :: |
| |
| period_bytes = frames_to_bytes(runtime, runtime->period_size); |
| |
| Also, many software parameters (sw_params) are stored in frames, too. |
| Please check the type of the field. ``snd_pcm_uframes_t`` is for the |
| frames as unsigned integer while ``snd_pcm_sframes_t`` is for the |
| frames as signed integer. |
| |
| DMA Buffer Information |
| ~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The DMA buffer is defined by the following four fields, ``dma_area``, |
| ``dma_addr``, ``dma_bytes`` and ``dma_private``. The ``dma_area`` |
| holds the buffer pointer (the logical address). You can call |
| :c:func:`memcpy()` from/to this pointer. Meanwhile, ``dma_addr`` holds |
| the physical address of the buffer. This field is specified only when |
| the buffer is a linear buffer. ``dma_bytes`` holds the size of buffer |
| in bytes. ``dma_private`` is used for the ALSA DMA allocator. |
| |
| If you use either the managed buffer allocation mode or the standard |
| API function :c:func:`snd_pcm_lib_malloc_pages()` for allocating the buffer, |
| these fields are set by the ALSA middle layer, and you should *not* |
| change them by yourself. You can read them but not write them. On the |
| other hand, if you want to allocate the buffer by yourself, you'll |
| need to manage it in hw_params callback. At least, ``dma_bytes`` is |
| mandatory. ``dma_area`` is necessary when the buffer is mmapped. If |
| your driver doesn't support mmap, this field is not |
| necessary. ``dma_addr`` is also optional. You can use dma_private as |
| you like, too. |
| |
| Running Status |
| ~~~~~~~~~~~~~~ |
| |
| The running status can be referred via ``runtime->status``. This is |
| the pointer to the :c:type:`struct snd_pcm_mmap_status |
| <snd_pcm_mmap_status>` record. For example, you can get the current |
| DMA hardware pointer via ``runtime->status->hw_ptr``. |
| |
| The DMA application pointer can be referred via ``runtime->control``, |
| which points to the :c:type:`struct snd_pcm_mmap_control |
| <snd_pcm_mmap_control>` record. However, accessing directly to |
| this value is not recommended. |
| |
| Private Data |
| ~~~~~~~~~~~~ |
| |
| You can allocate a record for the substream and store it in |
| ``runtime->private_data``. Usually, this is done in the `PCM open |
| callback`_. Don't mix this with ``pcm->private_data``. The |
| ``pcm->private_data`` usually points to the chip instance assigned |
| statically at the creation of PCM, while the ``runtime->private_data`` |
| points to a dynamic data structure created at the PCM open |
| callback. |
| |
| :: |
| |
| static int snd_xxx_open(struct snd_pcm_substream *substream) |
| { |
| struct my_pcm_data *data; |
| .... |
| data = kmalloc(sizeof(*data), GFP_KERNEL); |
| substream->runtime->private_data = data; |
| .... |
| } |
| |
| |
| The allocated object must be released in the `close callback`_. |
| |
| Operators |
| --------- |
| |
| OK, now let me give details about each pcm callback (``ops``). In |
| general, every callback must return 0 if successful, or a negative |
| error number such as ``-EINVAL``. To choose an appropriate error |
| number, it is advised to check what value other parts of the kernel |
| return when the same kind of request fails. |
| |
| The callback function takes at least the argument with :c:type:`struct |
| snd_pcm_substream <snd_pcm_substream>` pointer. To retrieve the chip |
| record from the given substream instance, you can use the following |
| macro. |
| |
| :: |
| |
| int xxx() { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| .... |
| } |
| |
| The macro reads ``substream->private_data``, which is a copy of |
| ``pcm->private_data``. You can override the former if you need to |
| assign different data records per PCM substream. For example, the |
| cmi8330 driver assigns different ``private_data`` for playback and |
| capture directions, because it uses two different codecs (SB- and |
| AD-compatible) for different directions. |
| |
| PCM open callback |
| ~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_open(struct snd_pcm_substream *substream); |
| |
| This is called when a pcm substream is opened. |
| |
| At least, here you have to initialize the ``runtime->hw`` |
| record. Typically, this is done by like this: |
| |
| :: |
| |
| static int snd_xxx_open(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| runtime->hw = snd_mychip_playback_hw; |
| return 0; |
| } |
| |
| where ``snd_mychip_playback_hw`` is the pre-defined hardware |
| description. |
| |
| You can allocate a private data in this callback, as described in |
| `Private Data`_ section. |
| |
| If the hardware configuration needs more constraints, set the hardware |
| constraints here, too. See Constraints_ for more details. |
| |
| close callback |
| ~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_close(struct snd_pcm_substream *substream); |
| |
| |
| Obviously, this is called when a pcm substream is closed. |
| |
| Any private instance for a pcm substream allocated in the ``open`` |
| callback will be released here. |
| |
| :: |
| |
| static int snd_xxx_close(struct snd_pcm_substream *substream) |
| { |
| .... |
| kfree(substream->runtime->private_data); |
| .... |
| } |
| |
| ioctl callback |
| ~~~~~~~~~~~~~~ |
| |
| This is used for any special call to pcm ioctls. But usually you can |
| leave it as NULL, then PCM core calls the generic ioctl callback |
| function :c:func:`snd_pcm_lib_ioctl()`. If you need to deal with the |
| unique setup of channel info or reset procedure, you can pass your own |
| callback function here. |
| |
| hw_params callback |
| ~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_hw_params(struct snd_pcm_substream *substream, |
| struct snd_pcm_hw_params *hw_params); |
| |
| This is called when the hardware parameter (``hw_params``) is set up |
| by the application, that is, once when the buffer size, the period |
| size, the format, etc. are defined for the pcm substream. |
| |
| Many hardware setups should be done in this callback, including the |
| allocation of buffers. |
| |
| Parameters to be initialized are retrieved by |
| :c:func:`params_xxx()` macros. |
| |
| When you set up the managed buffer allocation mode for the substream, |
| a buffer is already allocated before this callback gets |
| called. Alternatively, you can call a helper function below for |
| allocating the buffer, too. |
| |
| :: |
| |
| snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params)); |
| |
| :c:func:`snd_pcm_lib_malloc_pages()` is available only when the |
| DMA buffers have been pre-allocated. See the section `Buffer Types`_ |
| for more details. |
| |
| Note that this and ``prepare`` callbacks may be called multiple times |
| per initialization. For example, the OSS emulation may call these |
| callbacks at each change via its ioctl. |
| |
| Thus, you need to be careful not to allocate the same buffers many |
| times, which will lead to memory leaks! Calling the helper function |
| above many times is OK. It will release the previous buffer |
| automatically when it was already allocated. |
| |
| Another note is that this callback is non-atomic (schedulable) as |
| default, i.e. when no ``nonatomic`` flag set. This is important, |
| because the ``trigger`` callback is atomic (non-schedulable). That is, |
| mutexes or any schedule-related functions are not available in |
| ``trigger`` callback. Please see the subsection Atomicity_ for |
| details. |
| |
| hw_free callback |
| ~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_hw_free(struct snd_pcm_substream *substream); |
| |
| This is called to release the resources allocated via |
| ``hw_params``. |
| |
| This function is always called before the close callback is called. |
| Also, the callback may be called multiple times, too. Keep track |
| whether the resource was already released. |
| |
| When you have set up the managed buffer allocation mode for the PCM |
| substream, the allocated PCM buffer will be automatically released |
| after this callback gets called. Otherwise you'll have to release the |
| buffer manually. Typically, when the buffer was allocated from the |
| pre-allocated pool, you can use the standard API function |
| :c:func:`snd_pcm_lib_malloc_pages()` like: |
| |
| :: |
| |
| snd_pcm_lib_free_pages(substream); |
| |
| prepare callback |
| ~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_prepare(struct snd_pcm_substream *substream); |
| |
| This callback is called when the pcm is “prepared”. You can set the |
| format type, sample rate, etc. here. The difference from ``hw_params`` |
| is that the ``prepare`` callback will be called each time |
| :c:func:`snd_pcm_prepare()` is called, i.e. when recovering after |
| underruns, etc. |
| |
| Note that this callback is now non-atomic. You can use |
| schedule-related functions safely in this callback. |
| |
| In this and the following callbacks, you can refer to the values via |
| the runtime record, ``substream->runtime``. For example, to get the |
| current rate, format or channels, access to ``runtime->rate``, |
| ``runtime->format`` or ``runtime->channels``, respectively. The |
| physical address of the allocated buffer is set to |
| ``runtime->dma_area``. The buffer and period sizes are in |
| ``runtime->buffer_size`` and ``runtime->period_size``, respectively. |
| |
| Be careful that this callback will be called many times at each setup, |
| too. |
| |
| trigger callback |
| ~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd); |
| |
| This is called when the pcm is started, stopped or paused. |
| |
| Which action is specified in the second argument, |
| ``SNDRV_PCM_TRIGGER_XXX`` in ``<sound/pcm.h>``. At least, the ``START`` |
| and ``STOP`` commands must be defined in this callback. |
| |
| :: |
| |
| switch (cmd) { |
| case SNDRV_PCM_TRIGGER_START: |
| /* do something to start the PCM engine */ |
| break; |
| case SNDRV_PCM_TRIGGER_STOP: |
| /* do something to stop the PCM engine */ |
| break; |
| default: |
| return -EINVAL; |
| } |
| |
| When the pcm supports the pause operation (given in the info field of |
| the hardware table), the ``PAUSE_PUSH`` and ``PAUSE_RELEASE`` commands |
| must be handled here, too. The former is the command to pause the pcm, |
| and the latter to restart the pcm again. |
| |
| When the pcm supports the suspend/resume operation, regardless of full |
| or partial suspend/resume support, the ``SUSPEND`` and ``RESUME`` |
| commands must be handled, too. These commands are issued when the |
| power-management status is changed. Obviously, the ``SUSPEND`` and |
| ``RESUME`` commands suspend and resume the pcm substream, and usually, |
| they are identical to the ``STOP`` and ``START`` commands, respectively. |
| See the `Power Management`_ section for details. |
| |
| As mentioned, this callback is atomic as default unless ``nonatomic`` |
| flag set, and you cannot call functions which may sleep. The |
| ``trigger`` callback should be as minimal as possible, just really |
| triggering the DMA. The other stuff should be initialized |
| ``hw_params`` and ``prepare`` callbacks properly beforehand. |
| |
| sync_stop callback |
| ~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_sync_stop(struct snd_pcm_substream *substream); |
| |
| This callback is optional, and NULL can be passed. It's called after |
| the PCM core stops the stream and changes the stream state |
| ``prepare``, ``hw_params`` or ``hw_free``. |
| Since the IRQ handler might be still pending, we need to wait until |
| the pending task finishes before moving to the next step; otherwise it |
| might lead to a crash due to resource conflicts or access to the freed |
| resources. A typical behavior is to call a synchronization function |
| like :c:func:`synchronize_irq()` here. |
| |
| For majority of drivers that need only a call of |
| :c:func:`synchronize_irq()`, there is a simpler setup, too. |
| While keeping NULL to ``sync_stop`` PCM callback, the driver can set |
| ``card->sync_irq`` field to store the valid interrupt number after |
| requesting an IRQ, instead. Then PCM core will look call |
| :c:func:`synchronize_irq()` with the given IRQ appropriately. |
| |
| If the IRQ handler is released at the card destructor, you don't need |
| to clear ``card->sync_irq``, as the card itself is being released. |
| So, usually you'll need to add just a single line for assigning |
| ``card->sync_irq`` in the driver code unless the driver re-acquires |
| the IRQ. When the driver frees and re-acquires the IRQ dynamically |
| (e.g. for suspend/resume), it needs to clear and re-set |
| ``card->sync_irq`` again appropriately. |
| |
| pointer callback |
| ~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream) |
| |
| This callback is called when the PCM middle layer inquires the current |
| hardware position on the buffer. The position must be returned in |
| frames, ranging from 0 to ``buffer_size - 1``. |
| |
| This is called usually from the buffer-update routine in the pcm |
| middle layer, which is invoked when :c:func:`snd_pcm_period_elapsed()` |
| is called in the interrupt routine. Then the pcm middle layer updates |
| the position and calculates the available space, and wakes up the |
| sleeping poll threads, etc. |
| |
| This callback is also atomic as default. |
| |
| copy_user, copy_kernel and fill_silence ops |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| These callbacks are not mandatory, and can be omitted in most cases. |
| These callbacks are used when the hardware buffer cannot be in the |
| normal memory space. Some chips have their own buffer on the hardware |
| which is not mappable. In such a case, you have to transfer the data |
| manually from the memory buffer to the hardware buffer. Or, if the |
| buffer is non-contiguous on both physical and virtual memory spaces, |
| these callbacks must be defined, too. |
| |
| If these two callbacks are defined, copy and set-silence operations |
| are done by them. The detailed will be described in the later section |
| `Buffer and Memory Management`_. |
| |
| ack callback |
| ~~~~~~~~~~~~ |
| |
| This callback is also not mandatory. This callback is called when the |
| ``appl_ptr`` is updated in read or write operations. Some drivers like |
| emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the |
| internal buffer, and this callback is useful only for such a purpose. |
| |
| This callback is atomic as default. |
| |
| page callback |
| ~~~~~~~~~~~~~ |
| |
| This callback is optional too. The mmap calls this callback to get the |
| page fault address. |
| |
| Since the recent changes, you need no special callback any longer for |
| the standard SG-buffer or vmalloc-buffer. Hence this callback should |
| be rarely used. |
| |
| mmap calllback |
| ~~~~~~~~~~~~~~ |
| |
| This is another optional callback for controlling mmap behavior. |
| Once when defined, PCM core calls this callback when a page is |
| memory-mapped instead of dealing via the standard helper. |
| If you need special handling (due to some architecture or |
| device-specific issues), implement everything here as you like. |
| |
| |
| PCM Interrupt Handler |
| --------------------- |
| |
| The rest of pcm stuff is the PCM interrupt handler. The role of PCM |
| interrupt handler in the sound driver is to update the buffer position |
| and to tell the PCM middle layer when the buffer position goes across |
| the prescribed period size. To inform this, call the |
| :c:func:`snd_pcm_period_elapsed()` function. |
| |
| There are several types of sound chips to generate the interrupts. |
| |
| Interrupts at the period (fragment) boundary |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| This is the most frequently found type: the hardware generates an |
| interrupt at each period boundary. In this case, you can call |
| :c:func:`snd_pcm_period_elapsed()` at each interrupt. |
| |
| :c:func:`snd_pcm_period_elapsed()` takes the substream pointer as |
| its argument. Thus, you need to keep the substream pointer accessible |
| from the chip instance. For example, define ``substream`` field in the |
| chip record to hold the current running substream pointer, and set the |
| pointer value at ``open`` callback (and reset at ``close`` callback). |
| |
| If you acquire a spinlock in the interrupt handler, and the lock is used |
| in other pcm callbacks, too, then you have to release the lock before |
| calling :c:func:`snd_pcm_period_elapsed()`, because |
| :c:func:`snd_pcm_period_elapsed()` calls other pcm callbacks |
| inside. |
| |
| Typical code would be like: |
| |
| :: |
| |
| |
| static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id) |
| { |
| struct mychip *chip = dev_id; |
| spin_lock(&chip->lock); |
| .... |
| if (pcm_irq_invoked(chip)) { |
| /* call updater, unlock before it */ |
| spin_unlock(&chip->lock); |
| snd_pcm_period_elapsed(chip->substream); |
| spin_lock(&chip->lock); |
| /* acknowledge the interrupt if necessary */ |
| } |
| .... |
| spin_unlock(&chip->lock); |
| return IRQ_HANDLED; |
| } |
| |
| |
| |
| High frequency timer interrupts |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| This happens when the hardware doesn't generate interrupts at the period |
| boundary but issues timer interrupts at a fixed timer rate (e.g. es1968 |
| or ymfpci drivers). In this case, you need to check the current hardware |
| position and accumulate the processed sample length at each interrupt. |
| When the accumulated size exceeds the period size, call |
| :c:func:`snd_pcm_period_elapsed()` and reset the accumulator. |
| |
| Typical code would be like the following. |
| |
| :: |
| |
| |
| static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id) |
| { |
| struct mychip *chip = dev_id; |
| spin_lock(&chip->lock); |
| .... |
| if (pcm_irq_invoked(chip)) { |
| unsigned int last_ptr, size; |
| /* get the current hardware pointer (in frames) */ |
| last_ptr = get_hw_ptr(chip); |
| /* calculate the processed frames since the |
| * last update |
| */ |
| if (last_ptr < chip->last_ptr) |
| size = runtime->buffer_size + last_ptr |
| - chip->last_ptr; |
| else |
| size = last_ptr - chip->last_ptr; |
| /* remember the last updated point */ |
| chip->last_ptr = last_ptr; |
| /* accumulate the size */ |
| chip->size += size; |
| /* over the period boundary? */ |
| if (chip->size >= runtime->period_size) { |
| /* reset the accumulator */ |
| chip->size %= runtime->period_size; |
| /* call updater */ |
| spin_unlock(&chip->lock); |
| snd_pcm_period_elapsed(substream); |
| spin_lock(&chip->lock); |
| } |
| /* acknowledge the interrupt if necessary */ |
| } |
| .... |
| spin_unlock(&chip->lock); |
| return IRQ_HANDLED; |
| } |
| |
| |
| |
| On calling :c:func:`snd_pcm_period_elapsed()` |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| In both cases, even if more than one period are elapsed, you don't have |
| to call :c:func:`snd_pcm_period_elapsed()` many times. Call only |
| once. And the pcm layer will check the current hardware pointer and |
| update to the latest status. |
| |
| Atomicity |
| --------- |
| |
| One of the most important (and thus difficult to debug) problems in |
| kernel programming are race conditions. In the Linux kernel, they are |
| usually avoided via spin-locks, mutexes or semaphores. In general, if a |
| race condition can happen in an interrupt handler, it has to be managed |
| atomically, and you have to use a spinlock to protect the critical |
| session. If the critical section is not in interrupt handler code and if |
| taking a relatively long time to execute is acceptable, you should use |
| mutexes or semaphores instead. |
| |
| As already seen, some pcm callbacks are atomic and some are not. For |
| example, the ``hw_params`` callback is non-atomic, while ``trigger`` |
| callback is atomic. This means, the latter is called already in a |
| spinlock held by the PCM middle layer. Please take this atomicity into |
| account when you choose a locking scheme in the callbacks. |
| |
| In the atomic callbacks, you cannot use functions which may call |
| :c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and |
| mutexes can sleep, and hence they cannot be used inside the atomic |
| callbacks (e.g. ``trigger`` callback). To implement some delay in such a |
| callback, please use :c:func:`udelay()` or :c:func:`mdelay()`. |
| |
| All three atomic callbacks (trigger, pointer, and ack) are called with |
| local interrupts disabled. |
| |
| The recent changes in PCM core code, however, allow all PCM operations |
| to be non-atomic. This assumes that the all caller sides are in |
| non-atomic contexts. For example, the function |
| :c:func:`snd_pcm_period_elapsed()` is called typically from the |
| interrupt handler. But, if you set up the driver to use a threaded |
| interrupt handler, this call can be in non-atomic context, too. In such |
| a case, you can set ``nonatomic`` filed of :c:type:`struct snd_pcm |
| <snd_pcm>` object after creating it. When this flag is set, mutex |
| and rwsem are used internally in the PCM core instead of spin and |
| rwlocks, so that you can call all PCM functions safely in a non-atomic |
| context. |
| |
| Constraints |
| ----------- |
| |
| If your chip supports unconventional sample rates, or only the limited |
| samples, you need to set a constraint for the condition. |
| |
| For example, in order to restrict the sample rates in the some supported |
| values, use :c:func:`snd_pcm_hw_constraint_list()`. You need to |
| call this function in the open callback. |
| |
| :: |
| |
| static unsigned int rates[] = |
| {4000, 10000, 22050, 44100}; |
| static struct snd_pcm_hw_constraint_list constraints_rates = { |
| .count = ARRAY_SIZE(rates), |
| .list = rates, |
| .mask = 0, |
| }; |
| |
| static int snd_mychip_pcm_open(struct snd_pcm_substream *substream) |
| { |
| int err; |
| .... |
| err = snd_pcm_hw_constraint_list(substream->runtime, 0, |
| SNDRV_PCM_HW_PARAM_RATE, |
| &constraints_rates); |
| if (err < 0) |
| return err; |
| .... |
| } |
| |
| |
| |
| There are many different constraints. Look at ``sound/pcm.h`` for a |
| complete list. You can even define your own constraint rules. For |
| example, let's suppose my_chip can manage a substream of 1 channel if |
| and only if the format is ``S16_LE``, otherwise it supports any format |
| specified in the :c:type:`struct snd_pcm_hardware |
| <snd_pcm_hardware>` structure (or in any other |
| constraint_list). You can build a rule like this: |
| |
| :: |
| |
| static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params, |
| struct snd_pcm_hw_rule *rule) |
| { |
| struct snd_interval *c = hw_param_interval(params, |
| SNDRV_PCM_HW_PARAM_CHANNELS); |
| struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT); |
| struct snd_interval ch; |
| |
| snd_interval_any(&ch); |
| if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) { |
| ch.min = ch.max = 1; |
| ch.integer = 1; |
| return snd_interval_refine(c, &ch); |
| } |
| return 0; |
| } |
| |
| |
| Then you need to call this function to add your rule: |
| |
| :: |
| |
| snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS, |
| hw_rule_channels_by_format, NULL, |
| SNDRV_PCM_HW_PARAM_FORMAT, -1); |
| |
| The rule function is called when an application sets the PCM format, and |
| it refines the number of channels accordingly. But an application may |
| set the number of channels before setting the format. Thus you also need |
| to define the inverse rule: |
| |
| :: |
| |
| static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params, |
| struct snd_pcm_hw_rule *rule) |
| { |
| struct snd_interval *c = hw_param_interval(params, |
| SNDRV_PCM_HW_PARAM_CHANNELS); |
| struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT); |
| struct snd_mask fmt; |
| |
| snd_mask_any(&fmt); /* Init the struct */ |
| if (c->min < 2) { |
| fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE; |
| return snd_mask_refine(f, &fmt); |
| } |
| return 0; |
| } |
| |
| |
| ... and in the open callback: |
| |
| :: |
| |
| snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT, |
| hw_rule_format_by_channels, NULL, |
| SNDRV_PCM_HW_PARAM_CHANNELS, -1); |
| |
| One typical usage of the hw constraints is to align the buffer size |
| with the period size. As default, ALSA PCM core doesn't enforce the |
| buffer size to be aligned with the period size. For example, it'd be |
| possible to have a combination like 256 period bytes with 999 buffer |
| bytes. |
| |
| Many device chips, however, require the buffer to be a multiple of |
| periods. In such a case, call |
| :c:func:`snd_pcm_hw_constraint_integer()` for |
| ``SNDRV_PCM_HW_PARAM_PERIODS``. |
| |
| :: |
| |
| snd_pcm_hw_constraint_integer(substream->runtime, |
| SNDRV_PCM_HW_PARAM_PERIODS); |
| |
| This assures that the number of periods is integer, hence the buffer |
| size is aligned with the period size. |
| |
| The hw constraint is a very much powerful mechanism to define the |
| preferred PCM configuration, and there are relevant helpers. |
| I won't give more details here, rather I would like to say, “Luke, use |
| the source.” |
| |
| Control Interface |
| ================= |
| |
| General |
| ------- |
| |
| The control interface is used widely for many switches, sliders, etc. |
| which are accessed from user-space. Its most important use is the mixer |
| interface. In other words, since ALSA 0.9.x, all the mixer stuff is |
| implemented on the control kernel API. |
| |
| ALSA has a well-defined AC97 control module. If your chip supports only |
| the AC97 and nothing else, you can skip this section. |
| |
| The control API is defined in ``<sound/control.h>``. Include this file |
| if you want to add your own controls. |
| |
| Definition of Controls |
| ---------------------- |
| |
| To create a new control, you need to define the following three |
| callbacks: ``info``, ``get`` and ``put``. Then, define a |
| :c:type:`struct snd_kcontrol_new <snd_kcontrol_new>` record, such as: |
| |
| :: |
| |
| |
| static struct snd_kcontrol_new my_control = { |
| .iface = SNDRV_CTL_ELEM_IFACE_MIXER, |
| .name = "PCM Playback Switch", |
| .index = 0, |
| .access = SNDRV_CTL_ELEM_ACCESS_READWRITE, |
| .private_value = 0xffff, |
| .info = my_control_info, |
| .get = my_control_get, |
| .put = my_control_put |
| }; |
| |
| |
| The ``iface`` field specifies the control type, |
| ``SNDRV_CTL_ELEM_IFACE_XXX``, which is usually ``MIXER``. Use ``CARD`` |
| for global controls that are not logically part of the mixer. If the |
| control is closely associated with some specific device on the sound |
| card, use ``HWDEP``, ``PCM``, ``RAWMIDI``, ``TIMER``, or ``SEQUENCER``, |
| and specify the device number with the ``device`` and ``subdevice`` |
| fields. |
| |
| The ``name`` is the name identifier string. Since ALSA 0.9.x, the |
| control name is very important, because its role is classified from |
| its name. There are pre-defined standard control names. The details |
| are described in the `Control Names`_ subsection. |
| |
| The ``index`` field holds the index number of this control. If there |
| are several different controls with the same name, they can be |
| distinguished by the index number. This is the case when several |
| codecs exist on the card. If the index is zero, you can omit the |
| definition above. |
| |
| The ``access`` field contains the access type of this control. Give |
| the combination of bit masks, ``SNDRV_CTL_ELEM_ACCESS_XXX``, |
| there. The details will be explained in the `Access Flags`_ |
| subsection. |
| |
| The ``private_value`` field contains an arbitrary long integer value |
| for this record. When using the generic ``info``, ``get`` and ``put`` |
| callbacks, you can pass a value through this field. If several small |
| numbers are necessary, you can combine them in bitwise. Or, it's |
| possible to give a pointer (casted to unsigned long) of some record to |
| this field, too. |
| |
| The ``tlv`` field can be used to provide metadata about the control; |
| see the `Metadata`_ subsection. |
| |
| The other three are `Control Callbacks`_. |
| |
| Control Names |
| ------------- |
| |
| There are some standards to define the control names. A control is |
| usually defined from the three parts as “SOURCE DIRECTION FUNCTION”. |
| |
| The first, ``SOURCE``, specifies the source of the control, and is a |
| string such as “Master”, “PCM”, “CD” and “Line”. There are many |
| pre-defined sources. |
| |
| The second, ``DIRECTION``, is one of the following strings according to |
| the direction of the control: “Playback”, “Capture”, “Bypass Playback” |
| and “Bypass Capture”. Or, it can be omitted, meaning both playback and |
| capture directions. |
| |
| The third, ``FUNCTION``, is one of the following strings according to |
| the function of the control: “Switch”, “Volume” and “Route”. |
| |
| The example of control names are, thus, “Master Capture Switch” or “PCM |
| Playback Volume”. |
| |
| There are some exceptions: |
| |
| Global capture and playback |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| “Capture Source”, “Capture Switch” and “Capture Volume” are used for the |
| global capture (input) source, switch and volume. Similarly, “Playback |
| Switch” and “Playback Volume” are used for the global output gain switch |
| and volume. |
| |
| Tone-controls |
| ~~~~~~~~~~~~~ |
| |
| tone-control switch and volumes are specified like “Tone Control - XXX”, |
| e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control - |
| Center”. |
| |
| 3D controls |
| ~~~~~~~~~~~ |
| |
| 3D-control switches and volumes are specified like “3D Control - XXX”, |
| e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”. |
| |
| Mic boost |
| ~~~~~~~~~ |
| |
| Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”. |
| |
| More precise information can be found in |
| ``Documentation/sound/designs/control-names.rst``. |
| |
| Access Flags |
| ------------ |
| |
| The access flag is the bitmask which specifies the access type of the |
| given control. The default access type is |
| ``SNDRV_CTL_ELEM_ACCESS_READWRITE``, which means both read and write are |
| allowed to this control. When the access flag is omitted (i.e. = 0), it |
| is considered as ``READWRITE`` access as default. |
| |
| When the control is read-only, pass ``SNDRV_CTL_ELEM_ACCESS_READ`` |
| instead. In this case, you don't have to define the ``put`` callback. |
| Similarly, when the control is write-only (although it's a rare case), |
| you can use the ``WRITE`` flag instead, and you don't need the ``get`` |
| callback. |
| |
| If the control value changes frequently (e.g. the VU meter), |
| ``VOLATILE`` flag should be given. This means that the control may be |
| changed without `Change notification`_. Applications should poll such |
| a control constantly. |
| |
| When the control is inactive, set the ``INACTIVE`` flag, too. There are |
| ``LOCK`` and ``OWNER`` flags to change the write permissions. |
| |
| Control Callbacks |
| ----------------- |
| |
| info callback |
| ~~~~~~~~~~~~~ |
| |
| The ``info`` callback is used to get detailed information on this |
| control. This must store the values of the given :c:type:`struct |
| snd_ctl_elem_info <snd_ctl_elem_info>` object. For example, |
| for a boolean control with a single element: |
| |
| :: |
| |
| |
| static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_info *uinfo) |
| { |
| uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN; |
| uinfo->count = 1; |
| uinfo->value.integer.min = 0; |
| uinfo->value.integer.max = 1; |
| return 0; |
| } |
| |
| |
| |
| The ``type`` field specifies the type of the control. There are |
| ``BOOLEAN``, ``INTEGER``, ``ENUMERATED``, ``BYTES``, ``IEC958`` and |
| ``INTEGER64``. The ``count`` field specifies the number of elements in |
| this control. For example, a stereo volume would have count = 2. The |
| ``value`` field is a union, and the values stored are depending on the |
| type. The boolean and integer types are identical. |
| |
| The enumerated type is a bit different from others. You'll need to set |
| the string for the currently given item index. |
| |
| :: |
| |
| static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_info *uinfo) |
| { |
| static char *texts[4] = { |
| "First", "Second", "Third", "Fourth" |
| }; |
| uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED; |
| uinfo->count = 1; |
| uinfo->value.enumerated.items = 4; |
| if (uinfo->value.enumerated.item > 3) |
| uinfo->value.enumerated.item = 3; |
| strcpy(uinfo->value.enumerated.name, |
| texts[uinfo->value.enumerated.item]); |
| return 0; |
| } |
| |
| The above callback can be simplified with a helper function, |
| :c:func:`snd_ctl_enum_info()`. The final code looks like below. |
| (You can pass ``ARRAY_SIZE(texts)`` instead of 4 in the third argument; |
| it's a matter of taste.) |
| |
| :: |
| |
| static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_info *uinfo) |
| { |
| static char *texts[4] = { |
| "First", "Second", "Third", "Fourth" |
| }; |
| return snd_ctl_enum_info(uinfo, 1, 4, texts); |
| } |
| |
| |
| Some common info callbacks are available for your convenience: |
| :c:func:`snd_ctl_boolean_mono_info()` and |
| :c:func:`snd_ctl_boolean_stereo_info()`. Obviously, the former |
| is an info callback for a mono channel boolean item, just like |
| :c:func:`snd_myctl_mono_info()` above, and the latter is for a |
| stereo channel boolean item. |
| |
| get callback |
| ~~~~~~~~~~~~ |
| |
| This callback is used to read the current value of the control and to |
| return to user-space. |
| |
| For example, |
| |
| :: |
| |
| |
| static int snd_myctl_get(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_value *ucontrol) |
| { |
| struct mychip *chip = snd_kcontrol_chip(kcontrol); |
| ucontrol->value.integer.value[0] = get_some_value(chip); |
| return 0; |
| } |
| |
| |
| |
| The ``value`` field depends on the type of control as well as on the |
| info callback. For example, the sb driver uses this field to store the |
| register offset, the bit-shift and the bit-mask. The ``private_value`` |
| field is set as follows: |
| |
| :: |
| |
| .private_value = reg | (shift << 16) | (mask << 24) |
| |
| and is retrieved in callbacks like |
| |
| :: |
| |
| static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_value *ucontrol) |
| { |
| int reg = kcontrol->private_value & 0xff; |
| int shift = (kcontrol->private_value >> 16) & 0xff; |
| int mask = (kcontrol->private_value >> 24) & 0xff; |
| .... |
| } |
| |
| In the ``get`` callback, you have to fill all the elements if the |
| control has more than one elements, i.e. ``count > 1``. In the example |
| above, we filled only one element (``value.integer.value[0]``) since |
| it's assumed as ``count = 1``. |
| |
| put callback |
| ~~~~~~~~~~~~ |
| |
| This callback is used to write a value from user-space. |
| |
| For example, |
| |
| :: |
| |
| |
| static int snd_myctl_put(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_value *ucontrol) |
| { |
| struct mychip *chip = snd_kcontrol_chip(kcontrol); |
| int changed = 0; |
| if (chip->current_value != |
| ucontrol->value.integer.value[0]) { |
| change_current_value(chip, |
| ucontrol->value.integer.value[0]); |
| changed = 1; |
| } |
| return changed; |
| } |
| |
| |
| |
| As seen above, you have to return 1 if the value is changed. If the |
| value is not changed, return 0 instead. If any fatal error happens, |
| return a negative error code as usual. |
| |
| As in the ``get`` callback, when the control has more than one |
| elements, all elements must be evaluated in this callback, too. |
| |
| Callbacks are not atomic |
| ~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| All these three callbacks are basically not atomic. |
| |
| Control Constructor |
| ------------------- |
| |
| When everything is ready, finally we can create a new control. To create |
| a control, there are two functions to be called, |
| :c:func:`snd_ctl_new1()` and :c:func:`snd_ctl_add()`. |
| |
| In the simplest way, you can do like this: |
| |
| :: |
| |
| err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip)); |
| if (err < 0) |
| return err; |
| |
| where ``my_control`` is the :c:type:`struct snd_kcontrol_new |
| <snd_kcontrol_new>` object defined above, and chip is the object |
| pointer to be passed to kcontrol->private_data which can be referred |
| to in callbacks. |
| |
| :c:func:`snd_ctl_new1()` allocates a new :c:type:`struct |
| snd_kcontrol <snd_kcontrol>` instance, and |
| :c:func:`snd_ctl_add()` assigns the given control component to the |
| card. |
| |
| Change Notification |
| ------------------- |
| |
| If you need to change and update a control in the interrupt routine, you |
| can call :c:func:`snd_ctl_notify()`. For example, |
| |
| :: |
| |
| snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer); |
| |
| This function takes the card pointer, the event-mask, and the control id |
| pointer for the notification. The event-mask specifies the types of |
| notification, for example, in the above example, the change of control |
| values is notified. The id pointer is the pointer of :c:type:`struct |
| snd_ctl_elem_id <snd_ctl_elem_id>` to be notified. You can |
| find some examples in ``es1938.c`` or ``es1968.c`` for hardware volume |
| interrupts. |
| |
| Metadata |
| -------- |
| |
| To provide information about the dB values of a mixer control, use on of |
| the ``DECLARE_TLV_xxx`` macros from ``<sound/tlv.h>`` to define a |
| variable containing this information, set the ``tlv.p`` field to point to |
| this variable, and include the ``SNDRV_CTL_ELEM_ACCESS_TLV_READ`` flag |
| in the ``access`` field; like this: |
| |
| :: |
| |
| static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0); |
| |
| static struct snd_kcontrol_new my_control = { |
| ... |
| .access = SNDRV_CTL_ELEM_ACCESS_READWRITE | |
| SNDRV_CTL_ELEM_ACCESS_TLV_READ, |
| ... |
| .tlv.p = db_scale_my_control, |
| }; |
| |
| |
| The :c:func:`DECLARE_TLV_DB_SCALE()` macro defines information |
| about a mixer control where each step in the control's value changes the |
| dB value by a constant dB amount. The first parameter is the name of the |
| variable to be defined. The second parameter is the minimum value, in |
| units of 0.01 dB. The third parameter is the step size, in units of 0.01 |
| dB. Set the fourth parameter to 1 if the minimum value actually mutes |
| the control. |
| |
| The :c:func:`DECLARE_TLV_DB_LINEAR()` macro defines information |
| about a mixer control where the control's value affects the output |
| linearly. The first parameter is the name of the variable to be defined. |
| The second parameter is the minimum value, in units of 0.01 dB. The |
| third parameter is the maximum value, in units of 0.01 dB. If the |
| minimum value mutes the control, set the second parameter to |
| ``TLV_DB_GAIN_MUTE``. |
| |
| API for AC97 Codec |
| ================== |
| |
| General |
| ------- |
| |
| The ALSA AC97 codec layer is a well-defined one, and you don't have to |
| write much code to control it. Only low-level control routines are |
| necessary. The AC97 codec API is defined in ``<sound/ac97_codec.h>``. |
| |
| Full Code Example |
| ----------------- |
| |
| :: |
| |
| struct mychip { |
| .... |
| struct snd_ac97 *ac97; |
| .... |
| }; |
| |
| static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97, |
| unsigned short reg) |
| { |
| struct mychip *chip = ac97->private_data; |
| .... |
| /* read a register value here from the codec */ |
| return the_register_value; |
| } |
| |
| static void snd_mychip_ac97_write(struct snd_ac97 *ac97, |
| unsigned short reg, unsigned short val) |
| { |
| struct mychip *chip = ac97->private_data; |
| .... |
| /* write the given register value to the codec */ |
| } |
| |
| static int snd_mychip_ac97(struct mychip *chip) |
| { |
| struct snd_ac97_bus *bus; |
| struct snd_ac97_template ac97; |
| int err; |
| static struct snd_ac97_bus_ops ops = { |
| .write = snd_mychip_ac97_write, |
| .read = snd_mychip_ac97_read, |
| }; |
| |
| err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus); |
| if (err < 0) |
| return err; |
| memset(&ac97, 0, sizeof(ac97)); |
| ac97.private_data = chip; |
| return snd_ac97_mixer(bus, &ac97, &chip->ac97); |
| } |
| |
| |
| AC97 Constructor |
| ---------------- |
| |
| To create an ac97 instance, first call :c:func:`snd_ac97_bus()` |
| with an ``ac97_bus_ops_t`` record with callback functions. |
| |
| :: |
| |
| struct snd_ac97_bus *bus; |
| static struct snd_ac97_bus_ops ops = { |
| .write = snd_mychip_ac97_write, |
| .read = snd_mychip_ac97_read, |
| }; |
| |
| snd_ac97_bus(card, 0, &ops, NULL, &pbus); |
| |
| The bus record is shared among all belonging ac97 instances. |
| |
| And then call :c:func:`snd_ac97_mixer()` with an :c:type:`struct |
| snd_ac97_template <snd_ac97_template>` record together with |
| the bus pointer created above. |
| |
| :: |
| |
| struct snd_ac97_template ac97; |
| int err; |
| |
| memset(&ac97, 0, sizeof(ac97)); |
| ac97.private_data = chip; |
| snd_ac97_mixer(bus, &ac97, &chip->ac97); |
| |
| where chip->ac97 is a pointer to a newly created ``ac97_t`` |
| instance. In this case, the chip pointer is set as the private data, |
| so that the read/write callback functions can refer to this chip |
| instance. This instance is not necessarily stored in the chip |
| record. If you need to change the register values from the driver, or |
| need the suspend/resume of ac97 codecs, keep this pointer to pass to |
| the corresponding functions. |
| |
| AC97 Callbacks |
| -------------- |
| |
| The standard callbacks are ``read`` and ``write``. Obviously they |
| correspond to the functions for read and write accesses to the |
| hardware low-level codes. |
| |
| The ``read`` callback returns the register value specified in the |
| argument. |
| |
| :: |
| |
| static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97, |
| unsigned short reg) |
| { |
| struct mychip *chip = ac97->private_data; |
| .... |
| return the_register_value; |
| } |
| |
| Here, the chip can be cast from ``ac97->private_data``. |
| |
| Meanwhile, the ``write`` callback is used to set the register |
| value |
| |
| :: |
| |
| static void snd_mychip_ac97_write(struct snd_ac97 *ac97, |
| unsigned short reg, unsigned short val) |
| |
| |
| These callbacks are non-atomic like the control API callbacks. |
| |
| There are also other callbacks: ``reset``, ``wait`` and ``init``. |
| |
| The ``reset`` callback is used to reset the codec. If the chip |
| requires a special kind of reset, you can define this callback. |
| |
| The ``wait`` callback is used to add some waiting time in the standard |
| initialization of the codec. If the chip requires the extra waiting |
| time, define this callback. |
| |
| The ``init`` callback is used for additional initialization of the |
| codec. |
| |
| Updating Registers in The Driver |
| -------------------------------- |
| |
| If you need to access to the codec from the driver, you can call the |
| following functions: :c:func:`snd_ac97_write()`, |
| :c:func:`snd_ac97_read()`, :c:func:`snd_ac97_update()` and |
| :c:func:`snd_ac97_update_bits()`. |
| |
| Both :c:func:`snd_ac97_write()` and |
| :c:func:`snd_ac97_update()` functions are used to set a value to |
| the given register (``AC97_XXX``). The difference between them is that |
| :c:func:`snd_ac97_update()` doesn't write a value if the given |
| value has been already set, while :c:func:`snd_ac97_write()` |
| always rewrites the value. |
| |
| :: |
| |
| snd_ac97_write(ac97, AC97_MASTER, 0x8080); |
| snd_ac97_update(ac97, AC97_MASTER, 0x8080); |
| |
| :c:func:`snd_ac97_read()` is used to read the value of the given |
| register. For example, |
| |
| :: |
| |
| value = snd_ac97_read(ac97, AC97_MASTER); |
| |
| :c:func:`snd_ac97_update_bits()` is used to update some bits in |
| the given register. |
| |
| :: |
| |
| snd_ac97_update_bits(ac97, reg, mask, value); |
| |
| Also, there is a function to change the sample rate (of a given register |
| such as ``AC97_PCM_FRONT_DAC_RATE``) when VRA or DRA is supported by the |
| codec: :c:func:`snd_ac97_set_rate()`. |
| |
| :: |
| |
| snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100); |
| |
| |
| The following registers are available to set the rate: |
| ``AC97_PCM_MIC_ADC_RATE``, ``AC97_PCM_FRONT_DAC_RATE``, |
| ``AC97_PCM_LR_ADC_RATE``, ``AC97_SPDIF``. When ``AC97_SPDIF`` is |
| specified, the register is not really changed but the corresponding |
| IEC958 status bits will be updated. |
| |
| Clock Adjustment |
| ---------------- |
| |
| In some chips, the clock of the codec isn't 48000 but using a PCI clock |
| (to save a quartz!). In this case, change the field ``bus->clock`` to |
| the corresponding value. For example, intel8x0 and es1968 drivers have |
| their own function to read from the clock. |
| |
| Proc Files |
| ---------- |
| |
| The ALSA AC97 interface will create a proc file such as |
| ``/proc/asound/card0/codec97#0/ac97#0-0`` and ``ac97#0-0+regs``. You |
| can refer to these files to see the current status and registers of |
| the codec. |
| |
| Multiple Codecs |
| --------------- |
| |
| When there are several codecs on the same card, you need to call |
| :c:func:`snd_ac97_mixer()` multiple times with ``ac97.num=1`` or |
| greater. The ``num`` field specifies the codec number. |
| |
| If you set up multiple codecs, you either need to write different |
| callbacks for each codec or check ``ac97->num`` in the callback |
| routines. |
| |
| MIDI (MPU401-UART) Interface |
| ============================ |
| |
| General |
| ------- |
| |
| Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the |
| soundcard supports the standard MPU401-UART interface, most likely you |
| can use the ALSA MPU401-UART API. The MPU401-UART API is defined in |
| ``<sound/mpu401.h>``. |
| |
| Some soundchips have a similar but slightly different implementation of |
| mpu401 stuff. For example, emu10k1 has its own mpu401 routines. |
| |
| MIDI Constructor |
| ---------------- |
| |
| To create a rawmidi object, call :c:func:`snd_mpu401_uart_new()`. |
| |
| :: |
| |
| struct snd_rawmidi *rmidi; |
| snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags, |
| irq, &rmidi); |
| |
| |
| The first argument is the card pointer, and the second is the index of |
| this component. You can create up to 8 rawmidi devices. |
| |
| The third argument is the type of the hardware, ``MPU401_HW_XXX``. If |
| it's not a special one, you can use ``MPU401_HW_MPU401``. |
| |
| The 4th argument is the I/O port address. Many backward-compatible |
| MPU401 have an I/O port such as 0x330. Or, it might be a part of its own |
| PCI I/O region. It depends on the chip design. |
| |
| The 5th argument is a bitflag for additional information. When the I/O |
| port address above is part of the PCI I/O region, the MPU401 I/O port |
| might have been already allocated (reserved) by the driver itself. In |
| such a case, pass a bit flag ``MPU401_INFO_INTEGRATED``, and the |
| mpu401-uart layer will allocate the I/O ports by itself. |
| |
| When the controller supports only the input or output MIDI stream, pass |
| the ``MPU401_INFO_INPUT`` or ``MPU401_INFO_OUTPUT`` bitflag, |
| respectively. Then the rawmidi instance is created as a single stream. |
| |
| ``MPU401_INFO_MMIO`` bitflag is used to change the access method to MMIO |
| (via readb and writeb) instead of iob and outb. In this case, you have |
| to pass the iomapped address to :c:func:`snd_mpu401_uart_new()`. |
| |
| When ``MPU401_INFO_TX_IRQ`` is set, the output stream isn't checked in |
| the default interrupt handler. The driver needs to call |
| :c:func:`snd_mpu401_uart_interrupt_tx()` by itself to start |
| processing the output stream in the irq handler. |
| |
| If the MPU-401 interface shares its interrupt with the other logical |
| devices on the card, set ``MPU401_INFO_IRQ_HOOK`` (see |
| `below <#MIDI-Interrupt-Handler>`__). |
| |
| Usually, the port address corresponds to the command port and port + 1 |
| corresponds to the data port. If not, you may change the ``cport`` |
| field of :c:type:`struct snd_mpu401 <snd_mpu401>` manually afterward. |
| However, :c:type:`struct snd_mpu401 <snd_mpu401>` pointer is |
| not returned explicitly by :c:func:`snd_mpu401_uart_new()`. You |
| need to cast ``rmidi->private_data`` to :c:type:`struct snd_mpu401 |
| <snd_mpu401>` explicitly, |
| |
| :: |
| |
| struct snd_mpu401 *mpu; |
| mpu = rmidi->private_data; |
| |
| and reset the ``cport`` as you like: |
| |
| :: |
| |
| mpu->cport = my_own_control_port; |
| |
| The 6th argument specifies the ISA irq number that will be allocated. If |
| no interrupt is to be allocated (because your code is already allocating |
| a shared interrupt, or because the device does not use interrupts), pass |
| -1 instead. For a MPU-401 device without an interrupt, a polling timer |
| will be used instead. |
| |
| MIDI Interrupt Handler |
| ---------------------- |
| |
| When the interrupt is allocated in |
| :c:func:`snd_mpu401_uart_new()`, an exclusive ISA interrupt |
| handler is automatically used, hence you don't have anything else to do |
| than creating the mpu401 stuff. Otherwise, you have to set |
| ``MPU401_INFO_IRQ_HOOK``, and call |
| :c:func:`snd_mpu401_uart_interrupt()` explicitly from your own |
| interrupt handler when it has determined that a UART interrupt has |
| occurred. |
| |
| In this case, you need to pass the private_data of the returned rawmidi |
| object from :c:func:`snd_mpu401_uart_new()` as the second |
| argument of :c:func:`snd_mpu401_uart_interrupt()`. |
| |
| :: |
| |
| snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs); |
| |
| |
| RawMIDI Interface |
| ================= |
| |
| Overview |
| -------- |
| |
| The raw MIDI interface is used for hardware MIDI ports that can be |
| accessed as a byte stream. It is not used for synthesizer chips that do |
| not directly understand MIDI. |
| |
| ALSA handles file and buffer management. All you have to do is to write |
| some code to move data between the buffer and the hardware. |
| |
| The rawmidi API is defined in ``<sound/rawmidi.h>``. |
| |
| RawMIDI Constructor |
| ------------------- |
| |
| To create a rawmidi device, call the :c:func:`snd_rawmidi_new()` |
| function: |
| |
| :: |
| |
| struct snd_rawmidi *rmidi; |
| err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi); |
| if (err < 0) |
| return err; |
| rmidi->private_data = chip; |
| strcpy(rmidi->name, "My MIDI"); |
| rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT | |
| SNDRV_RAWMIDI_INFO_INPUT | |
| SNDRV_RAWMIDI_INFO_DUPLEX; |
| |
| The first argument is the card pointer, the second argument is the ID |
| string. |
| |
| The third argument is the index of this component. You can create up to |
| 8 rawmidi devices. |
| |
| The fourth and fifth arguments are the number of output and input |
| substreams, respectively, of this device (a substream is the equivalent |
| of a MIDI port). |
| |
| Set the ``info_flags`` field to specify the capabilities of the |
| device. Set ``SNDRV_RAWMIDI_INFO_OUTPUT`` if there is at least one |
| output port, ``SNDRV_RAWMIDI_INFO_INPUT`` if there is at least one |
| input port, and ``SNDRV_RAWMIDI_INFO_DUPLEX`` if the device can handle |
| output and input at the same time. |
| |
| After the rawmidi device is created, you need to set the operators |
| (callbacks) for each substream. There are helper functions to set the |
| operators for all the substreams of a device: |
| |
| :: |
| |
| snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops); |
| snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops); |
| |
| The operators are usually defined like this: |
| |
| :: |
| |
| static struct snd_rawmidi_ops snd_mymidi_output_ops = { |
| .open = snd_mymidi_output_open, |
| .close = snd_mymidi_output_close, |
| .trigger = snd_mymidi_output_trigger, |
| }; |
| |
| These callbacks are explained in the `RawMIDI Callbacks`_ section. |
| |
| If there are more than one substream, you should give a unique name to |
| each of them: |
| |
| :: |
| |
| struct snd_rawmidi_substream *substream; |
| list_for_each_entry(substream, |
| &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams, |
| list { |
| sprintf(substream->name, "My MIDI Port %d", substream->number + 1); |
| } |
| /* same for SNDRV_RAWMIDI_STREAM_INPUT */ |
| |
| RawMIDI Callbacks |
| ----------------- |
| |
| In all the callbacks, the private data that you've set for the rawmidi |
| device can be accessed as ``substream->rmidi->private_data``. |
| |
| If there is more than one port, your callbacks can determine the port |
| index from the struct snd_rawmidi_substream data passed to each |
| callback: |
| |
| :: |
| |
| struct snd_rawmidi_substream *substream; |
| int index = substream->number; |
| |
| RawMIDI open callback |
| ~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_open(struct snd_rawmidi_substream *substream); |
| |
| |
| This is called when a substream is opened. You can initialize the |
| hardware here, but you shouldn't start transmitting/receiving data yet. |
| |
| RawMIDI close callback |
| ~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static int snd_xxx_close(struct snd_rawmidi_substream *substream); |
| |
| Guess what. |
| |
| The ``open`` and ``close`` callbacks of a rawmidi device are |
| serialized with a mutex, and can sleep. |
| |
| Rawmidi trigger callback for output substreams |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up); |
| |
| |
| This is called with a nonzero ``up`` parameter when there is some data |
| in the substream buffer that must be transmitted. |
| |
| To read data from the buffer, call |
| :c:func:`snd_rawmidi_transmit_peek()`. It will return the number |
| of bytes that have been read; this will be less than the number of bytes |
| requested when there are no more data in the buffer. After the data have |
| been transmitted successfully, call |
| :c:func:`snd_rawmidi_transmit_ack()` to remove the data from the |
| substream buffer: |
| |
| :: |
| |
| unsigned char data; |
| while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) { |
| if (snd_mychip_try_to_transmit(data)) |
| snd_rawmidi_transmit_ack(substream, 1); |
| else |
| break; /* hardware FIFO full */ |
| } |
| |
| If you know beforehand that the hardware will accept data, you can use |
| the :c:func:`snd_rawmidi_transmit()` function which reads some |
| data and removes them from the buffer at once: |
| |
| :: |
| |
| while (snd_mychip_transmit_possible()) { |
| unsigned char data; |
| if (snd_rawmidi_transmit(substream, &data, 1) != 1) |
| break; /* no more data */ |
| snd_mychip_transmit(data); |
| } |
| |
| If you know beforehand how many bytes you can accept, you can use a |
| buffer size greater than one with the |
| :c:func:`snd_rawmidi_transmit\*()` functions. |
| |
| The ``trigger`` callback must not sleep. If the hardware FIFO is full |
| before the substream buffer has been emptied, you have to continue |
| transmitting data later, either in an interrupt handler, or with a |
| timer if the hardware doesn't have a MIDI transmit interrupt. |
| |
| The ``trigger`` callback is called with a zero ``up`` parameter when |
| the transmission of data should be aborted. |
| |
| RawMIDI trigger callback for input substreams |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up); |
| |
| |
| This is called with a nonzero ``up`` parameter to enable receiving data, |
| or with a zero ``up`` parameter do disable receiving data. |
| |
| The ``trigger`` callback must not sleep; the actual reading of data |
| from the device is usually done in an interrupt handler. |
| |
| When data reception is enabled, your interrupt handler should call |
| :c:func:`snd_rawmidi_receive()` for all received data: |
| |
| :: |
| |
| void snd_mychip_midi_interrupt(...) |
| { |
| while (mychip_midi_available()) { |
| unsigned char data; |
| data = mychip_midi_read(); |
| snd_rawmidi_receive(substream, &data, 1); |
| } |
| } |
| |
| |
| drain callback |
| ~~~~~~~~~~~~~~ |
| |
| :: |
| |
| static void snd_xxx_drain(struct snd_rawmidi_substream *substream); |
| |
| |
| This is only used with output substreams. This function should wait |
| until all data read from the substream buffer have been transmitted. |
| This ensures that the device can be closed and the driver unloaded |
| without losing data. |
| |
| This callback is optional. If you do not set ``drain`` in the struct |
| snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds |
| instead. |
| |
| Miscellaneous Devices |
| ===================== |
| |
| FM OPL3 |
| ------- |
| |
| The FM OPL3 is still used in many chips (mainly for backward |
| compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API |
| is defined in ``<sound/opl3.h>``. |
| |
| FM registers can be directly accessed through the direct-FM API, defined |
| in ``<sound/asound_fm.h>``. In ALSA native mode, FM registers are |
| accessed through the Hardware-Dependent Device direct-FM extension API, |
| whereas in OSS compatible mode, FM registers can be accessed with the |
| OSS direct-FM compatible API in ``/dev/dmfmX`` device. |
| |
| To create the OPL3 component, you have two functions to call. The first |
| one is a constructor for the ``opl3_t`` instance. |
| |
| :: |
| |
| struct snd_opl3 *opl3; |
| snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX, |
| integrated, &opl3); |
| |
| The first argument is the card pointer, the second one is the left port |
| address, and the third is the right port address. In most cases, the |
| right port is placed at the left port + 2. |
| |
| The fourth argument is the hardware type. |
| |
| When the left and right ports have been already allocated by the card |
| driver, pass non-zero to the fifth argument (``integrated``). Otherwise, |
| the opl3 module will allocate the specified ports by itself. |
| |
| When the accessing the hardware requires special method instead of the |
| standard I/O access, you can create opl3 instance separately with |
| :c:func:`snd_opl3_new()`. |
| |
| :: |
| |
| struct snd_opl3 *opl3; |
| snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3); |
| |
| Then set ``command``, ``private_data`` and ``private_free`` for the |
| private access function, the private data and the destructor. The |
| ``l_port`` and ``r_port`` are not necessarily set. Only the command |
| must be set properly. You can retrieve the data from the |
| ``opl3->private_data`` field. |
| |
| After creating the opl3 instance via :c:func:`snd_opl3_new()`, |
| call :c:func:`snd_opl3_init()` to initialize the chip to the |
| proper state. Note that :c:func:`snd_opl3_create()` always calls |
| it internally. |
| |
| If the opl3 instance is created successfully, then create a hwdep device |
| for this opl3. |
| |
| :: |
| |
| struct snd_hwdep *opl3hwdep; |
| snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep); |
| |
| The first argument is the ``opl3_t`` instance you created, and the |
| second is the index number, usually 0. |
| |
| The third argument is the index-offset for the sequencer client assigned |
| to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART |
| always takes 0). |
| |
| Hardware-Dependent Devices |
| -------------------------- |
| |
| Some chips need user-space access for special controls or for loading |
| the micro code. In such a case, you can create a hwdep |
| (hardware-dependent) device. The hwdep API is defined in |
| ``<sound/hwdep.h>``. You can find examples in opl3 driver or |
| ``isa/sb/sb16_csp.c``. |
| |
| The creation of the ``hwdep`` instance is done via |
| :c:func:`snd_hwdep_new()`. |
| |
| :: |
| |
| struct snd_hwdep *hw; |
| snd_hwdep_new(card, "My HWDEP", 0, &hw); |
| |
| where the third argument is the index number. |
| |
| You can then pass any pointer value to the ``private_data``. If you |
| assign a private data, you should define the destructor, too. The |
| destructor function is set in the ``private_free`` field. |
| |
| :: |
| |
| struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL); |
| hw->private_data = p; |
| hw->private_free = mydata_free; |
| |
| and the implementation of the destructor would be: |
| |
| :: |
| |
| static void mydata_free(struct snd_hwdep *hw) |
| { |
| struct mydata *p = hw->private_data; |
| kfree(p); |
| } |
| |
| The arbitrary file operations can be defined for this instance. The file |
| operators are defined in the ``ops`` table. For example, assume that |
| this chip needs an ioctl. |
| |
| :: |
| |
| hw->ops.open = mydata_open; |
| hw->ops.ioctl = mydata_ioctl; |
| hw->ops.release = mydata_release; |
| |
| And implement the callback functions as you like. |
| |
| IEC958 (S/PDIF) |
| --------------- |
| |
| Usually the controls for IEC958 devices are implemented via the control |
| interface. There is a macro to compose a name string for IEC958 |
| controls, :c:func:`SNDRV_CTL_NAME_IEC958()` defined in |
| ``<include/asound.h>``. |
| |
| There are some standard controls for IEC958 status bits. These controls |
| use the type ``SNDRV_CTL_ELEM_TYPE_IEC958``, and the size of element is |
| fixed as 4 bytes array (value.iec958.status[x]). For the ``info`` |
| callback, you don't specify the value field for this type (the count |
| field must be set, though). |
| |
| “IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958 |
| status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask” |
| returns the bitmask for professional mode. They are read-only controls, |
| and are defined as MIXER controls (iface = |
| ``SNDRV_CTL_ELEM_IFACE_MIXER``). |
| |
| Meanwhile, “IEC958 Playback Default” control is defined for getting and |
| setting the current default IEC958 bits. Note that this one is usually |
| defined as a PCM control (iface = ``SNDRV_CTL_ELEM_IFACE_PCM``), |
| although in some places it's defined as a MIXER control. |
| |
| In addition, you can define the control switches to enable/disable or to |
| set the raw bit mode. The implementation will depend on the chip, but |
| the control should be named as “IEC958 xxx”, preferably using the |
| :c:func:`SNDRV_CTL_NAME_IEC958()` macro. |
| |
| You can find several cases, for example, ``pci/emu10k1``, |
| ``pci/ice1712``, or ``pci/cmipci.c``. |
| |
| Buffer and Memory Management |
| ============================ |
| |
| Buffer Types |
| ------------ |
| |
| ALSA provides several different buffer allocation functions depending on |
| the bus and the architecture. All these have a consistent API. The |
| allocation of physically-contiguous pages is done via |
| :c:func:`snd_malloc_xxx_pages()` function, where xxx is the bus |
| type. |
| |
| The allocation of pages with fallback is |
| :c:func:`snd_malloc_xxx_pages_fallback()`. This function tries |
| to allocate the specified pages but if the pages are not available, it |
| tries to reduce the page sizes until enough space is found. |
| |
| The release the pages, call :c:func:`snd_free_xxx_pages()` |
| function. |
| |
| Usually, ALSA drivers try to allocate and reserve a large contiguous |
| physical space at the time the module is loaded for the later use. This |
| is called “pre-allocation”. As already written, you can call the |
| following function at pcm instance construction time (in the case of PCI |
| bus). |
| |
| :: |
| |
| snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV, |
| &pci->dev, size, max); |
| |
| where ``size`` is the byte size to be pre-allocated and the ``max`` is |
| the maximum size to be changed via the ``prealloc`` proc file. The |
| allocator will try to get an area as large as possible within the |
| given size. |
| |
| The second argument (type) and the third argument (device pointer) are |
| dependent on the bus. For normal devices, pass the device pointer |
| (typically identical as ``card->dev``) to the third argument with |
| ``SNDRV_DMA_TYPE_DEV`` type. For the continuous buffer unrelated to the |
| bus can be pre-allocated with ``SNDRV_DMA_TYPE_CONTINUOUS`` type. |
| You can pass NULL to the device pointer in that case, which is the |
| default mode implying to allocate with ``GFP_KRENEL`` flag. |
| If you need a different GFP flag, you can pass it by encoding the flag |
| into the device pointer via a special macro |
| :c:func:`snd_dma_continuous_data()`. |
| For the scatter-gather buffers, use ``SNDRV_DMA_TYPE_DEV_SG`` with the |
| device pointer (see the `Non-Contiguous Buffers`_ section). |
| |
| Once the buffer is pre-allocated, you can use the allocator in the |
| ``hw_params`` callback: |
| |
| :: |
| |
| snd_pcm_lib_malloc_pages(substream, size); |
| |
| Note that you have to pre-allocate to use this function. |
| |
| Most of drivers use, though, rather the newly introduced "managed |
| buffer allocation mode" instead of the manual allocation or release. |
| This is done by calling :c:func:`snd_pcm_set_managed_buffer_all()` |
| instead of :c:func:`snd_pcm_lib_preallocate_pages_for_all()`. |
| |
| :: |
| |
| snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV, |
| &pci->dev, size, max); |
| |
| where passed arguments are identical in both functions. |
| The difference in the managed mode is that PCM core will call |
| :c:func:`snd_pcm_lib_malloc_pages()` internally already before calling |
| the PCM ``hw_params`` callback, and call :c:func:`snd_pcm_lib_free_pages()` |
| after the PCM ``hw_free`` callback automatically. So the driver |
| doesn't have to call these functions explicitly in its callback any |
| longer. This made many driver code having NULL ``hw_params`` and |
| ``hw_free`` entries. |
| |
| External Hardware Buffers |
| ------------------------- |
| |
| Some chips have their own hardware buffers and the DMA transfer from the |
| host memory is not available. In such a case, you need to either 1) |
| copy/set the audio data directly to the external hardware buffer, or 2) |
| make an intermediate buffer and copy/set the data from it to the |
| external hardware buffer in interrupts (or in tasklets, preferably). |
| |
| The first case works fine if the external hardware buffer is large |
| enough. This method doesn't need any extra buffers and thus is more |
| effective. You need to define the ``copy_user`` and ``copy_kernel`` |
| callbacks for the data transfer, in addition to ``fill_silence`` |
| callback for playback. However, there is a drawback: it cannot be |
| mmapped. The examples are GUS's GF1 PCM or emu8000's wavetable PCM. |
| |
| The second case allows for mmap on the buffer, although you have to |
| handle an interrupt or a tasklet to transfer the data from the |
| intermediate buffer to the hardware buffer. You can find an example in |
| the vxpocket driver. |
| |
| Another case is when the chip uses a PCI memory-map region for the |
| buffer instead of the host memory. In this case, mmap is available only |
| on certain architectures like the Intel one. In non-mmap mode, the data |
| cannot be transferred as in the normal way. Thus you need to define the |
| ``copy_user``, ``copy_kernel`` and ``fill_silence`` callbacks as well, |
| as in the cases above. The examples are found in ``rme32.c`` and |
| ``rme96.c``. |
| |
| The implementation of the ``copy_user``, ``copy_kernel`` and |
| ``silence`` callbacks depends upon whether the hardware supports |
| interleaved or non-interleaved samples. The ``copy_user`` callback is |
| defined like below, a bit differently depending whether the direction |
| is playback or capture: |
| |
| :: |
| |
| static int playback_copy_user(struct snd_pcm_substream *substream, |
| int channel, unsigned long pos, |
| void __user *src, unsigned long count); |
| static int capture_copy_user(struct snd_pcm_substream *substream, |
| int channel, unsigned long pos, |
| void __user *dst, unsigned long count); |
| |
| In the case of interleaved samples, the second argument (``channel``) is |
| not used. The third argument (``pos``) points the current position |
| offset in bytes. |
| |
| The meaning of the fourth argument is different between playback and |
| capture. For playback, it holds the source data pointer, and for |
| capture, it's the destination data pointer. |
| |
| The last argument is the number of bytes to be copied. |
| |
| What you have to do in this callback is again different between playback |
| and capture directions. In the playback case, you copy the given amount |
| of data (``count``) at the specified pointer (``src``) to the specified |
| offset (``pos``) on the hardware buffer. When coded like memcpy-like |
| way, the copy would be like: |
| |
| :: |
| |
| my_memcpy_from_user(my_buffer + pos, src, count); |
| |
| For the capture direction, you copy the given amount of data (``count``) |
| at the specified offset (``pos``) on the hardware buffer to the |
| specified pointer (``dst``). |
| |
| :: |
| |
| my_memcpy_to_user(dst, my_buffer + pos, count); |
| |
| Here the functions are named as ``from_user`` and ``to_user`` because |
| it's the user-space buffer that is passed to these callbacks. That |
| is, the callback is supposed to copy from/to the user-space data |
| directly to/from the hardware buffer. |
| |
| Careful readers might notice that these callbacks receive the |
| arguments in bytes, not in frames like other callbacks. It's because |
| it would make coding easier like the examples above, and also it makes |
| easier to unify both the interleaved and non-interleaved cases, as |
| explained in the following. |
| |
| In the case of non-interleaved samples, the implementation will be a bit |
| more complicated. The callback is called for each channel, passed by |
| the second argument, so totally it's called for N-channels times per |
| transfer. |
| |
| The meaning of other arguments are almost same as the interleaved |
| case. The callback is supposed to copy the data from/to the given |
| user-space buffer, but only for the given channel. For the detailed |
| implementations, please check ``isa/gus/gus_pcm.c`` or |
| "pci/rme9652/rme9652.c" as examples. |
| |
| The above callbacks are the copy from/to the user-space buffer. There |
| are some cases where we want copy from/to the kernel-space buffer |
| instead. In such a case, ``copy_kernel`` callback is called. It'd |
| look like: |
| |
| :: |
| |
| static int playback_copy_kernel(struct snd_pcm_substream *substream, |
| int channel, unsigned long pos, |
| void *src, unsigned long count); |
| static int capture_copy_kernel(struct snd_pcm_substream *substream, |
| int channel, unsigned long pos, |
| void *dst, unsigned long count); |
| |
| As found easily, the only difference is that the buffer pointer is |
| without ``__user`` prefix; that is, a kernel-buffer pointer is passed |
| in the fourth argument. Correspondingly, the implementation would be |
| a version without the user-copy, such as: |
| |
| :: |
| |
| my_memcpy(my_buffer + pos, src, count); |
| |
| Usually for the playback, another callback ``fill_silence`` is |
| defined. It's implemented in a similar way as the copy callbacks |
| above: |
| |
| :: |
| |
| static int silence(struct snd_pcm_substream *substream, int channel, |
| unsigned long pos, unsigned long count); |
| |
| The meanings of arguments are the same as in the ``copy_user`` and |
| ``copy_kernel`` callbacks, although there is no buffer pointer |
| argument. In the case of interleaved samples, the channel argument has |
| no meaning, as well as on ``copy_*`` callbacks. |
| |
| The role of ``fill_silence`` callback is to set the given amount |
| (``count``) of silence data at the specified offset (``pos``) on the |
| hardware buffer. Suppose that the data format is signed (that is, the |
| silent-data is 0), and the implementation using a memset-like function |
| would be like: |
| |
| :: |
| |
| my_memset(my_buffer + pos, 0, count); |
| |
| In the case of non-interleaved samples, again, the implementation |
| becomes a bit more complicated, as it's called N-times per transfer |
| for each channel. See, for example, ``isa/gus/gus_pcm.c``. |
| |
| Non-Contiguous Buffers |
| ---------------------- |
| |
| If your hardware supports the page table as in emu10k1 or the buffer |
| descriptors as in via82xx, you can use the scatter-gather (SG) DMA. ALSA |
| provides an interface for handling SG-buffers. The API is provided in |
| ``<sound/pcm.h>``. |
| |
| For creating the SG-buffer handler, call |
| :c:func:`snd_pcm_set_managed_buffer()` or |
| :c:func:`snd_pcm_set_managed_buffer_all()` with |
| ``SNDRV_DMA_TYPE_DEV_SG`` in the PCM constructor like other PCI |
| pre-allocator. You need to pass ``&pci->dev``, where pci is |
| the :c:type:`struct pci_dev <pci_dev>` pointer of the chip as |
| well. |
| |
| :: |
| |
| snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV_SG, |
| &pci->dev, size, max); |
| |
| The ``struct snd_sg_buf`` instance is created as |
| ``substream->dma_private`` in turn. You can cast the pointer like: |
| |
| :: |
| |
| struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private; |
| |
| Then in :c:func:`snd_pcm_lib_malloc_pages()` call, the common SG-buffer |
| handler will allocate the non-contiguous kernel pages of the given size |
| and map them onto the virtually contiguous memory. The virtual pointer |
| is addressed in runtime->dma_area. The physical address |
| (``runtime->dma_addr``) is set to zero, because the buffer is |
| physically non-contiguous. The physical address table is set up in |
| ``sgbuf->table``. You can get the physical address at a certain offset |
| via :c:func:`snd_pcm_sgbuf_get_addr()`. |
| |
| If you need to release the SG-buffer data explicitly, call the |
| standard API function :c:func:`snd_pcm_lib_free_pages()` as usual. |
| |
| Vmalloc'ed Buffers |
| ------------------ |
| |
| It's possible to use a buffer allocated via :c:func:`vmalloc()`, for |
| example, for an intermediate buffer. In the recent version of kernel, |
| you can simply allocate it via standard |
| :c:func:`snd_pcm_lib_malloc_pages()` and co after setting up the |
| buffer preallocation with ``SNDRV_DMA_TYPE_VMALLOC`` type. |
| |
| :: |
| |
| snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_VMALLOC, |
| NULL, 0, 0); |
| |
| The NULL is passed to the device pointer argument, which indicates |
| that the default pages (GFP_KERNEL and GFP_HIGHMEM) will be |
| allocated. |
| |
| Also, note that zero is passed to both the size and the max size |
| arguments here. Since each vmalloc call should succeed at any time, |
| we don't need to pre-allocate the buffers like other continuous |
| pages. |
| |
| If you need the 32bit DMA allocation, pass the device pointer encoded |
| by :c:func:`snd_dma_continuous_data()` with ``GFP_KERNEL|__GFP_DMA32`` |
| argument. |
| |
| :: |
| |
| snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_VMALLOC, |
| snd_dma_continuous_data(GFP_KERNEL | __GFP_DMA32), 0, 0); |
| |
| Proc Interface |
| ============== |
| |
| ALSA provides an easy interface for procfs. The proc files are very |
| useful for debugging. I recommend you set up proc files if you write a |
| driver and want to get a running status or register dumps. The API is |
| found in ``<sound/info.h>``. |
| |
| To create a proc file, call :c:func:`snd_card_proc_new()`. |
| |
| :: |
| |
| struct snd_info_entry *entry; |
| int err = snd_card_proc_new(card, "my-file", &entry); |
| |
| where the second argument specifies the name of the proc file to be |
| created. The above example will create a file ``my-file`` under the |
| card directory, e.g. ``/proc/asound/card0/my-file``. |
| |
| Like other components, the proc entry created via |
| :c:func:`snd_card_proc_new()` will be registered and released |
| automatically in the card registration and release functions. |
| |
| When the creation is successful, the function stores a new instance in |
| the pointer given in the third argument. It is initialized as a text |
| proc file for read only. To use this proc file as a read-only text file |
| as it is, set the read callback with a private data via |
| :c:func:`snd_info_set_text_ops()`. |
| |
| :: |
| |
| snd_info_set_text_ops(entry, chip, my_proc_read); |
| |
| where the second argument (``chip``) is the private data to be used in |
| the callbacks. The third parameter specifies the read buffer size and |
| the fourth (``my_proc_read``) is the callback function, which is |
| defined like |
| |
| :: |
| |
| static void my_proc_read(struct snd_info_entry *entry, |
| struct snd_info_buffer *buffer); |
| |
| In the read callback, use :c:func:`snd_iprintf()` for output |
| strings, which works just like normal :c:func:`printf()`. For |
| example, |
| |
| :: |
| |
| static void my_proc_read(struct snd_info_entry *entry, |
| struct snd_info_buffer *buffer) |
| { |
| struct my_chip *chip = entry->private_data; |
| |
| snd_iprintf(buffer, "This is my chip!\n"); |
| snd_iprintf(buffer, "Port = %ld\n", chip->port); |
| } |
| |
| The file permissions can be changed afterwards. As default, it's set as |
| read only for all users. If you want to add write permission for the |
| user (root as default), do as follows: |
| |
| :: |
| |
| entry->mode = S_IFREG | S_IRUGO | S_IWUSR; |
| |
| and set the write buffer size and the callback |
| |
| :: |
| |
| entry->c.text.write = my_proc_write; |
| |
| For the write callback, you can use :c:func:`snd_info_get_line()` |
| to get a text line, and :c:func:`snd_info_get_str()` to retrieve |
| a string from the line. Some examples are found in |
| ``core/oss/mixer_oss.c``, core/oss/and ``pcm_oss.c``. |
| |
| For a raw-data proc-file, set the attributes as follows: |
| |
| :: |
| |
| static struct snd_info_entry_ops my_file_io_ops = { |
| .read = my_file_io_read, |
| }; |
| |
| entry->content = SNDRV_INFO_CONTENT_DATA; |
| entry->private_data = chip; |
| entry->c.ops = &my_file_io_ops; |
| entry->size = 4096; |
| entry->mode = S_IFREG | S_IRUGO; |
| |
| For the raw data, ``size`` field must be set properly. This specifies |
| the maximum size of the proc file access. |
| |
| The read/write callbacks of raw mode are more direct than the text mode. |
| You need to use a low-level I/O functions such as |
| :c:func:`copy_from/to_user()` to transfer the data. |
| |
| :: |
| |
| static ssize_t my_file_io_read(struct snd_info_entry *entry, |
| void *file_private_data, |
| struct file *file, |
| char *buf, |
| size_t count, |
| loff_t pos) |
| { |
| if (copy_to_user(buf, local_data + pos, count)) |
| return -EFAULT; |
| return count; |
| } |
| |
| If the size of the info entry has been set up properly, ``count`` and |
| ``pos`` are guaranteed to fit within 0 and the given size. You don't |
| have to check the range in the callbacks unless any other condition is |
| required. |
| |
| Power Management |
| ================ |
| |
| If the chip is supposed to work with suspend/resume functions, you need |
| to add power-management code to the driver. The additional code for |
| power-management should be ifdef-ed with ``CONFIG_PM``, or annotated |
| with __maybe_unused attribute; otherwise the compiler will complain |
| you. |
| |
| If the driver *fully* supports suspend/resume that is, the device can be |
| properly resumed to its state when suspend was called, you can set the |
| ``SNDRV_PCM_INFO_RESUME`` flag in the pcm info field. Usually, this is |
| possible when the registers of the chip can be safely saved and restored |
| to RAM. If this is set, the trigger callback is called with |
| ``SNDRV_PCM_TRIGGER_RESUME`` after the resume callback completes. |
| |
| Even if the driver doesn't support PM fully but partial suspend/resume |
| is still possible, it's still worthy to implement suspend/resume |
| callbacks. In such a case, applications would reset the status by |
| calling :c:func:`snd_pcm_prepare()` and restart the stream |
| appropriately. Hence, you can define suspend/resume callbacks below but |
| don't set ``SNDRV_PCM_INFO_RESUME`` info flag to the PCM. |
| |
| Note that the trigger with SUSPEND can always be called when |
| :c:func:`snd_pcm_suspend_all()` is called, regardless of the |
| ``SNDRV_PCM_INFO_RESUME`` flag. The ``RESUME`` flag affects only the |
| behavior of :c:func:`snd_pcm_resume()`. (Thus, in theory, |
| ``SNDRV_PCM_TRIGGER_RESUME`` isn't needed to be handled in the trigger |
| callback when no ``SNDRV_PCM_INFO_RESUME`` flag is set. But, it's better |
| to keep it for compatibility reasons.) |
| |
| In the earlier version of ALSA drivers, a common power-management layer |
| was provided, but it has been removed. The driver needs to define the |
| suspend/resume hooks according to the bus the device is connected to. In |
| the case of PCI drivers, the callbacks look like below: |
| |
| :: |
| |
| static int __maybe_unused snd_my_suspend(struct device *dev) |
| { |
| .... /* do things for suspend */ |
| return 0; |
| } |
| static int __maybe_unused snd_my_resume(struct device *dev) |
| { |
| .... /* do things for suspend */ |
| return 0; |
| } |
| |
| The scheme of the real suspend job is as follows. |
| |
| 1. Retrieve the card and the chip data. |
| |
| 2. Call :c:func:`snd_power_change_state()` with |
| ``SNDRV_CTL_POWER_D3hot`` to change the power status. |
| |
| 3. If AC97 codecs are used, call :c:func:`snd_ac97_suspend()` for |
| each codec. |
| |
| 4. Save the register values if necessary. |
| |
| 5. Stop the hardware if necessary. |
| |
| A typical code would be like: |
| |
| :: |
| |
| static int __maybe_unused mychip_suspend(struct device *dev) |
| { |
| /* (1) */ |
| struct snd_card *card = dev_get_drvdata(dev); |
| struct mychip *chip = card->private_data; |
| /* (2) */ |
| snd_power_change_state(card, SNDRV_CTL_POWER_D3hot); |
| /* (3) */ |
| snd_ac97_suspend(chip->ac97); |
| /* (4) */ |
| snd_mychip_save_registers(chip); |
| /* (5) */ |
| snd_mychip_stop_hardware(chip); |
| return 0; |
| } |
| |
| |
| The scheme of the real resume job is as follows. |
| |
| 1. Retrieve the card and the chip data. |
| |
| 2. Re-initialize the chip. |
| |
| 3. Restore the saved registers if necessary. |
| |
| 4. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`. |
| |
| 5. Restart the hardware (if any). |
| |
| 6. Call :c:func:`snd_power_change_state()` with |
| ``SNDRV_CTL_POWER_D0`` to notify the processes. |
| |
| A typical code would be like: |
| |
| :: |
| |
| static int __maybe_unused mychip_resume(struct pci_dev *pci) |
| { |
| /* (1) */ |
| struct snd_card *card = dev_get_drvdata(dev); |
| struct mychip *chip = card->private_data; |
| /* (2) */ |
| snd_mychip_reinit_chip(chip); |
| /* (3) */ |
| snd_mychip_restore_registers(chip); |
| /* (4) */ |
| snd_ac97_resume(chip->ac97); |
| /* (5) */ |
| snd_mychip_restart_chip(chip); |
| /* (6) */ |
| snd_power_change_state(card, SNDRV_CTL_POWER_D0); |
| return 0; |
| } |
| |
| Note that, at the time this callback gets called, the PCM stream has |
| been already suspended via its own PM ops calling |
| :c:func:`snd_pcm_suspend_all()` internally. |
| |
| OK, we have all callbacks now. Let's set them up. In the initialization |
| of the card, make sure that you can get the chip data from the card |
| instance, typically via ``private_data`` field, in case you created the |
| chip data individually. |
| |
| :: |
| |
| static int snd_mychip_probe(struct pci_dev *pci, |
| const struct pci_device_id *pci_id) |
| { |
| .... |
| struct snd_card *card; |
| struct mychip *chip; |
| int err; |
| .... |
| err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE, |
| 0, &card); |
| .... |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| .... |
| card->private_data = chip; |
| .... |
| } |
| |
| When you created the chip data with :c:func:`snd_card_new()`, it's |
| anyway accessible via ``private_data`` field. |
| |
| :: |
| |
| static int snd_mychip_probe(struct pci_dev *pci, |
| const struct pci_device_id *pci_id) |
| { |
| .... |
| struct snd_card *card; |
| struct mychip *chip; |
| int err; |
| .... |
| err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE, |
| sizeof(struct mychip), &card); |
| .... |
| chip = card->private_data; |
| .... |
| } |
| |
| If you need a space to save the registers, allocate the buffer for it |
| here, too, since it would be fatal if you cannot allocate a memory in |
| the suspend phase. The allocated buffer should be released in the |
| corresponding destructor. |
| |
| And next, set suspend/resume callbacks to the pci_driver. |
| |
| :: |
| |
| static SIMPLE_DEV_PM_OPS(snd_my_pm_ops, mychip_suspend, mychip_resume); |
| |
| static struct pci_driver driver = { |
| .name = KBUILD_MODNAME, |
| .id_table = snd_my_ids, |
| .probe = snd_my_probe, |
| .remove = snd_my_remove, |
| .driver.pm = &snd_my_pm_ops, |
| }; |
| |
| Module Parameters |
| ================= |
| |
| There are standard module options for ALSA. At least, each module should |
| have the ``index``, ``id`` and ``enable`` options. |
| |
| If the module supports multiple cards (usually up to 8 = ``SNDRV_CARDS`` |
| cards), they should be arrays. The default initial values are defined |
| already as constants for easier programming: |
| |
| :: |
| |
| static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX; |
| static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR; |
| static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP; |
| |
| If the module supports only a single card, they could be single |
| variables, instead. ``enable`` option is not always necessary in this |
| case, but it would be better to have a dummy option for compatibility. |
| |
| The module parameters must be declared with the standard |
| ``module_param()``, ``module_param_array()`` and |
| :c:func:`MODULE_PARM_DESC()` macros. |
| |
| The typical coding would be like below: |
| |
| :: |
| |
| #define CARD_NAME "My Chip" |
| |
| module_param_array(index, int, NULL, 0444); |
| MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard."); |
| module_param_array(id, charp, NULL, 0444); |
| MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard."); |
| module_param_array(enable, bool, NULL, 0444); |
| MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard."); |
| |
| Also, don't forget to define the module description and the license. |
| Especially, the recent modprobe requires to define the |
| module license as GPL, etc., otherwise the system is shown as “tainted”. |
| |
| :: |
| |
| MODULE_DESCRIPTION("Sound driver for My Chip"); |
| MODULE_LICENSE("GPL"); |
| |
| |
| How To Put Your Driver Into ALSA Tree |
| ===================================== |
| |
| General |
| ------- |
| |
| So far, you've learned how to write the driver codes. And you might have |
| a question now: how to put my own driver into the ALSA driver tree? Here |
| (finally :) the standard procedure is described briefly. |
| |
| Suppose that you create a new PCI driver for the card “xyz”. The card |
| module name would be snd-xyz. The new driver is usually put into the |
| alsa-driver tree, ``sound/pci`` directory in the case of PCI |
| cards. |
| |
| In the following sections, the driver code is supposed to be put into |
| Linux kernel tree. The two cases are covered: a driver consisting of a |
| single source file and one consisting of several source files. |
| |
| Driver with A Single Source File |
| -------------------------------- |
| |
| 1. Modify sound/pci/Makefile |
| |
| Suppose you have a file xyz.c. Add the following two lines |
| |
| :: |
| |
| snd-xyz-objs := xyz.o |
| obj-$(CONFIG_SND_XYZ) += snd-xyz.o |
| |
| 2. Create the Kconfig entry |
| |
| Add the new entry of Kconfig for your xyz driver. config SND_XYZ |
| tristate "Foobar XYZ" depends on SND select SND_PCM help Say Y here |
| to include support for Foobar XYZ soundcard. To compile this driver |
| as a module, choose M here: the module will be called snd-xyz. the |
| line, select SND_PCM, specifies that the driver xyz supports PCM. In |
| addition to SND_PCM, the following components are supported for |
| select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP, |
| SND_MPU401_UART, SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, |
| SND_AC97_CODEC. Add the select command for each supported |
| component. |
| |
| Note that some selections imply the lowlevel selections. For example, |
| PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC |
| includes PCM, and OPL3_LIB includes HWDEP. You don't need to give |
| the lowlevel selections again. |
| |
| For the details of Kconfig script, refer to the kbuild documentation. |
| |
| Drivers with Several Source Files |
| --------------------------------- |
| |
| Suppose that the driver snd-xyz have several source files. They are |
| located in the new subdirectory, sound/pci/xyz. |
| |
| 1. Add a new directory (``sound/pci/xyz``) in ``sound/pci/Makefile`` |
| as below |
| |
| :: |
| |
| obj-$(CONFIG_SND) += sound/pci/xyz/ |
| |
| |
| 2. Under the directory ``sound/pci/xyz``, create a Makefile |
| |
| :: |
| |
| snd-xyz-objs := xyz.o abc.o def.o |
| obj-$(CONFIG_SND_XYZ) += snd-xyz.o |
| |
| 3. Create the Kconfig entry |
| |
| This procedure is as same as in the last section. |
| |
| |
| Useful Functions |
| ================ |
| |
| :c:func:`snd_printk()` and friends |
| ---------------------------------- |
| |
| .. note:: This subsection describes a few helper functions for |
| decorating a bit more on the standard :c:func:`printk()` & co. |
| However, in general, the use of such helpers is no longer recommended. |
| If possible, try to stick with the standard functions like |
| :c:func:`dev_err()` or :c:func:`pr_err()`. |
| |
| ALSA provides a verbose version of the :c:func:`printk()` function. |
| If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function |
| prints the given message together with the file name and the line of the |
| caller. The ``KERN_XXX`` prefix is processed as well as the original |
| :c:func:`printk()` does, so it's recommended to add this prefix, |
| e.g. snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\\n"); |
| |
| There are also :c:func:`printk()`'s for debugging. |
| :c:func:`snd_printd()` can be used for general debugging purposes. |
| If ``CONFIG_SND_DEBUG`` is set, this function is compiled, and works |
| just like :c:func:`snd_printk()`. If the ALSA is compiled without |
| the debugging flag, it's ignored. |
| |
| :c:func:`snd_printdd()` is compiled in only when |
| ``CONFIG_SND_DEBUG_VERBOSE`` is set. |
| |
| :c:func:`snd_BUG()` |
| ------------------- |
| |
| It shows the ``BUG?`` message and stack trace as well as |
| :c:func:`snd_BUG_ON()` at the point. It's useful to show that a |
| fatal error happens there. |
| |
| When no debug flag is set, this macro is ignored. |
| |
| :c:func:`snd_BUG_ON()` |
| ---------------------- |
| |
| :c:func:`snd_BUG_ON()` macro is similar with |
| :c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or |
| it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug)) |
| return -EINVAL; |
| |
| The macro takes an conditional expression to evaluate. When |
| ``CONFIG_SND_DEBUG``, is set, if the expression is non-zero, it shows |
| the warning message such as ``BUG? (xxx)`` normally followed by stack |
| trace. In both cases it returns the evaluated value. |
| |
| Acknowledgments |
| =============== |
| |
| I would like to thank Phil Kerr for his help for improvement and |
| corrections of this document. |
| |
| Kevin Conder reformatted the original plain-text to the DocBook format. |
| |
| Giuliano Pochini corrected typos and contributed the example codes in |
| the hardware constraints section. |