.. SPDX-License-Identifier: GPL-2.0

================
EISA bus support
================

:Author: Marc Zyngier <maz@wild-wind.fr.eu.org>

This document groups random notes about porting EISA drivers to the
new EISA/sysfs API.

Starting from version 2.5.59, the EISA bus is almost given the same
status as other much more mainstream buses such as PCI or USB. This
has been possible through sysfs, which defines a nice enough set of
abstractions to manage buses, devices and drivers.

Although the new API is quite simple to use, converting existing
drivers to the new infrastructure is not an easy task (mostly because
detection code is generally also used to probe ISA cards). Moreover,
most EISA drivers are among the oldest Linux drivers so, as you can
imagine, some dust has settled here over the years.

The EISA infrastructure is made up of three parts:

    - The bus code implements most of the generic code. It is shared
      among all the architectures that the EISA code runs on. It
      implements bus probing (detecting EISA cards available on the bus),
      allocates I/O resources, allows fancy naming through sysfs, and
      offers interfaces for driver to register.

    - The bus root driver implements the glue between the bus hardware
      and the generic bus code. It is responsible for discovering the
      device implementing the bus, and setting it up to be latter probed
      by the bus code. This can go from something as simple as reserving
      an I/O region on x86, to the rather more complex, like the hppa
      EISA code. This is the part to implement in order to have EISA
      running on an "new" platform.

    - The driver offers the bus a list of devices that it manages, and
      implements the necessary callbacks to probe and release devices
      whenever told to.

Every function/structure below lives in <linux/eisa.h>, which depends
heavily on <linux/device.h>.

Bus root driver
===============

::

	int eisa_root_register (struct eisa_root_device *root);

The eisa_root_register function is used to declare a device as the
root of an EISA bus. The eisa_root_device structure holds a reference
to this device, as well as some parameters for probing purposes::

	struct eisa_root_device {
		struct device   *dev;	 /* Pointer to bridge device */
		struct resource *res;
		unsigned long    bus_base_addr;
		int		 slots;  /* Max slot number */
		int		 force_probe; /* Probe even when no slot 0 */
		u64		 dma_mask; /* from bridge device */
		int              bus_nr; /* Set by eisa_root_register */
		struct resource  eisa_root_res;	/* ditto */
	};

============= ======================================================
node          used for eisa_root_register internal purpose
dev           pointer to the root device
res           root device I/O resource
bus_base_addr slot 0 address on this bus
slots	      max slot number to probe
force_probe   Probe even when slot 0 is empty (no EISA mainboard)
dma_mask      Default DMA mask. Usually the bridge device dma_mask.
bus_nr	      unique bus id, set by eisa_root_register
============= ======================================================

Driver
======

::

	int eisa_driver_register (struct eisa_driver *edrv);
	void eisa_driver_unregister (struct eisa_driver *edrv);

Clear enough ?

::

	struct eisa_device_id {
		char sig[EISA_SIG_LEN];
		unsigned long driver_data;
	};

	struct eisa_driver {
		const struct eisa_device_id *id_table;
		struct device_driver         driver;
	};

=============== ====================================================
id_table	an array of NULL terminated EISA id strings,
		followed by an empty string. Each string can
		optionally be paired with a driver-dependent value
		(driver_data).

driver		a generic driver, such as described in
		Documentation/driver-api/driver-model/driver.rst. Only .name,
		.probe and .remove members are mandatory.
=============== ====================================================

An example is the 3c59x driver::

	static struct eisa_device_id vortex_eisa_ids[] = {
		{ "TCM5920", EISA_3C592_OFFSET },
		{ "TCM5970", EISA_3C597_OFFSET },
		{ "" }
	};

	static struct eisa_driver vortex_eisa_driver = {
		.id_table = vortex_eisa_ids,
		.driver   = {
			.name    = "3c59x",
			.probe   = vortex_eisa_probe,
			.remove  = vortex_eisa_remove
		}
	};

Device
======

The sysfs framework calls .probe and .remove functions upon device
discovery and removal (note that the .remove function is only called
when driver is built as a module).

Both functions are passed a pointer to a 'struct device', which is
encapsulated in a 'struct eisa_device' described as follows::

	struct eisa_device {
		struct eisa_device_id id;
		int                   slot;
		int                   state;
		unsigned long         base_addr;
		struct resource       res[EISA_MAX_RESOURCES];
		u64                   dma_mask;
		struct device         dev; /* generic device */
	};

======== ============================================================
id	 EISA id, as read from device. id.driver_data is set from the
	 matching driver EISA id.
slot	 slot number which the device was detected on
state    set of flags indicating the state of the device. Current
	 flags are EISA_CONFIG_ENABLED and EISA_CONFIG_FORCED.
res	 set of four 256 bytes I/O regions allocated to this device
dma_mask DMA mask set from the parent device.
dev	 generic device (see Documentation/driver-api/driver-model/device.rst)
======== ============================================================

You can get the 'struct eisa_device' from 'struct device' using the
'to_eisa_device' macro.

Misc stuff
==========

::

	void eisa_set_drvdata (struct eisa_device *edev, void *data);

Stores data into the device's driver_data area.

::

	void *eisa_get_drvdata (struct eisa_device *edev):

Gets the pointer previously stored into the device's driver_data area.

::

	int eisa_get_region_index (void *addr);

Returns the region number (0 <= x < EISA_MAX_RESOURCES) of a given
address.

Kernel parameters
=================

eisa_bus.enable_dev
	A comma-separated list of slots to be enabled, even if the firmware
	set the card as disabled. The driver must be able to properly
	initialize the device in such conditions.

eisa_bus.disable_dev
	A comma-separated list of slots to be disabled, even if the firmware
	set the card as enabled. The driver won't be called to handle this
	device.

virtual_root.force_probe
	Force the probing code to probe EISA slots even when it cannot find an
	EISA compliant mainboard (nothing appears on slot 0). Defaults to 0
	(don't force), and set to 1 (force probing) when
	CONFIG_EISA_VLB_PRIMING is set.

Random notes
============

Converting an EISA driver to the new API mostly involves *deleting*
code (since probing is now in the core EISA code). Unfortunately, most
drivers share their probing routine between ISA, and EISA. Special
care must be taken when ripping out the EISA code, so other buses
won't suffer from these surgical strikes...

You *must not* expect any EISA device to be detected when returning
from eisa_driver_register, since the chances are that the bus has not
yet been probed. In fact, that's what happens most of the time (the
bus root driver usually kicks in rather late in the boot process).
Unfortunately, most drivers are doing the probing by themselves, and
expect to have explored the whole machine when they exit their probe
routine.

For example, switching your favorite EISA SCSI card to the "hotplug"
model is "the right thing"(tm).

Thanks
======

I'd like to thank the following people for their help:

- Xavier Benigni for lending me a wonderful Alpha Jensen,
- James Bottomley, Jeff Garzik for getting this stuff into the kernel,
- Andries Brouwer for contributing numerous EISA ids,
- Catrin Jones for coping with far too many machines at home.