Documentation/driver-api/serial-interfaces.rst - linux - Git at Google

 Serial Peripheral Interface (SPI)
 =================================

 SPI is the "Serial Peripheral Interface", widely used with embedded
 systems because it is a simple and efficient interface: basically a
 multiplexed shift register. Its three signal wires hold a clock (SCK,
 often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
 line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
 duplex protocol; for each bit shifted out the MOSI line (one per clock)
 another is shifted in on the MISO line. Those bits are assembled into
 words of various sizes on the way to and from system memory. An
 additional chipselect line is usually active-low (nCS); four signals are
 normally used for each peripheral, plus sometimes an interrupt.

 The SPI bus facilities listed here provide a generalized interface to
 declare SPI busses and devices, manage them according to the standard
 Linux driver model, and perform input/output operations. At this time,
 only "master" side interfaces are supported, where Linux talks to SPI
 peripherals and does not implement such a peripheral itself. (Interfaces
 to support implementing SPI slaves would necessarily look different.)

 The programming interface is structured around two kinds of driver, and
 two kinds of device. A "Controller Driver" abstracts the controller
 hardware, which may be as simple as a set of GPIO pins or as complex as
 a pair of FIFOs connected to dual DMA engines on the other side of the
 SPI shift register (maximizing throughput). Such drivers bridge between
 whatever bus they sit on (often the platform bus) and SPI, and expose
 the SPI side of their device as a :c:type:`struct spi_master
 <spi_master>`. SPI devices are children of that master,
 represented as a :c:type:`struct spi_device <spi_device>` and
 manufactured from :c:type:`struct spi_board_info
 <spi_board_info>` descriptors which are usually provided by
 board-specific initialization code. A :c:type:`struct spi_driver
 <spi_driver>` is called a "Protocol Driver", and is bound to a
 spi_device using normal driver model calls.

 The I/O model is a set of queued messages. Protocol drivers submit one
 or more :c:type:`struct spi_message <spi_message>` objects,
 which are processed and completed asynchronously. (There are synchronous
 wrappers, however.) Messages are built from one or more
 :c:type:`struct spi_transfer <spi_transfer>` objects, each of
 which wraps a full duplex SPI transfer. A variety of protocol tweaking
 options are needed, because different chips adopt very different
 policies for how they use the bits transferred with SPI.

 .. kernel-doc:: include/linux/spi/spi.h
    :internal:

 .. kernel-doc:: drivers/spi/spi.c
    :functions: spi_register_board_info

 .. kernel-doc:: drivers/spi/spi.c
    :export:

 I\ :sup:`2`\ C and SMBus Subsystem
 ==================================

 I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
 the "Inter-IC" bus, a simple bus protocol which is widely used where low
 data rate communications suffice. Since it's also a licensed trademark,
 some vendors use another name (such as "Two-Wire Interface", TWI) for
 the same bus. I2C only needs two signals (SCL for clock, SDA for data),
 conserving board real estate and minimizing signal quality issues. Most
 I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
 there's a high speed extension (3.4 MHz) that's not yet found wide use.
 I2C is a multi-master bus; open drain signaling is used to arbitrate
 between masters, as well as to handshake and to synchronize clocks from
 slower clients.

 The Linux I2C programming interfaces support only the master side of bus
 interactions, not the slave side. The programming interface is
 structured around two kinds of driver, and two kinds of device. An I2C
 "Adapter Driver" abstracts the controller hardware; it binds to a
 physical device (perhaps a PCI device or platform_device) and exposes a
 :c:type:`struct i2c_adapter <i2c_adapter>` representing each
 I2C bus segment it manages. On each I2C bus segment will be I2C devices
 represented by a :c:type:`struct i2c_client <i2c_client>`.
 Those devices will be bound to a :c:type:`struct i2c_driver
 <i2c_driver>`, which should follow the standard Linux driver
 model. (At this writing, a legacy model is more widely used.) There are
 functions to perform various I2C protocol operations; at this writing
 all such functions are usable only from task context.

 The System Management Bus (SMBus) is a sibling protocol. Most SMBus
 systems are also I2C conformant. The electrical constraints are tighter
 for SMBus, and it standardizes particular protocol messages and idioms.
 Controllers that support I2C can also support most SMBus operations, but
 SMBus controllers don't support all the protocol options that an I2C
 controller will. There are functions to perform various SMBus protocol
 operations, either using I2C primitives or by issuing SMBus commands to
 i2c_adapter devices which don't support those I2C operations.

 .. kernel-doc:: include/linux/i2c.h
    :internal:

 .. kernel-doc:: drivers/i2c/i2c-boardinfo.c
    :functions: i2c_register_board_info

 .. kernel-doc:: drivers/i2c/i2c-core.c
    :export:

 High Speed Synchronous Serial Interface (HSI)
 =============================================

 High Speed Synchronous Serial Interface (HSI) is a serial interface
 mainly used for connecting application engines (APE) with cellular modem
 engines (CMT) in cellular handsets. HSI provides multiplexing for up to
 16 logical channels, low-latency and full duplex communication.

 .. kernel-doc:: include/linux/hsi/hsi.h
    :internal:

 .. kernel-doc:: drivers/hsi/hsi_core.c
    :export:
	Serial Peripheral Interface (SPI)
	=================================

	SPI is the "Serial Peripheral Interface", widely used with embedded
	systems because it is a simple and efficient interface: basically a
	multiplexed shift register. Its three signal wires hold a clock (SCK,
	often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
	line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
	duplex protocol; for each bit shifted out the MOSI line (one per clock)
	another is shifted in on the MISO line. Those bits are assembled into
	words of various sizes on the way to and from system memory. An
	additional chipselect line is usually active-low (nCS); four signals are
	normally used for each peripheral, plus sometimes an interrupt.

	The SPI bus facilities listed here provide a generalized interface to
	declare SPI busses and devices, manage them according to the standard
	Linux driver model, and perform input/output operations. At this time,
	only "master" side interfaces are supported, where Linux talks to SPI
	peripherals and does not implement such a peripheral itself. (Interfaces
	to support implementing SPI slaves would necessarily look different.)

	The programming interface is structured around two kinds of driver, and
	two kinds of device. A "Controller Driver" abstracts the controller
	hardware, which may be as simple as a set of GPIO pins or as complex as
	a pair of FIFOs connected to dual DMA engines on the other side of the
	SPI shift register (maximizing throughput). Such drivers bridge between
	whatever bus they sit on (often the platform bus) and SPI, and expose
	the SPI side of their device as a :c:type:`struct spi_master
	<spi_master>`. SPI devices are children of that master,
	represented as a :c:type:`struct spi_device <spi_device>` and
	manufactured from :c:type:`struct spi_board_info
	<spi_board_info>` descriptors which are usually provided by
	board-specific initialization code. A :c:type:`struct spi_driver
	<spi_driver>` is called a "Protocol Driver", and is bound to a
	spi_device using normal driver model calls.

	The I/O model is a set of queued messages. Protocol drivers submit one
	or more :c:type:`struct spi_message <spi_message>` objects,
	which are processed and completed asynchronously. (There are synchronous
	wrappers, however.) Messages are built from one or more
	:c:type:`struct spi_transfer <spi_transfer>` objects, each of
	which wraps a full duplex SPI transfer. A variety of protocol tweaking
	options are needed, because different chips adopt very different
	policies for how they use the bits transferred with SPI.

	.. kernel-doc:: include/linux/spi/spi.h
	:internal:

	.. kernel-doc:: drivers/spi/spi.c
	:functions: spi_register_board_info

	.. kernel-doc:: drivers/spi/spi.c
	:export:

	I\ :sup:`2`\ C and SMBus Subsystem
	==================================

	I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
	the "Inter-IC" bus, a simple bus protocol which is widely used where low
	data rate communications suffice. Since it's also a licensed trademark,
	some vendors use another name (such as "Two-Wire Interface", TWI) for
	the same bus. I2C only needs two signals (SCL for clock, SDA for data),
	conserving board real estate and minimizing signal quality issues. Most
	I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
	there's a high speed extension (3.4 MHz) that's not yet found wide use.
	I2C is a multi-master bus; open drain signaling is used to arbitrate
	between masters, as well as to handshake and to synchronize clocks from
	slower clients.

	The Linux I2C programming interfaces support only the master side of bus
	interactions, not the slave side. The programming interface is
	structured around two kinds of driver, and two kinds of device. An I2C
	"Adapter Driver" abstracts the controller hardware; it binds to a
	physical device (perhaps a PCI device or platform_device) and exposes a
	:c:type:`struct i2c_adapter <i2c_adapter>` representing each
	I2C bus segment it manages. On each I2C bus segment will be I2C devices
	represented by a :c:type:`struct i2c_client <i2c_client>`.
	Those devices will be bound to a :c:type:`struct i2c_driver
	<i2c_driver>`, which should follow the standard Linux driver
	model. (At this writing, a legacy model is more widely used.) There are
	functions to perform various I2C protocol operations; at this writing
	all such functions are usable only from task context.

	The System Management Bus (SMBus) is a sibling protocol. Most SMBus
	systems are also I2C conformant. The electrical constraints are tighter
	for SMBus, and it standardizes particular protocol messages and idioms.
	Controllers that support I2C can also support most SMBus operations, but
	SMBus controllers don't support all the protocol options that an I2C
	controller will. There are functions to perform various SMBus protocol
	operations, either using I2C primitives or by issuing SMBus commands to
	i2c_adapter devices which don't support those I2C operations.

	.. kernel-doc:: include/linux/i2c.h
	:internal:

	.. kernel-doc:: drivers/i2c/i2c-boardinfo.c
	:functions: i2c_register_board_info

	.. kernel-doc:: drivers/i2c/i2c-core.c
	:export:

	High Speed Synchronous Serial Interface (HSI)
	=============================================

	High Speed Synchronous Serial Interface (HSI) is a serial interface
	mainly used for connecting application engines (APE) with cellular modem
	engines (CMT) in cellular handsets. HSI provides multiplexing for up to
	16 logical channels, low-latency and full duplex communication.

	.. kernel-doc:: include/linux/hsi/hsi.h
	:internal:

	.. kernel-doc:: drivers/hsi/hsi_core.c
	:export: