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We don't need to keep it as a single large file anymore; split it up so that it is easier to manage and the individual sections can be read directly as plain files. Signed-off-by: Jonathan Corbet <corbet@lwn.net>
116 lines
5.5 KiB
ReStructuredText
116 lines
5.5 KiB
ReStructuredText
Serial Peripheral Interface (SPI)
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=================================
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SPI is the "Serial Peripheral Interface", widely used with embedded
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systems because it is a simple and efficient interface: basically a
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multiplexed shift register. Its three signal wires hold a clock (SCK,
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often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
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line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
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duplex protocol; for each bit shifted out the MOSI line (one per clock)
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another is shifted in on the MISO line. Those bits are assembled into
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words of various sizes on the way to and from system memory. An
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additional chipselect line is usually active-low (nCS); four signals are
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normally used for each peripheral, plus sometimes an interrupt.
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The SPI bus facilities listed here provide a generalized interface to
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declare SPI busses and devices, manage them according to the standard
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Linux driver model, and perform input/output operations. At this time,
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only "master" side interfaces are supported, where Linux talks to SPI
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peripherals and does not implement such a peripheral itself. (Interfaces
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to support implementing SPI slaves would necessarily look different.)
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The programming interface is structured around two kinds of driver, and
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two kinds of device. A "Controller Driver" abstracts the controller
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hardware, which may be as simple as a set of GPIO pins or as complex as
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a pair of FIFOs connected to dual DMA engines on the other side of the
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SPI shift register (maximizing throughput). Such drivers bridge between
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whatever bus they sit on (often the platform bus) and SPI, and expose
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the SPI side of their device as a :c:type:`struct spi_master
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<spi_master>`. SPI devices are children of that master,
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represented as a :c:type:`struct spi_device <spi_device>` and
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manufactured from :c:type:`struct spi_board_info
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<spi_board_info>` descriptors which are usually provided by
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board-specific initialization code. A :c:type:`struct spi_driver
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<spi_driver>` is called a "Protocol Driver", and is bound to a
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spi_device using normal driver model calls.
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The I/O model is a set of queued messages. Protocol drivers submit one
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or more :c:type:`struct spi_message <spi_message>` objects,
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which are processed and completed asynchronously. (There are synchronous
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wrappers, however.) Messages are built from one or more
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:c:type:`struct spi_transfer <spi_transfer>` objects, each of
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which wraps a full duplex SPI transfer. A variety of protocol tweaking
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options are needed, because different chips adopt very different
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policies for how they use the bits transferred with SPI.
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.. kernel-doc:: include/linux/spi/spi.h
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:internal:
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.. kernel-doc:: drivers/spi/spi.c
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:functions: spi_register_board_info
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.. kernel-doc:: drivers/spi/spi.c
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:export:
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I\ :sup:`2`\ C and SMBus Subsystem
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==================================
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I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
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the "Inter-IC" bus, a simple bus protocol which is widely used where low
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data rate communications suffice. Since it's also a licensed trademark,
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some vendors use another name (such as "Two-Wire Interface", TWI) for
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the same bus. I2C only needs two signals (SCL for clock, SDA for data),
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conserving board real estate and minimizing signal quality issues. Most
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I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
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there's a high speed extension (3.4 MHz) that's not yet found wide use.
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I2C is a multi-master bus; open drain signaling is used to arbitrate
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between masters, as well as to handshake and to synchronize clocks from
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slower clients.
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The Linux I2C programming interfaces support only the master side of bus
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interactions, not the slave side. The programming interface is
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structured around two kinds of driver, and two kinds of device. An I2C
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"Adapter Driver" abstracts the controller hardware; it binds to a
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physical device (perhaps a PCI device or platform_device) and exposes a
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:c:type:`struct i2c_adapter <i2c_adapter>` representing each
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I2C bus segment it manages. On each I2C bus segment will be I2C devices
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represented by a :c:type:`struct i2c_client <i2c_client>`.
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Those devices will be bound to a :c:type:`struct i2c_driver
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<i2c_driver>`, which should follow the standard Linux driver
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model. (At this writing, a legacy model is more widely used.) There are
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functions to perform various I2C protocol operations; at this writing
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all such functions are usable only from task context.
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The System Management Bus (SMBus) is a sibling protocol. Most SMBus
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systems are also I2C conformant. The electrical constraints are tighter
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for SMBus, and it standardizes particular protocol messages and idioms.
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Controllers that support I2C can also support most SMBus operations, but
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SMBus controllers don't support all the protocol options that an I2C
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controller will. There are functions to perform various SMBus protocol
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operations, either using I2C primitives or by issuing SMBus commands to
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i2c_adapter devices which don't support those I2C operations.
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.. kernel-doc:: include/linux/i2c.h
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:internal:
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.. kernel-doc:: drivers/i2c/i2c-boardinfo.c
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:functions: i2c_register_board_info
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.. kernel-doc:: drivers/i2c/i2c-core.c
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:export:
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High Speed Synchronous Serial Interface (HSI)
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=============================================
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High Speed Synchronous Serial Interface (HSI) is a serial interface
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mainly used for connecting application engines (APE) with cellular modem
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engines (CMT) in cellular handsets. HSI provides multiplexing for up to
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16 logical channels, low-latency and full duplex communication.
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.. kernel-doc:: include/linux/hsi/hsi.h
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:internal:
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.. kernel-doc:: drivers/hsi/hsi_core.c
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:export:
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