1Serial Peripheral Interface (SPI)
2=================================
3
4SPI is the "Serial Peripheral Interface", widely used with embedded
5systems because it is a simple and efficient interface: basically a
6multiplexed shift register. Its three signal wires hold a clock (SCK,
7often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
8line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
9duplex protocol; for each bit shifted out the MOSI line (one per clock)
10another is shifted in on the MISO line. Those bits are assembled into
11words of various sizes on the way to and from system memory. An
12additional chipselect line is usually active-low (nCS); four signals are
13normally used for each peripheral, plus sometimes an interrupt.
14
15The SPI bus facilities listed here provide a generalized interface to
16declare SPI busses and devices, manage them according to the standard
17Linux driver model, and perform input/output operations. At this time,
18only "master" side interfaces are supported, where Linux talks to SPI
19peripherals and does not implement such a peripheral itself. (Interfaces
20to support implementing SPI slaves would necessarily look different.)
21
22The programming interface is structured around two kinds of driver, and
23two kinds of device. A "Controller Driver" abstracts the controller
24hardware, which may be as simple as a set of GPIO pins or as complex as
25a pair of FIFOs connected to dual DMA engines on the other side of the
26SPI shift register (maximizing throughput). Such drivers bridge between
27whatever bus they sit on (often the platform bus) and SPI, and expose
28the SPI side of their device as a :c:type:`struct spi_master
29<spi_master>`. SPI devices are children of that master,
30represented as a :c:type:`struct spi_device <spi_device>` and
31manufactured from :c:type:`struct spi_board_info
32<spi_board_info>` descriptors which are usually provided by
33board-specific initialization code. A :c:type:`struct spi_driver
34<spi_driver>` is called a "Protocol Driver", and is bound to a
35spi_device using normal driver model calls.
36
37The I/O model is a set of queued messages. Protocol drivers submit one
38or more :c:type:`struct spi_message <spi_message>` objects,
39which are processed and completed asynchronously. (There are synchronous
40wrappers, however.) Messages are built from one or more
41:c:type:`struct spi_transfer <spi_transfer>` objects, each of
42which wraps a full duplex SPI transfer. A variety of protocol tweaking
43options are needed, because different chips adopt very different
44policies for how they use the bits transferred with SPI.
45
46.. kernel-doc:: include/linux/spi/spi.h
47   :internal:
48
49.. kernel-doc:: drivers/spi/spi.c
50   :functions: spi_register_board_info
51
52.. kernel-doc:: drivers/spi/spi.c
53   :export:
54