| 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: |