| 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: |