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diff --git a/Documentation/DocBook/kernel-api.tmpl b/Documentation/DocBook/kernel-api.tmpl index b61dfc79e1b8..a2b2b4d187c5 100644 --- a/Documentation/DocBook/kernel-api.tmpl +++ b/Documentation/DocBook/kernel-api.tmpl @@ -576,4 +576,67 @@ X!Idrivers/video/console/fonts.c !Edrivers/input/ff-core.c !Edrivers/input/ff-memless.c </chapter> + + <chapter id="spi"> + <title>Serial Peripheral Interface (SPI)</title> + <para> + 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. + </para> + <para> + 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.) + </para> + <para> + 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 + <structname>struct spi_master</structname>. + SPI devices are children of that master, represented as a + <structname>struct spi_device</structname> and manufactured from + <structname>struct spi_board_info</structname> descriptors which + are usually provided by board-specific initialization code. + A <structname>struct spi_driver</structname> is called a + "Protocol Driver", and is bound to a spi_device using normal + driver model calls. + </para> + <para> + The I/O model is a set of queued messages. Protocol drivers + submit one or more <structname>struct spi_message</structname> + objects, which are processed and completed asynchronously. + (There are synchronous wrappers, however.) Messages are + built from one or more <structname>struct spi_transfer</structname> + 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. + </para> +!Iinclude/linux/spi/spi.h +!Fdrivers/spi/spi.c spi_register_board_info +!Edrivers/spi/spi.c + </chapter> + </book> |