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@@ -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>