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+Overview of Linux kernel SPI support
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+====================================
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+
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+02-Dec-2005
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+
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+What is SPI?
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+------------
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+The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
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+link used to connect microcontrollers to sensors, memory, and peripherals.
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+
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+The three signal wires hold a clock (SCLK, often on the order of 10 MHz),
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+and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
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+Slave Out" (MISO) signals. (Other names are also used.) There are four
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+clocking modes through which data is exchanged; mode-0 and mode-3 are most
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+commonly used. Each clock cycle shifts data out and data in; the clock
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+doesn't cycle except when there is data to shift.
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+
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+SPI masters may use a "chip select" line to activate a given SPI slave
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+device, so those three signal wires may be connected to several chips
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+in parallel. All SPI slaves support chipselects. Some devices have
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+other signals, often including an interrupt to the master.
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+
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+Unlike serial busses like USB or SMBUS, even low level protocols for
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+SPI slave functions are usually not interoperable between vendors
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+(except for cases like SPI memory chips).
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+
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+ - SPI may be used for request/response style device protocols, as with
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+ touchscreen sensors and memory chips.
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+
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+ - It may also be used to stream data in either direction (half duplex),
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+ or both of them at the same time (full duplex).
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+
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+ - Some devices may use eight bit words. Others may different word
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+ lengths, such as streams of 12-bit or 20-bit digital samples.
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+
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+In the same way, SPI slaves will only rarely support any kind of automatic
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+discovery/enumeration protocol. The tree of slave devices accessible from
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+a given SPI master will normally be set up manually, with configuration
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+tables.
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+
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+SPI is only one of the names used by such four-wire protocols, and
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+most controllers have no problem handling "MicroWire" (think of it as
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+half-duplex SPI, for request/response protocols), SSP ("Synchronous
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+Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
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+related protocols.
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+
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+Microcontrollers often support both master and slave sides of the SPI
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+protocol. This document (and Linux) currently only supports the master
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+side of SPI interactions.
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+
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+
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+Who uses it? On what kinds of systems?
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+---------------------------------------
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+Linux developers using SPI are probably writing device drivers for embedded
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+systems boards. SPI is used to control external chips, and it is also a
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+protocol supported by every MMC or SD memory card. (The older "DataFlash"
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+cards, predating MMC cards but using the same connectors and card shape,
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+support only SPI.) Some PC hardware uses SPI flash for BIOS code.
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+
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+SPI slave chips range from digital/analog converters used for analog
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+sensors and codecs, to memory, to peripherals like USB controllers
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+or Ethernet adapters; and more.
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+
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+Most systems using SPI will integrate a few devices on a mainboard.
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+Some provide SPI links on expansion connectors; in cases where no
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+dedicated SPI controller exists, GPIO pins can be used to create a
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+low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI
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+controller; the reasons to use SPI focus on low cost and simple operation,
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+and if dynamic reconfiguration is important, USB will often be a more
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+appropriate low-pincount peripheral bus.
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+
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+Many microcontrollers that can run Linux integrate one or more I/O
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+interfaces with SPI modes. Given SPI support, they could use MMC or SD
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+cards without needing a special purpose MMC/SD/SDIO controller.
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+
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+
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+How do these driver programming interfaces work?
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+------------------------------------------------
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+The <linux/spi/spi.h> header file includes kerneldoc, as does the
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+main source code, and you should certainly read that. This is just
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+an overview, so you get the big picture before the details.
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+
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+SPI requests always go into I/O queues. Requests for a given SPI device
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+are always executed in FIFO order, and complete asynchronously through
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+completion callbacks. There are also some simple synchronous wrappers
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+for those calls, including ones for common transaction types like writing
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+a command and then reading its response.
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+
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+There are two types of SPI driver, here called:
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+
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+ Controller drivers ... these are often built in to System-On-Chip
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+ processors, and often support both Master and Slave roles.
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+ These drivers touch hardware registers and may use DMA.
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+ Or they can be PIO bitbangers, needing just GPIO pins.
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+
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+ Protocol drivers ... these pass messages through the controller
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+ driver to communicate with a Slave or Master device on the
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+ other side of an SPI link.
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+
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+So for example one protocol driver might talk to the MTD layer to export
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+data to filesystems stored on SPI flash like DataFlash; and others might
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+control audio interfaces, present touchscreen sensors as input interfaces,
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+or monitor temperature and voltage levels during industrial processing.
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+And those might all be sharing the same controller driver.
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+
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+A "struct spi_device" encapsulates the master-side interface between
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+those two types of driver. At this writing, Linux has no slave side
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+programming interface.
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+
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+There is a minimal core of SPI programming interfaces, focussing on
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+using driver model to connect controller and protocol drivers using
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+device tables provided by board specific initialization code. SPI
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+shows up in sysfs in several locations:
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+
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+ /sys/devices/.../CTLR/spiB.C ... spi_device for on bus "B",
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+ chipselect C, accessed through CTLR.
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+
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+ /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver
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+ that should be used with this device (for hotplug/coldplug)
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+
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+ /sys/bus/spi/devices/spiB.C ... symlink to the physical
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+ spiB-C device
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+
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+ /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
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+
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+ /sys/class/spi_master/spiB ... class device for the controller
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+ managing bus "B". All the spiB.* devices share the same
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+ physical SPI bus segment, with SCLK, MOSI, and MISO.
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+
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+
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+How does board-specific init code declare SPI devices?
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+------------------------------------------------------
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+Linux needs several kinds of information to properly configure SPI devices.
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+That information is normally provided by board-specific code, even for
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+chips that do support some of automated discovery/enumeration.
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+
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+DECLARE CONTROLLERS
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+
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+The first kind of information is a list of what SPI controllers exist.
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+For System-on-Chip (SOC) based boards, these will usually be platform
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+devices, and the controller may need some platform_data in order to
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+operate properly. The "struct platform_device" will include resources
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+like the physical address of the controller's first register and its IRQ.
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+
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+Platforms will often abstract the "register SPI controller" operation,
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+maybe coupling it with code to initialize pin configurations, so that
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+the arch/.../mach-*/board-*.c files for several boards can all share the
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+same basic controller setup code. This is because most SOCs have several
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+SPI-capable controllers, and only the ones actually usable on a given
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+board should normally be set up and registered.
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+
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+So for example arch/.../mach-*/board-*.c files might have code like:
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+
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+ #include <asm/arch/spi.h> /* for mysoc_spi_data */
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+
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+ /* if your mach-* infrastructure doesn't support kernels that can
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+ * run on multiple boards, pdata wouldn't benefit from "__init".
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+ */
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+ static struct mysoc_spi_data __init pdata = { ... };
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+
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+ static __init board_init(void)
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+ {
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+ ...
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+ /* this board only uses SPI controller #2 */
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+ mysoc_register_spi(2, &pdata);
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+ ...
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+ }
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+
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+And SOC-specific utility code might look something like:
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+
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+ #include <asm/arch/spi.h>
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+
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+ static struct platform_device spi2 = { ... };
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+
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+ void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
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+ {
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+ struct mysoc_spi_data *pdata2;
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+
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+ pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
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+ *pdata2 = pdata;
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+ ...
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+ if (n == 2) {
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+ spi2->dev.platform_data = pdata2;
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+ register_platform_device(&spi2);
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+
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+ /* also: set up pin modes so the spi2 signals are
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+ * visible on the relevant pins ... bootloaders on
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+ * production boards may already have done this, but
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+ * developer boards will often need Linux to do it.
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+ */
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+ }
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+ ...
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+ }
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+
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+Notice how the platform_data for boards may be different, even if the
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+same SOC controller is used. For example, on one board SPI might use
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+an external clock, where another derives the SPI clock from current
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+settings of some master clock.
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+
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+
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+DECLARE SLAVE DEVICES
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+
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+The second kind of information is a list of what SPI slave devices exist
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+on the target board, often with some board-specific data needed for the
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+driver to work correctly.
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+
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+Normally your arch/.../mach-*/board-*.c files would provide a small table
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+listing the SPI devices on each board. (This would typically be only a
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+small handful.) That might look like:
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+
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+ static struct ads7846_platform_data ads_info = {
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+ .vref_delay_usecs = 100,
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+ .x_plate_ohms = 580,
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+ .y_plate_ohms = 410,
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+ };
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+
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+ static struct spi_board_info spi_board_info[] __initdata = {
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+ {
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+ .modalias = "ads7846",
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+ .platform_data = &ads_info,
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+ .mode = SPI_MODE_0,
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+ .irq = GPIO_IRQ(31),
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+ .max_speed_hz = 120000 /* max sample rate at 3V */ * 16,
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+ .bus_num = 1,
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+ .chip_select = 0,
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+ },
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+ };
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+
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+Again, notice how board-specific information is provided; each chip may need
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+several types. This example shows generic constraints like the fastest SPI
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+clock to allow (a function of board voltage in this case) or how an IRQ pin
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+is wired, plus chip-specific constraints like an important delay that's
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+changed by the capacitance at one pin.
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+
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+(There's also "controller_data", information that may be useful to the
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+controller driver. An example would be peripheral-specific DMA tuning
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+data or chipselect callbacks. This is stored in spi_device later.)
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+
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+The board_info should provide enough information to let the system work
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+without the chip's driver being loaded. The most troublesome aspect of
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+that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
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+sharing a bus with a device that interprets chipselect "backwards" is
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+not possible.
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+
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+Then your board initialization code would register that table with the SPI
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+infrastructure, so that it's available later when the SPI master controller
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+driver is registered:
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+
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+ spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
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+
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+Like with other static board-specific setup, you won't unregister those.
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+
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+The widely used "card" style computers bundle memory, cpu, and little else
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+onto a card that's maybe just thirty square centimeters. On such systems,
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+your arch/.../mach-.../board-*.c file would primarily provide information
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+about the devices on the mainboard into which such a card is plugged. That
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+certainly includes SPI devices hooked up through the card connectors!
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+
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+
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+NON-STATIC CONFIGURATIONS
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+
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+Developer boards often play by different rules than product boards, and one
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+example is the potential need to hotplug SPI devices and/or controllers.
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+
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+For those cases you might need to use use spi_busnum_to_master() to look
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+up the spi bus master, and will likely need spi_new_device() to provide the
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+board info based on the board that was hotplugged. Of course, you'd later
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+call at least spi_unregister_device() when that board is removed.
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+
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+When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those
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+configurations will also be dynamic. Fortunately, those devices all support
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+basic device identification probes, so that support should hotplug normally.
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+
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+
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+How do I write an "SPI Protocol Driver"?
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+----------------------------------------
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+All SPI drivers are currently kernel drivers. A userspace driver API
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+would just be another kernel driver, probably offering some lowlevel
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+access through aio_read(), aio_write(), and ioctl() calls and using the
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+standard userspace sysfs mechanisms to bind to a given SPI device.
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+
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+SPI protocol drivers somewhat resemble platform device drivers:
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+
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+ static struct spi_driver CHIP_driver = {
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+ .driver = {
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+ .name = "CHIP",
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+ .bus = &spi_bus_type,
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+ .owner = THIS_MODULE,
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+ },
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+
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+ .probe = CHIP_probe,
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+ .remove = __devexit_p(CHIP_remove),
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+ .suspend = CHIP_suspend,
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+ .resume = CHIP_resume,
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+ };
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+
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+The driver core will autmatically attempt to bind this driver to any SPI
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+device whose board_info gave a modalias of "CHIP". Your probe() code
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+might look like this unless you're creating a class_device:
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+
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+ static int __devinit CHIP_probe(struct spi_device *spi)
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+ {
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+ struct CHIP *chip;
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+ struct CHIP_platform_data *pdata;
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+
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+ /* assuming the driver requires board-specific data: */
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+ pdata = &spi->dev.platform_data;
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+ if (!pdata)
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+ return -ENODEV;
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+
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+ /* get memory for driver's per-chip state */
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+ chip = kzalloc(sizeof *chip, GFP_KERNEL);
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+ if (!chip)
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+ return -ENOMEM;
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+ dev_set_drvdata(&spi->dev, chip);
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+
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+ ... etc
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+ return 0;
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+ }
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+
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+As soon as it enters probe(), the driver may issue I/O requests to
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+the SPI device using "struct spi_message". When remove() returns,
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+the driver guarantees that it won't submit any more such messages.
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+
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+ - An spi_message is a sequence of of protocol operations, executed
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+ as one atomic sequence. SPI driver controls include:
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+
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+ + when bidirectional reads and writes start ... by how its
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+ sequence of spi_transfer requests is arranged;
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+
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+ + optionally defining short delays after transfers ... using
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+ the spi_transfer.delay_usecs setting;
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+
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+ + whether the chipselect becomes inactive after a transfer and
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+ any delay ... by using the spi_transfer.cs_change flag;
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+
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+ + hinting whether the next message is likely to go to this same
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+ device ... using the spi_transfer.cs_change flag on the last
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+ transfer in that atomic group, and potentially saving costs
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+ for chip deselect and select operations.
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+
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+ - Follow standard kernel rules, and provide DMA-safe buffers in
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+ your messages. That way controller drivers using DMA aren't forced
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+ to make extra copies unless the hardware requires it (e.g. working
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+ around hardware errata that force the use of bounce buffering).
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+
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+ If standard dma_map_single() handling of these buffers is inappropriate,
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+ you can use spi_message.is_dma_mapped to tell the controller driver
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+ that you've already provided the relevant DMA addresses.
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+
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+ - The basic I/O primitive is spi_async(). Async requests may be
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+ issued in any context (irq handler, task, etc) and completion
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+ is reported using a callback provided with the message.
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+ After any detected error, the chip is deselected and processing
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+ of that spi_message is aborted.
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+
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+ - There are also synchronous wrappers like spi_sync(), and wrappers
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+ like spi_read(), spi_write(), and spi_write_then_read(). These
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+ may be issued only in contexts that may sleep, and they're all
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+ clean (and small, and "optional") layers over spi_async().
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+
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+ - The spi_write_then_read() call, and convenience wrappers around
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+ it, should only be used with small amounts of data where the
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+ cost of an extra copy may be ignored. It's designed to support
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+ common RPC-style requests, such as writing an eight bit command
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+ and reading a sixteen bit response -- spi_w8r16() being one its
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+ wrappers, doing exactly that.
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+
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+Some drivers may need to modify spi_device characteristics like the
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+transfer mode, wordsize, or clock rate. This is done with spi_setup(),
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+which would normally be called from probe() before the first I/O is
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+done to the device.
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+
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+While "spi_device" would be the bottom boundary of the driver, the
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+upper boundaries might include sysfs (especially for sensor readings),
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+the input layer, ALSA, networking, MTD, the character device framework,
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+or other Linux subsystems.
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+
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+Note that there are two types of memory your driver must manage as part
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+of interacting with SPI devices.
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+
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+ - I/O buffers use the usual Linux rules, and must be DMA-safe.
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+ You'd normally allocate them from the heap or free page pool.
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+ Don't use the stack, or anything that's declared "static".
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+
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+ - The spi_message and spi_transfer metadata used to glue those
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+ I/O buffers into a group of protocol transactions. These can
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+ be allocated anywhere it's convenient, including as part of
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+ other allocate-once driver data structures. Zero-init these.
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+
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+If you like, spi_message_alloc() and spi_message_free() convenience
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+routines are available to allocate and zero-initialize an spi_message
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+with several transfers.
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+
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+
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+How do I write an "SPI Master Controller Driver"?
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+-------------------------------------------------
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+An SPI controller will probably be registered on the platform_bus; write
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+a driver to bind to the device, whichever bus is involved.
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+
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+The main task of this type of driver is to provide an "spi_master".
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+Use spi_alloc_master() to allocate the master, and class_get_devdata()
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+to get the driver-private data allocated for that device.
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+
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+ struct spi_master *master;
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+ struct CONTROLLER *c;
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+
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+ master = spi_alloc_master(dev, sizeof *c);
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+ if (!master)
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+ return -ENODEV;
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+
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+ c = class_get_devdata(&master->cdev);
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+
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+The driver will initialize the fields of that spi_master, including the
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+bus number (maybe the same as the platform device ID) and three methods
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+used to interact with the SPI core and SPI protocol drivers. It will
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+also initialize its own internal state.
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+
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+ master->setup(struct spi_device *spi)
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+ This sets up the device clock rate, SPI mode, and word sizes.
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+ Drivers may change the defaults provided by board_info, and then
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+ call spi_setup(spi) to invoke this routine. It may sleep.
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+
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+ master->transfer(struct spi_device *spi, struct spi_message *message)
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+ This must not sleep. Its responsibility is arrange that the
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+ transfer happens and its complete() callback is issued; the two
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+ will normally happen later, after other transfers complete.
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+
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+ master->cleanup(struct spi_device *spi)
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+ Your controller driver may use spi_device.controller_state to hold
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+ state it dynamically associates with that device. If you do that,
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+ be sure to provide the cleanup() method to free that state.
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+
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+The bulk of the driver will be managing the I/O queue fed by transfer().
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+
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+That queue could be purely conceptual. For example, a driver used only
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+for low-frequency sensor acess might be fine using synchronous PIO.
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+
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+But the queue will probably be very real, using message->queue, PIO,
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+often DMA (especially if the root filesystem is in SPI flash), and
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+execution contexts like IRQ handlers, tasklets, or workqueues (such
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|
+as keventd). Your driver can be as fancy, or as simple, as you need.
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+
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+
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+THANKS TO
|
|
|
+---------
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|
+Contributors to Linux-SPI discussions include (in alphabetical order,
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|
+by last name):
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|
+
|
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|
+David Brownell
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|
+Russell King
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|
+Dmitry Pervushin
|
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|
+Stephen Street
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+Mark Underwood
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+Andrew Victor
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+Vitaly Wool
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+
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