Overview of Linux kernel SPI support
  ====================================

  21-May-2007

  
  What is SPI?
  ------------

  The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
  link used to connect microcontrollers to sensors, memory, and peripherals.

  It's a simple "de facto" standard, not complicated enough to acquire a
  standardization body.  SPI uses a master/slave configuration.

  The three signal wires hold a clock (SCK, often on the order of 10 MHz),

  and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
  Slave Out" (MISO) signals.  (Other names are also used.)  There are four
  clocking modes through which data is exchanged; mode-0 and mode-3 are most

  commonly used.  Each clock cycle shifts data out and data in; the clock

  doesn't cycle except when there is a data bit to shift.  Not all data bits
  are used though; not every protocol uses those full duplex capabilities.

  SPI masters use a fourth "chip select" line to activate a given SPI slave

  device, so those three signal wires may be connected to several chips

  in parallel.  All SPI slaves support chipselects; they are usually active
  low signals, labeled nCSx for slave 'x' (e.g. nCS0).  Some devices have

  other signals, often including an interrupt to the master.

  Unlike serial busses like USB or SMBus, even low level protocols for

  SPI slave functions are usually not interoperable between vendors

  (except for commodities like SPI memory chips).

  
    - SPI may be used for request/response style device protocols, as with
      touchscreen sensors and memory chips.
  
    - It may also be used to stream data in either direction (half duplex),
      or both of them at the same time (full duplex).
  
    - Some devices may use eight bit words.  Others may different word
      lengths, such as streams of 12-bit or 20-bit digital samples.

    - Words are usually sent with their most significant bit (MSB) first,
      but sometimes the least significant bit (LSB) goes first instead.
  
    - Sometimes SPI is used to daisy-chain devices, like shift registers.

  In the same way, SPI slaves will only rarely support any kind of automatic
  discovery/enumeration protocol.  The tree of slave devices accessible from
  a given SPI master will normally be set up manually, with configuration
  tables.
  
  SPI is only one of the names used by such four-wire protocols, and
  most controllers have no problem handling "MicroWire" (think of it as
  half-duplex SPI, for request/response protocols), SSP ("Synchronous
  Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
  related protocols.

  Some chips eliminate a signal line by combining MOSI and MISO, and
  limiting themselves to half-duplex at the hardware level.  In fact
  some SPI chips have this signal mode as a strapping option.  These
  can be accessed using the same programming interface as SPI, but of
  course they won't handle full duplex transfers.  You may find such
  chips described as using "three wire" signaling: SCK, data, nCSx.
  (That data line is sometimes called MOMI or SISO.)

  Microcontrollers often support both master and slave sides of the SPI
  protocol.  This document (and Linux) currently only supports the master
  side of SPI interactions.
  
  
  Who uses it?  On what kinds of systems?
  ---------------------------------------
  Linux developers using SPI are probably writing device drivers for embedded
  systems boards.  SPI is used to control external chips, and it is also a
  protocol supported by every MMC or SD memory card.  (The older "DataFlash"
  cards, predating MMC cards but using the same connectors and card shape,
  support only SPI.)  Some PC hardware uses SPI flash for BIOS code.
  
  SPI slave chips range from digital/analog converters used for analog
  sensors and codecs, to memory, to peripherals like USB controllers
  or Ethernet adapters; and more.
  
  Most systems using SPI will integrate a few devices on a mainboard.
  Some provide SPI links on expansion connectors; in cases where no
  dedicated SPI controller exists, GPIO pins can be used to create a
  low speed "bitbanging" adapter.  Very few systems will "hotplug" an SPI
  controller; the reasons to use SPI focus on low cost and simple operation,
  and if dynamic reconfiguration is important, USB will often be a more
  appropriate low-pincount peripheral bus.
  
  Many microcontrollers that can run Linux integrate one or more I/O
  interfaces with SPI modes.  Given SPI support, they could use MMC or SD
  cards without needing a special purpose MMC/SD/SDIO controller.

  I'm confused.  What are these four SPI "clock modes"?
  -----------------------------------------------------
  It's easy to be confused here, and the vendor documentation you'll
  find isn't necessarily helpful.  The four modes combine two mode bits:
  
   - CPOL indicates the initial clock polarity.  CPOL=0 means the
     clock starts low, so the first (leading) edge is rising, and
     the second (trailing) edge is falling.  CPOL=1 means the clock
     starts high, so the first (leading) edge is falling.
  
   - CPHA indicates the clock phase used to sample data; CPHA=0 says
     sample on the leading edge, CPHA=1 means the trailing edge.
  
     Since the signal needs to stablize before it's sampled, CPHA=0
     implies that its data is written half a clock before the first
     clock edge.  The chipselect may have made it become available.
  
  Chip specs won't always say "uses SPI mode X" in as many words,
  but their timing diagrams will make the CPOL and CPHA modes clear.
  
  In the SPI mode number, CPOL is the high order bit and CPHA is the
  low order bit.  So when a chip's timing diagram shows the clock
  starting low (CPOL=0) and data stabilized for sampling during the
  trailing clock edge (CPHA=1), that's SPI mode 1.

  Note that the clock mode is relevant as soon as the chipselect goes
  active.  So the master must set the clock to inactive before selecting
  a slave, and the slave can tell the chosen polarity by sampling the
  clock level when its select line goes active.  That's why many devices
  support for example both modes 0 and 3:  they don't care about polarity,
  and alway clock data in/out on rising clock edges.

  How do these driver programming interfaces work?
  ------------------------------------------------
  The <linux/spi/spi.h> header file includes kerneldoc, as does the

  main source code, and you should certainly read that chapter of the
  kernel API document.  This is just an overview, so you get the big
  picture before those details.

  SPI requests always go into I/O queues.  Requests for a given SPI device
  are always executed in FIFO order, and complete asynchronously through
  completion callbacks.  There are also some simple synchronous wrappers
  for those calls, including ones for common transaction types like writing
  a command and then reading its response.

  There are two types of SPI driver, here called:

    Controller drivers ... controllers may be built in to System-On-Chip

  	processors, and often support both Master and Slave roles.
  	These drivers touch hardware registers and may use DMA.

  	Or they can be PIO bitbangers, needing just GPIO pins.

  
    Protocol drivers ... these pass messages through the controller
  	driver to communicate with a Slave or Master device on the
  	other side of an SPI link.
  
  So for example one protocol driver might talk to the MTD layer to export
  data to filesystems stored on SPI flash like DataFlash; and others might
  control audio interfaces, present touchscreen sensors as input interfaces,
  or monitor temperature and voltage levels during industrial processing.
  And those might all be sharing the same controller driver.
  
  A "struct spi_device" encapsulates the master-side interface between
  those two types of driver.  At this writing, Linux has no slave side
  programming interface.
  
  There is a minimal core of SPI programming interfaces, focussing on

  using the driver model to connect controller and protocol drivers using

  device tables provided by board specific initialization code.  SPI
  shows up in sysfs in several locations:

     /sys/devices/.../CTLR ... physical node for a given SPI controller

     /sys/devices/.../CTLR/spiB.C ... spi_device on bus "B",

  	chipselect C, accessed through CTLR.

     /sys/bus/spi/devices/spiB.C ... symlink to that physical
     	.../CTLR/spiB.C device

     /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver
  	that should be used with this device (for hotplug/coldplug)

     /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices

     /sys/class/spi_master/spiB ... symlink (or actual device node) to
  	a logical node which could hold class related state for the
  	controller managing bus "B".  All spiB.* devices share one

  	physical SPI bus segment, with SCLK, MOSI, and MISO.

  Note that the actual location of the controller's class state depends
  on whether you enabled CONFIG_SYSFS_DEPRECATED or not.  At this time,
  the only class-specific state is the bus number ("B" in "spiB"), so
  those /sys/class entries are only useful to quickly identify busses.

  
  How does board-specific init code declare SPI devices?
  ------------------------------------------------------
  Linux needs several kinds of information to properly configure SPI devices.
  That information is normally provided by board-specific code, even for
  chips that do support some of automated discovery/enumeration.
  
  DECLARE CONTROLLERS
  
  The first kind of information is a list of what SPI controllers exist.
  For System-on-Chip (SOC) based boards, these will usually be platform
  devices, and the controller may need some platform_data in order to
  operate properly.  The "struct platform_device" will include resources
  like the physical address of the controller's first register and its IRQ.
  
  Platforms will often abstract the "register SPI controller" operation,
  maybe coupling it with code to initialize pin configurations, so that
  the arch/.../mach-*/board-*.c files for several boards can all share the
  same basic controller setup code.  This is because most SOCs have several
  SPI-capable controllers, and only the ones actually usable on a given
  board should normally be set up and registered.
  
  So for example arch/.../mach-*/board-*.c files might have code like:

  	#include <mach/spi.h>	/* for mysoc_spi_data */

  
  	/* if your mach-* infrastructure doesn't support kernels that can
  	 * run on multiple boards, pdata wouldn't benefit from "__init".
  	 */

  	static struct mysoc_spi_data __initdata pdata = { ... };

  
  	static __init board_init(void)
  	{
  		...
  		/* this board only uses SPI controller #2 */
  		mysoc_register_spi(2, &pdata);
  		...
  	}
  
  And SOC-specific utility code might look something like:

  	#include <mach/spi.h>

  
  	static struct platform_device spi2 = { ... };
  
  	void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
  	{
  		struct mysoc_spi_data *pdata2;
  
  		pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
  		*pdata2 = pdata;
  		...
  		if (n == 2) {
  			spi2->dev.platform_data = pdata2;
  			register_platform_device(&spi2);
  
  			/* also: set up pin modes so the spi2 signals are
  			 * visible on the relevant pins ... bootloaders on
  			 * production boards may already have done this, but
  			 * developer boards will often need Linux to do it.
  			 */
  		}
  		...
  	}
  
  Notice how the platform_data for boards may be different, even if the
  same SOC controller is used.  For example, on one board SPI might use
  an external clock, where another derives the SPI clock from current
  settings of some master clock.
  
  
  DECLARE SLAVE DEVICES
  
  The second kind of information is a list of what SPI slave devices exist
  on the target board, often with some board-specific data needed for the
  driver to work correctly.
  
  Normally your arch/.../mach-*/board-*.c files would provide a small table
  listing the SPI devices on each board.  (This would typically be only a
  small handful.)  That might look like:
  
  	static struct ads7846_platform_data ads_info = {
  		.vref_delay_usecs	= 100,
  		.x_plate_ohms		= 580,
  		.y_plate_ohms		= 410,
  	};
  
  	static struct spi_board_info spi_board_info[] __initdata = {
  	{
  		.modalias	= "ads7846",
  		.platform_data	= &ads_info,
  		.mode		= SPI_MODE_0,
  		.irq		= GPIO_IRQ(31),
  		.max_speed_hz	= 120000 /* max sample rate at 3V */ * 16,
  		.bus_num	= 1,
  		.chip_select	= 0,
  	},
  	};
  
  Again, notice how board-specific information is provided; each chip may need
  several types.  This example shows generic constraints like the fastest SPI
  clock to allow (a function of board voltage in this case) or how an IRQ pin
  is wired, plus chip-specific constraints like an important delay that's
  changed by the capacitance at one pin.
  
  (There's also "controller_data", information that may be useful to the
  controller driver.  An example would be peripheral-specific DMA tuning
  data or chipselect callbacks.  This is stored in spi_device later.)
  
  The board_info should provide enough information to let the system work
  without the chip's driver being loaded.  The most troublesome aspect of
  that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
  sharing a bus with a device that interprets chipselect "backwards" is

  not possible until the infrastructure knows how to deselect it.

  
  Then your board initialization code would register that table with the SPI
  infrastructure, so that it's available later when the SPI master controller
  driver is registered:
  
  	spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
  
  Like with other static board-specific setup, you won't unregister those.

  The widely used "card" style computers bundle memory, cpu, and little else
  onto a card that's maybe just thirty square centimeters.  On such systems,
  your arch/.../mach-.../board-*.c file would primarily provide information
  about the devices on the mainboard into which such a card is plugged.  That
  certainly includes SPI devices hooked up through the card connectors!

  
  NON-STATIC CONFIGURATIONS
  
  Developer boards often play by different rules than product boards, and one
  example is the potential need to hotplug SPI devices and/or controllers.

  For those cases you might need to use spi_busnum_to_master() to look

  up the spi bus master, and will likely need spi_new_device() to provide the
  board info based on the board that was hotplugged.  Of course, you'd later
  call at least spi_unregister_device() when that board is removed.

  When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those

  configurations will also be dynamic.  Fortunately, such devices all support
  basic device identification probes, so they should hotplug normally.

  
  How do I write an "SPI Protocol Driver"?
  ----------------------------------------

  Most SPI drivers are currently kernel drivers, but there's also support
  for userspace drivers.  Here we talk only about kernel drivers.

  SPI protocol drivers somewhat resemble platform device drivers:
  
  	static struct spi_driver CHIP_driver = {
  		.driver = {
  			.name		= "CHIP",

  			.owner		= THIS_MODULE,
  		},

  		.probe		= CHIP_probe,

  		.remove		= __devexit_p(CHIP_remove),

  		.suspend	= CHIP_suspend,
  		.resume		= CHIP_resume,
  	};

  The driver core will automatically attempt to bind this driver to any SPI

  device whose board_info gave a modalias of "CHIP".  Your probe() code

  might look like this unless you're creating a device which is managing
  a bus (appearing under /sys/class/spi_master).

  	static int __devinit CHIP_probe(struct spi_device *spi)

  	{

  		struct CHIP			*chip;

  		struct CHIP_platform_data	*pdata;
  
  		/* assuming the driver requires board-specific data: */
  		pdata = &spi->dev.platform_data;
  		if (!pdata)
  			return -ENODEV;

  
  		/* get memory for driver's per-chip state */
  		chip = kzalloc(sizeof *chip, GFP_KERNEL);
  		if (!chip)
  			return -ENOMEM;

  		spi_set_drvdata(spi, chip);

  
  		... etc
  		return 0;
  	}
  
  As soon as it enters probe(), the driver may issue I/O requests to
  the SPI device using "struct spi_message".  When remove() returns,

  or after probe() fails, the driver guarantees that it won't submit
  any more such messages.

    - An spi_message is a sequence of protocol operations, executed

      as one atomic sequence.  SPI driver controls include:
  
        + when bidirectional reads and writes start ... by how its
          sequence of spi_transfer requests is arranged;

        + which I/O buffers are used ... each spi_transfer wraps a
          buffer for each transfer direction, supporting full duplex
          (two pointers, maybe the same one in both cases) and half
          duplex (one pointer is NULL) transfers;

        + optionally defining short delays after transfers ... using

          the spi_transfer.delay_usecs setting (this delay can be the
          only protocol effect, if the buffer length is zero);

  
        + whether the chipselect becomes inactive after a transfer and
          any delay ... by using the spi_transfer.cs_change flag;
  
        + hinting whether the next message is likely to go to this same
          device ... using the spi_transfer.cs_change flag on the last
  	transfer in that atomic group, and potentially saving costs
  	for chip deselect and select operations.
  
    - Follow standard kernel rules, and provide DMA-safe buffers in
      your messages.  That way controller drivers using DMA aren't forced
      to make extra copies unless the hardware requires it (e.g. working
      around hardware errata that force the use of bounce buffering).
  
      If standard dma_map_single() handling of these buffers is inappropriate,
      you can use spi_message.is_dma_mapped to tell the controller driver
      that you've already provided the relevant DMA addresses.
  
    - The basic I/O primitive is spi_async().  Async requests may be
      issued in any context (irq handler, task, etc) and completion
      is reported using a callback provided with the message.

      After any detected error, the chip is deselected and processing
      of that spi_message is aborted.

  
    - There are also synchronous wrappers like spi_sync(), and wrappers
      like spi_read(), spi_write(), and spi_write_then_read().  These
      may be issued only in contexts that may sleep, and they're all
      clean (and small, and "optional") layers over spi_async().
  
    - The spi_write_then_read() call, and convenience wrappers around
      it, should only be used with small amounts of data where the
      cost of an extra copy may be ignored.  It's designed to support
      common RPC-style requests, such as writing an eight bit command
      and reading a sixteen bit response -- spi_w8r16() being one its
      wrappers, doing exactly that.
  
  Some drivers may need to modify spi_device characteristics like the
  transfer mode, wordsize, or clock rate.  This is done with spi_setup(),
  which would normally be called from probe() before the first I/O is

  done to the device.  However, that can also be called at any time
  that no message is pending for that device.

  
  While "spi_device" would be the bottom boundary of the driver, the
  upper boundaries might include sysfs (especially for sensor readings),
  the input layer, ALSA, networking, MTD, the character device framework,
  or other Linux subsystems.

  Note that there are two types of memory your driver must manage as part
  of interacting with SPI devices.
  
    - I/O buffers use the usual Linux rules, and must be DMA-safe.
      You'd normally allocate them from the heap or free page pool.
      Don't use the stack, or anything that's declared "static".
  
    - The spi_message and spi_transfer metadata used to glue those
      I/O buffers into a group of protocol transactions.  These can
      be allocated anywhere it's convenient, including as part of
      other allocate-once driver data structures.  Zero-init these.
  
  If you like, spi_message_alloc() and spi_message_free() convenience
  routines are available to allocate and zero-initialize an spi_message
  with several transfers.

  
  How do I write an "SPI Master Controller Driver"?
  -------------------------------------------------
  An SPI controller will probably be registered on the platform_bus; write
  a driver to bind to the device, whichever bus is involved.
  
  The main task of this type of driver is to provide an "spi_master".

  Use spi_alloc_master() to allocate the master, and spi_master_get_devdata()

  to get the driver-private data allocated for that device.
  
  	struct spi_master	*master;
  	struct CONTROLLER	*c;
  
  	master = spi_alloc_master(dev, sizeof *c);
  	if (!master)
  		return -ENODEV;

  	c = spi_master_get_devdata(master);

  
  The driver will initialize the fields of that spi_master, including the
  bus number (maybe the same as the platform device ID) and three methods
  used to interact with the SPI core and SPI protocol drivers.  It will

  also initialize its own internal state.  (See below about bus numbering
  and those methods.)
  
  After you initialize the spi_master, then use spi_register_master() to
  publish it to the rest of the system.  At that time, device nodes for
  the controller and any predeclared spi devices will be made available,
  and the driver model core will take care of binding them to drivers.
  
  If you need to remove your SPI controller driver, spi_unregister_master()
  will reverse the effect of spi_register_master().
  
  
  BUS NUMBERING
  
  Bus numbering is important, since that's how Linux identifies a given
  SPI bus (shared SCK, MOSI, MISO).  Valid bus numbers start at zero.  On
  SOC systems, the bus numbers should match the numbers defined by the chip
  manufacturer.  For example, hardware controller SPI2 would be bus number 2,
  and spi_board_info for devices connected to it would use that number.
  
  If you don't have such hardware-assigned bus number, and for some reason
  you can't just assign them, then provide a negative bus number.  That will
  then be replaced by a dynamically assigned number. You'd then need to treat
  this as a non-static configuration (see above).
  
  
  SPI MASTER METHODS

  
      master->setup(struct spi_device *spi)
  	This sets up the device clock rate, SPI mode, and word sizes.
  	Drivers may change the defaults provided by board_info, and then
  	call spi_setup(spi) to invoke this routine.  It may sleep.

  	Unless each SPI slave has its own configuration registers, don't
  	change them right away ... otherwise drivers could corrupt I/O
  	that's in progress for other SPI devices.

  		** BUG ALERT:  for some reason the first version of
  		** many spi_master drivers seems to get this wrong.
  		** When you code setup(), ASSUME that the controller
  		** is actively processing transfers for another device.

      master->transfer(struct spi_device *spi, struct spi_message *message)
      	This must not sleep.  Its responsibility is arrange that the

  	transfer happens and its complete() callback is issued.  The two
  	will normally happen later, after other transfers complete, and
  	if the controller is idle it will need to be kickstarted.

  
      master->cleanup(struct spi_device *spi)
  	Your controller driver may use spi_device.controller_state to hold
  	state it dynamically associates with that device.  If you do that,
  	be sure to provide the cleanup() method to free that state.

  
  SPI MESSAGE QUEUE

  The bulk of the driver will be managing the I/O queue fed by transfer().
  
  That queue could be purely conceptual.  For example, a driver used only

  for low-frequency sensor access might be fine using synchronous PIO.

  
  But the queue will probably be very real, using message->queue, PIO,
  often DMA (especially if the root filesystem is in SPI flash), and
  execution contexts like IRQ handlers, tasklets, or workqueues (such
  as keventd).  Your driver can be as fancy, or as simple, as you need.

  Such a transfer() method would normally just add the message to a
  queue, and then start some asynchronous transfer engine (unless it's
  already running).

  
  
  THANKS TO
  ---------
  Contributors to Linux-SPI discussions include (in alphabetical order,
  by last name):
  
  David Brownell
  Russell King
  Dmitry Pervushin
  Stephen Street
  Mark Underwood
  Andrew Victor
  Vitaly Wool