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

02-Feb-2012

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 use 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) supports both the master and slave
sides 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 always 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 into 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 controller-side interface between
those two types of drivers.

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 SPI
        master controller managing bus "B".  All spiB.* devices share one
        physical SPI bus segment, with SCLK, MOSI, and MISO.

   /sys/devices/.../CTLR/slave ... virtual file for (un)registering the
        slave device for an SPI slave controller.
        Writing the driver name of an SPI slave handler to this file
        registers the slave device; writing "(null)" unregisters the slave
        device.
        Reading from this file shows the name of the slave device ("(null)"
        if not registered).

   /sys/class/spi_slave/spiB ... symlink (or actual device node) to
        a logical node which could hold class related state for the SPI
        slave controller on bus "B".  When registered, a single spiB.*
        device is present here, possible sharing the physical SPI bus
        segment with other SPI slave devices.

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 pdata __initdata = { ... };

        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,
                        .pm             = &CHIP_pm_ops,
                },

                .probe          = CHIP_probe,
                .remove         = CHIP_remove,
        };

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 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->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.

    master->prepare_transfer_hardware(struct spi_master *master)
        This will be called by the queue mechanism to signal to the driver
        that a message is coming in soon, so the subsystem requests the
        driver to prepare the transfer hardware by issuing this call.
        This may sleep.

    master->unprepare_transfer_hardware(struct spi_master *master)
        This will be called by the queue mechanism to signal to the driver
        that there are no more messages pending in the queue and it may
        relax the hardware (e.g. by power management calls). This may sleep.

    master->transfer_one_message(struct spi_master *master,
                                 struct spi_message *mesg)
        The subsystem calls the driver to transfer a single message while
        queuing transfers that arrive in the meantime. When the driver is
        finished with this message, it must call
        spi_finalize_current_message() so the subsystem can issue the next
        message. This may sleep.

    master->transfer_one(struct spi_master *master, struct spi_device *spi,
                         struct spi_transfer *transfer)
        The subsystem calls the driver to transfer a single transfer while
        queuing transfers that arrive in the meantime. When the driver is
        finished with this transfer, it must call
        spi_finalize_current_transfer() so the subsystem can issue the next
        transfer. This may sleep. Note: transfer_one and transfer_one_message
        are mutually exclusive; when both are set, the generic subsystem does
        not call your transfer_one callback.

        Return values:
        negative errno: error
        0: transfer is finished
        1: transfer is still in progress

    DEPRECATED METHODS

    master->transfer(struct spi_device *spi, struct spi_message *message)
        This must not sleep. Its responsibility is to 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. This
        method is not used on queued controllers and must be NULL if
        transfer_one_message() and (un)prepare_transfer_hardware() are
        implemented.


SPI MESSAGE QUEUE

If you are happy with the standard queueing mechanism provided by the
SPI subsystem, just implement the queued methods specified above. Using
the message queue has the upside of centralizing a lot of code and
providing pure process-context execution of methods. The message queue
can also be elevated to realtime priority on high-priority SPI traffic.

Unless the queueing mechanism in the SPI subsystem is selected, the bulk
of the driver will be managing the I/O queue fed by the now deprecated
function 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):

Mark Brown
David Brownell
Russell King
Grant Likely
Dmitry Pervushin
Stephen Street
Mark Underwood
Andrew Victor
Linus Walleij
Vitaly Wool