spi-summary

linux 内核源代码

		.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 autmatically 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;

      + optionally defining short delays after transfers ... using
        the spi_transfer.delay_usecs setting;

      + 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.

    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 acess 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