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-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 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->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
- transfer. This may sleep.
-
- DEPRECATED METHODS
-
- 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. 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):
-
-David Brownell
-Russell King
-Dmitry Pervushin
-Stephen Street
-Mark Underwood
-Andrew Victor
-Vitaly Wool
-Grant Likely
-Mark Brown
-Linus Walleij