diff options
Diffstat (limited to 'kernel/Documentation/spi/spi-summary')
| -rw-r--r-- | kernel/Documentation/spi/spi-summary | 612 |
1 files changed, 612 insertions, 0 deletions
diff --git a/kernel/Documentation/spi/spi-summary b/kernel/Documentation/spi/spi-summary new file mode 100644 index 000000000..d1824b399 --- /dev/null +++ b/kernel/Documentation/spi/spi-summary @@ -0,0 +1,612 @@ +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) 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 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 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 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 |