1 Overview of Linux kernel SPI support
2 ====================================
8 The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
9 link used to connect microcontrollers to sensors, memory, and peripherals.
11 The three signal wires hold a clock (SCLK, often on the order of 10 MHz),
12 and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
13 Slave Out" (MISO) signals. (Other names are also used.) There are four
14 clocking modes through which data is exchanged; mode-0 and mode-3 are most
15 commonly used. Each clock cycle shifts data out and data in; the clock
16 doesn't cycle except when there is data to shift.
18 SPI masters may use a "chip select" line to activate a given SPI slave
19 device, so those three signal wires may be connected to several chips
20 in parallel. All SPI slaves support chipselects. Some devices have
21 other signals, often including an interrupt to the master.
23 Unlike serial busses like USB or SMBUS, even low level protocols for
24 SPI slave functions are usually not interoperable between vendors
25 (except for cases like SPI memory chips).
27 - SPI may be used for request/response style device protocols, as with
28 touchscreen sensors and memory chips.
30 - It may also be used to stream data in either direction (half duplex),
31 or both of them at the same time (full duplex).
33 - Some devices may use eight bit words. Others may different word
34 lengths, such as streams of 12-bit or 20-bit digital samples.
36 In the same way, SPI slaves will only rarely support any kind of automatic
37 discovery/enumeration protocol. The tree of slave devices accessible from
38 a given SPI master will normally be set up manually, with configuration
41 SPI is only one of the names used by such four-wire protocols, and
42 most controllers have no problem handling "MicroWire" (think of it as
43 half-duplex SPI, for request/response protocols), SSP ("Synchronous
44 Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
47 Microcontrollers often support both master and slave sides of the SPI
48 protocol. This document (and Linux) currently only supports the master
49 side of SPI interactions.
52 Who uses it? On what kinds of systems?
53 ---------------------------------------
54 Linux developers using SPI are probably writing device drivers for embedded
55 systems boards. SPI is used to control external chips, and it is also a
56 protocol supported by every MMC or SD memory card. (The older "DataFlash"
57 cards, predating MMC cards but using the same connectors and card shape,
58 support only SPI.) Some PC hardware uses SPI flash for BIOS code.
60 SPI slave chips range from digital/analog converters used for analog
61 sensors and codecs, to memory, to peripherals like USB controllers
62 or Ethernet adapters; and more.
64 Most systems using SPI will integrate a few devices on a mainboard.
65 Some provide SPI links on expansion connectors; in cases where no
66 dedicated SPI controller exists, GPIO pins can be used to create a
67 low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI
68 controller; the reasons to use SPI focus on low cost and simple operation,
69 and if dynamic reconfiguration is important, USB will often be a more
70 appropriate low-pincount peripheral bus.
72 Many microcontrollers that can run Linux integrate one or more I/O
73 interfaces with SPI modes. Given SPI support, they could use MMC or SD
74 cards without needing a special purpose MMC/SD/SDIO controller.
77 How do these driver programming interfaces work?
78 ------------------------------------------------
79 The <linux/spi/spi.h> header file includes kerneldoc, as does the
80 main source code, and you should certainly read that. This is just
81 an overview, so you get the big picture before the details.
83 SPI requests always go into I/O queues. Requests for a given SPI device
84 are always executed in FIFO order, and complete asynchronously through
85 completion callbacks. There are also some simple synchronous wrappers
86 for those calls, including ones for common transaction types like writing
87 a command and then reading its response.
89 There are two types of SPI driver, here called:
91 Controller drivers ... these are often built in to System-On-Chip
92 processors, and often support both Master and Slave roles.
93 These drivers touch hardware registers and may use DMA.
94 Or they can be PIO bitbangers, needing just GPIO pins.
96 Protocol drivers ... these pass messages through the controller
97 driver to communicate with a Slave or Master device on the
98 other side of an SPI link.
100 So for example one protocol driver might talk to the MTD layer to export
101 data to filesystems stored on SPI flash like DataFlash; and others might
102 control audio interfaces, present touchscreen sensors as input interfaces,
103 or monitor temperature and voltage levels during industrial processing.
104 And those might all be sharing the same controller driver.
106 A "struct spi_device" encapsulates the master-side interface between
107 those two types of driver. At this writing, Linux has no slave side
108 programming interface.
110 There is a minimal core of SPI programming interfaces, focussing on
111 using driver model to connect controller and protocol drivers using
112 device tables provided by board specific initialization code. SPI
113 shows up in sysfs in several locations:
115 /sys/devices/.../CTLR/spiB.C ... spi_device for on bus "B",
116 chipselect C, accessed through CTLR.
118 /sys/bus/spi/devices/spiB.C ... symlink to the physical
121 /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
123 /sys/class/spi_master/spiB ... class device for the controller
124 managing bus "B". All the spiB.* devices share the same
125 physical SPI bus segment, with SCLK, MOSI, and MISO.
128 How does board-specific init code declare SPI devices?
129 ------------------------------------------------------
130 Linux needs several kinds of information to properly configure SPI devices.
131 That information is normally provided by board-specific code, even for
132 chips that do support some of automated discovery/enumeration.
136 The first kind of information is a list of what SPI controllers exist.
137 For System-on-Chip (SOC) based boards, these will usually be platform
138 devices, and the controller may need some platform_data in order to
139 operate properly. The "struct platform_device" will include resources
140 like the physical address of the controller's first register and its IRQ.
142 Platforms will often abstract the "register SPI controller" operation,
143 maybe coupling it with code to initialize pin configurations, so that
144 the arch/.../mach-*/board-*.c files for several boards can all share the
145 same basic controller setup code. This is because most SOCs have several
146 SPI-capable controllers, and only the ones actually usable on a given
147 board should normally be set up and registered.
149 So for example arch/.../mach-*/board-*.c files might have code like:
151 #include <asm/arch/spi.h> /* for mysoc_spi_data */
153 /* if your mach-* infrastructure doesn't support kernels that can
154 * run on multiple boards, pdata wouldn't benefit from "__init".
156 static struct mysoc_spi_data __init pdata = { ... };
158 static __init board_init(void)
161 /* this board only uses SPI controller #2 */
162 mysoc_register_spi(2, &pdata);
166 And SOC-specific utility code might look something like:
168 #include <asm/arch/spi.h>
170 static struct platform_device spi2 = { ... };
172 void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
174 struct mysoc_spi_data *pdata2;
176 pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
180 spi2->dev.platform_data = pdata2;
181 register_platform_device(&spi2);
183 /* also: set up pin modes so the spi2 signals are
184 * visible on the relevant pins ... bootloaders on
185 * production boards may already have done this, but
186 * developer boards will often need Linux to do it.
192 Notice how the platform_data for boards may be different, even if the
193 same SOC controller is used. For example, on one board SPI might use
194 an external clock, where another derives the SPI clock from current
195 settings of some master clock.
198 DECLARE SLAVE DEVICES
200 The second kind of information is a list of what SPI slave devices exist
201 on the target board, often with some board-specific data needed for the
202 driver to work correctly.
204 Normally your arch/.../mach-*/board-*.c files would provide a small table
205 listing the SPI devices on each board. (This would typically be only a
206 small handful.) That might look like:
208 static struct ads7846_platform_data ads_info = {
209 .vref_delay_usecs = 100,
214 static struct spi_board_info spi_board_info[] __initdata = {
216 .modalias = "ads7846",
217 .platform_data = &ads_info,
220 .max_speed_hz = 120000 /* max sample rate at 3V */ * 16,
226 Again, notice how board-specific information is provided; each chip may need
227 several types. This example shows generic constraints like the fastest SPI
228 clock to allow (a function of board voltage in this case) or how an IRQ pin
229 is wired, plus chip-specific constraints like an important delay that's
230 changed by the capacitance at one pin.
232 (There's also "controller_data", information that may be useful to the
233 controller driver. An example would be peripheral-specific DMA tuning
234 data or chipselect callbacks. This is stored in spi_device later.)
236 The board_info should provide enough information to let the system work
237 without the chip's driver being loaded. The most troublesome aspect of
238 that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
239 sharing a bus with a device that interprets chipselect "backwards" is
242 Then your board initialization code would register that table with the SPI
243 infrastructure, so that it's available later when the SPI master controller
244 driver is registered:
246 spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
248 Like with other static board-specific setup, you won't unregister those.
251 NON-STATIC CONFIGURATIONS
253 Developer boards often play by different rules than product boards, and one
254 example is the potential need to hotplug SPI devices and/or controllers.
256 For those cases you might need to use use spi_busnum_to_master() to look
257 up the spi bus master, and will likely need spi_new_device() to provide the
258 board info based on the board that was hotplugged. Of course, you'd later
259 call at least spi_unregister_device() when that board is removed.
262 How do I write an "SPI Protocol Driver"?
263 ----------------------------------------
264 All SPI drivers are currently kernel drivers. A userspace driver API
265 would just be another kernel driver, probably offering some lowlevel
266 access through aio_read(), aio_write(), and ioctl() calls and using the
267 standard userspace sysfs mechanisms to bind to a given SPI device.
269 SPI protocol drivers somewhat resemble platform device drivers:
271 static struct spi_driver CHIP_driver = {
274 .bus = &spi_bus_type,
275 .owner = THIS_MODULE,
279 .remove = __devexit_p(CHIP_remove),
280 .suspend = CHIP_suspend,
281 .resume = CHIP_resume,
284 The driver core will autmatically attempt to bind this driver to any SPI
285 device whose board_info gave a modalias of "CHIP". Your probe() code
286 might look like this unless you're creating a class_device:
288 static int __devinit CHIP_probe(struct spi_device *spi)
291 struct CHIP_platform_data *pdata;
293 /* assuming the driver requires board-specific data: */
294 pdata = &spi->dev.platform_data;
298 /* get memory for driver's per-chip state */
299 chip = kzalloc(sizeof *chip, GFP_KERNEL);
302 dev_set_drvdata(&spi->dev, chip);
308 As soon as it enters probe(), the driver may issue I/O requests to
309 the SPI device using "struct spi_message". When remove() returns,
310 the driver guarantees that it won't submit any more such messages.
312 - An spi_message is a sequence of of protocol operations, executed
313 as one atomic sequence. SPI driver controls include:
315 + when bidirectional reads and writes start ... by how its
316 sequence of spi_transfer requests is arranged;
318 + optionally defining short delays after transfers ... using
319 the spi_transfer.delay_usecs setting;
321 + whether the chipselect becomes inactive after a transfer and
322 any delay ... by using the spi_transfer.cs_change flag;
324 + hinting whether the next message is likely to go to this same
325 device ... using the spi_transfer.cs_change flag on the last
326 transfer in that atomic group, and potentially saving costs
327 for chip deselect and select operations.
329 - Follow standard kernel rules, and provide DMA-safe buffers in
330 your messages. That way controller drivers using DMA aren't forced
331 to make extra copies unless the hardware requires it (e.g. working
332 around hardware errata that force the use of bounce buffering).
334 If standard dma_map_single() handling of these buffers is inappropriate,
335 you can use spi_message.is_dma_mapped to tell the controller driver
336 that you've already provided the relevant DMA addresses.
338 - The basic I/O primitive is spi_async(). Async requests may be
339 issued in any context (irq handler, task, etc) and completion
340 is reported using a callback provided with the message.
341 After any detected error, the chip is deselected and processing
342 of that spi_message is aborted.
344 - There are also synchronous wrappers like spi_sync(), and wrappers
345 like spi_read(), spi_write(), and spi_write_then_read(). These
346 may be issued only in contexts that may sleep, and they're all
347 clean (and small, and "optional") layers over spi_async().
349 - The spi_write_then_read() call, and convenience wrappers around
350 it, should only be used with small amounts of data where the
351 cost of an extra copy may be ignored. It's designed to support
352 common RPC-style requests, such as writing an eight bit command
353 and reading a sixteen bit response -- spi_w8r16() being one its
354 wrappers, doing exactly that.
356 Some drivers may need to modify spi_device characteristics like the
357 transfer mode, wordsize, or clock rate. This is done with spi_setup(),
358 which would normally be called from probe() before the first I/O is
361 While "spi_device" would be the bottom boundary of the driver, the
362 upper boundaries might include sysfs (especially for sensor readings),
363 the input layer, ALSA, networking, MTD, the character device framework,
364 or other Linux subsystems.
367 How do I write an "SPI Master Controller Driver"?
368 -------------------------------------------------
369 An SPI controller will probably be registered on the platform_bus; write
370 a driver to bind to the device, whichever bus is involved.
372 The main task of this type of driver is to provide an "spi_master".
373 Use spi_alloc_master() to allocate the master, and class_get_devdata()
374 to get the driver-private data allocated for that device.
376 struct spi_master *master;
377 struct CONTROLLER *c;
379 master = spi_alloc_master(dev, sizeof *c);
383 c = class_get_devdata(&master->cdev);
385 The driver will initialize the fields of that spi_master, including the
386 bus number (maybe the same as the platform device ID) and three methods
387 used to interact with the SPI core and SPI protocol drivers. It will
388 also initialize its own internal state.
390 master->setup(struct spi_device *spi)
391 This sets up the device clock rate, SPI mode, and word sizes.
392 Drivers may change the defaults provided by board_info, and then
393 call spi_setup(spi) to invoke this routine. It may sleep.
395 master->transfer(struct spi_device *spi, struct spi_message *message)
396 This must not sleep. Its responsibility is arrange that the
397 transfer happens and its complete() callback is issued; the two
398 will normally happen later, after other transfers complete.
400 master->cleanup(struct spi_device *spi)
401 Your controller driver may use spi_device.controller_state to hold
402 state it dynamically associates with that device. If you do that,
403 be sure to provide the cleanup() method to free that state.
405 The bulk of the driver will be managing the I/O queue fed by transfer().
407 That queue could be purely conceptual. For example, a driver used only
408 for low-frequency sensor acess might be fine using synchronous PIO.
410 But the queue will probably be very real, using message->queue, PIO,
411 often DMA (especially if the root filesystem is in SPI flash), and
412 execution contexts like IRQ handlers, tasklets, or workqueues (such
413 as keventd). Your driver can be as fancy, or as simple, as you need.
418 Contributors to Linux-SPI discussions include (in alphabetical order,