1 Overview of Linux kernel SPI support
2 ====================================
8 The "Serial Peripheral Interface" (SPI) is a four-wire point-to-point
9 serial link used to connect microcontrollers to sensors and memory.
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
17 SPI masters may use a "chip select" line to activate a given SPI slave
18 device, so those three signal wires may be connected to several chips
19 in parallel. All SPI slaves support chipselects. Some devices have
20 other signals, often including an interrupt to the master.
22 Unlike serial busses like USB or SMBUS, even low level protocols for
23 SPI slave functions are usually not interoperable between vendors
24 (except for cases like SPI memory chips).
26 - SPI may be used for request/response style device protocols, as with
27 touchscreen sensors and memory chips.
29 - It may also be used to stream data in either direction (half duplex),
30 or both of them at the same time (full duplex).
32 - Some devices may use eight bit words. Others may different word
33 lengths, such as streams of 12-bit or 20-bit digital samples.
35 In the same way, SPI slaves will only rarely support any kind of automatic
36 discovery/enumeration protocol. The tree of slave devices accessible from
37 a given SPI master will normally be set up manually, with configuration
40 SPI is only one of the names used by such four-wire protocols, and
41 most controllers have no problem handling "MicroWire" (think of it as
42 half-duplex SPI, for request/response protocols), SSP ("Synchronous
43 Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
46 Microcontrollers often support both master and slave sides of the SPI
47 protocol. This document (and Linux) currently only supports the master
48 side of SPI interactions.
51 Who uses it? On what kinds of systems?
52 ---------------------------------------
53 Linux developers using SPI are probably writing device drivers for embedded
54 systems boards. SPI is used to control external chips, and it is also a
55 protocol supported by every MMC or SD memory card. (The older "DataFlash"
56 cards, predating MMC cards but using the same connectors and card shape,
57 support only SPI.) Some PC hardware uses SPI flash for BIOS code.
59 SPI slave chips range from digital/analog converters used for analog
60 sensors and codecs, to memory, to peripherals like USB controllers
61 or Ethernet adapters; and more.
63 Most systems using SPI will integrate a few devices on a mainboard.
64 Some provide SPI links on expansion connectors; in cases where no
65 dedicated SPI controller exists, GPIO pins can be used to create a
66 low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI
67 controller; the reasons to use SPI focus on low cost and simple operation,
68 and if dynamic reconfiguration is important, USB will often be a more
69 appropriate low-pincount peripheral bus.
71 Many microcontrollers that can run Linux integrate one or more I/O
72 interfaces with SPI modes. Given SPI support, they could use MMC or SD
73 cards without needing a special purpose MMC/SD/SDIO controller.
76 How do these driver programming interfaces work?
77 ------------------------------------------------
78 The <linux/spi/spi.h> header file includes kerneldoc, as does the
79 main source code, and you should certainly read that. This is just
80 an overview, so you get the big picture before the details.
82 There are two types of SPI driver, here called:
84 Controller drivers ... these are often built in to System-On-Chip
85 processors, and often support both Master and Slave roles.
86 These drivers touch hardware registers and may use DMA.
88 Protocol drivers ... these pass messages through the controller
89 driver to communicate with a Slave or Master device on the
90 other side of an SPI link.
92 So for example one protocol driver might talk to the MTD layer to export
93 data to filesystems stored on SPI flash like DataFlash; and others might
94 control audio interfaces, present touchscreen sensors as input interfaces,
95 or monitor temperature and voltage levels during industrial processing.
96 And those might all be sharing the same controller driver.
98 A "struct spi_device" encapsulates the master-side interface between
99 those two types of driver. At this writing, Linux has no slave side
100 programming interface.
102 There is a minimal core of SPI programming interfaces, focussing on
103 using driver model to connect controller and protocol drivers using
104 device tables provided by board specific initialization code. SPI
105 shows up in sysfs in several locations:
107 /sys/devices/.../CTLR/spiB.C ... spi_device for on bus "B",
108 chipselect C, accessed through CTLR.
110 /sys/bus/spi/devices/spiB.C ... symlink to the physical
113 /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
115 /sys/class/spi_master/spiB ... class device for the controller
116 managing bus "B". All the spiB.* devices share the same
117 physical SPI bus segment, with SCLK, MOSI, and MISO.
119 The basic I/O primitive submits an asynchronous message to an I/O queue
120 maintained by the controller driver. A completion callback is issued
121 asynchronously when the data transfer(s) in that message completes.
122 There are also some simple synchronous wrappers for those calls.
125 How does board-specific init code declare SPI devices?
126 ------------------------------------------------------
127 Linux needs several kinds of information to properly configure SPI devices.
128 That information is normally provided by board-specific code, even for
129 chips that do support some of automated discovery/enumeration.
133 The first kind of information is a list of what SPI controllers exist.
134 For System-on-Chip (SOC) based boards, these will usually be platform
135 devices, and the controller may need some platform_data in order to
136 operate properly. The "struct platform_device" will include resources
137 like the physical address of the controller's first register and its IRQ.
139 Platforms will often abstract the "register SPI controller" operation,
140 maybe coupling it with code to initialize pin configurations, so that
141 the arch/.../mach-*/board-*.c files for several boards can all share the
142 same basic controller setup code. This is because most SOCs have several
143 SPI-capable controllers, and only the ones actually usable on a given
144 board should normally be set up and registered.
146 So for example arch/.../mach-*/board-*.c files might have code like:
148 #include <asm/arch/spi.h> /* for mysoc_spi_data */
150 /* if your mach-* infrastructure doesn't support kernels that can
151 * run on multiple boards, pdata wouldn't benefit from "__init".
153 static struct mysoc_spi_data __init pdata = { ... };
155 static __init board_init(void)
158 /* this board only uses SPI controller #2 */
159 mysoc_register_spi(2, &pdata);
163 And SOC-specific utility code might look something like:
165 #include <asm/arch/spi.h>
167 static struct platform_device spi2 = { ... };
169 void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
171 struct mysoc_spi_data *pdata2;
173 pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
177 spi2->dev.platform_data = pdata2;
178 register_platform_device(&spi2);
180 /* also: set up pin modes so the spi2 signals are
181 * visible on the relevant pins ... bootloaders on
182 * production boards may already have done this, but
183 * developer boards will often need Linux to do it.
189 Notice how the platform_data for boards may be different, even if the
190 same SOC controller is used. For example, on one board SPI might use
191 an external clock, where another derives the SPI clock from current
192 settings of some master clock.
195 DECLARE SLAVE DEVICES
197 The second kind of information is a list of what SPI slave devices exist
198 on the target board, often with some board-specific data needed for the
199 driver to work correctly.
201 Normally your arch/.../mach-*/board-*.c files would provide a small table
202 listing the SPI devices on each board. (This would typically be only a
203 small handful.) That might look like:
205 static struct ads7846_platform_data ads_info = {
206 .vref_delay_usecs = 100,
211 static struct spi_board_info spi_board_info[] __initdata = {
213 .modalias = "ads7846",
214 .platform_data = &ads_info,
217 .max_speed_hz = 120000 /* max sample rate at 3V */ * 16,
223 Again, notice how board-specific information is provided; each chip may need
224 several types. This example shows generic constraints like the fastest SPI
225 clock to allow (a function of board voltage in this case) or how an IRQ pin
226 is wired, plus chip-specific constraints like an important delay that's
227 changed by the capacitance at one pin.
229 (There's also "controller_data", information that may be useful to the
230 controller driver. An example would be peripheral-specific DMA tuning
231 data or chipselect callbacks. This is stored in spi_device later.)
233 The board_info should provide enough information to let the system work
234 without the chip's driver being loaded. The most troublesome aspect of
235 that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
236 sharing a bus with a device that interprets chipselect "backwards" is
239 Then your board initialization code would register that table with the SPI
240 infrastructure, so that it's available later when the SPI master controller
241 driver is registered:
243 spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
245 Like with other static board-specific setup, you won't unregister those.
248 NON-STATIC CONFIGURATIONS
250 Developer boards often play by different rules than product boards, and one
251 example is the potential need to hotplug SPI devices and/or controllers.
253 For those cases you might need to use use spi_busnum_to_master() to look
254 up the spi bus master, and will likely need spi_new_device() to provide the
255 board info based on the board that was hotplugged. Of course, you'd later
256 call at least spi_unregister_device() when that board is removed.
259 How do I write an "SPI Protocol Driver"?
260 ----------------------------------------
261 All SPI drivers are currently kernel drivers. A userspace driver API
262 would just be another kernel driver, probably offering some lowlevel
263 access through aio_read(), aio_write(), and ioctl() calls and using the
264 standard userspace sysfs mechanisms to bind to a given SPI device.
266 SPI protocol drivers are normal device drivers, with no more wrapper
267 than needed by platform devices:
269 static struct device_driver CHIP_driver = {
271 .bus = &spi_bus_type,
273 .remove = __exit_p(CHIP_remove),
274 .suspend = CHIP_suspend,
275 .resume = CHIP_resume,
278 The SPI core will autmatically attempt to bind this driver to any SPI
279 device whose board_info gave a modalias of "CHIP". Your probe() code
280 might look like this unless you're creating a class_device:
282 static int __init CHIP_probe(struct device *dev)
284 struct spi_device *spi = to_spi_device(dev);
286 struct CHIP_platform_data *pdata = dev->platform_data;
288 /* get memory for driver's per-chip state */
289 chip = kzalloc(sizeof *chip, GFP_KERNEL);
292 dev_set_drvdata(dev, chip);
298 As soon as it enters probe(), the driver may issue I/O requests to
299 the SPI device using "struct spi_message". When remove() returns,
300 the driver guarantees that it won't submit any more such messages.
302 - An spi_message is a sequence of of protocol operations, executed
303 as one atomic sequence. SPI driver controls include:
305 + when bidirectional reads and writes start ... by how its
306 sequence of spi_transfer requests is arranged;
308 + optionally defining short delays after transfers ... using
309 the spi_transfer.delay_usecs setting;
311 + whether the chipselect becomes inactive after a transfer and
312 any delay ... by using the spi_transfer.cs_change flag;
314 + hinting whether the next message is likely to go to this same
315 device ... using the spi_transfer.cs_change flag on the last
316 transfer in that atomic group, and potentially saving costs
317 for chip deselect and select operations.
319 - Follow standard kernel rules, and provide DMA-safe buffers in
320 your messages. That way controller drivers using DMA aren't forced
321 to make extra copies unless the hardware requires it (e.g. working
322 around hardware errata that force the use of bounce buffering).
324 If standard dma_map_single() handling of these buffers is inappropriate,
325 you can use spi_message.is_dma_mapped to tell the controller driver
326 that you've already provided the relevant DMA addresses.
328 - The basic I/O primitive is spi_async(). Async requests may be
329 issued in any context (irq handler, task, etc) and completion
330 is reported using a callback provided with the message.
332 - There are also synchronous wrappers like spi_sync(), and wrappers
333 like spi_read(), spi_write(), and spi_write_then_read(). These
334 may be issued only in contexts that may sleep, and they're all
335 clean (and small, and "optional") layers over spi_async().
337 - The spi_write_then_read() call, and convenience wrappers around
338 it, should only be used with small amounts of data where the
339 cost of an extra copy may be ignored. It's designed to support
340 common RPC-style requests, such as writing an eight bit command
341 and reading a sixteen bit response -- spi_w8r16() being one its
342 wrappers, doing exactly that.
344 Some drivers may need to modify spi_device characteristics like the
345 transfer mode, wordsize, or clock rate. This is done with spi_setup(),
346 which would normally be called from probe() before the first I/O is
349 While "spi_device" would be the bottom boundary of the driver, the
350 upper boundaries might include sysfs (especially for sensor readings),
351 the input layer, ALSA, networking, MTD, the character device framework,
352 or other Linux subsystems.
355 How do I write an "SPI Master Controller Driver"?
356 -------------------------------------------------
357 An SPI controller will probably be registered on the platform_bus; write
358 a driver to bind to the device, whichever bus is involved.
360 The main task of this type of driver is to provide an "spi_master".
361 Use spi_alloc_master() to allocate the master, and class_get_devdata()
362 to get the driver-private data allocated for that device.
364 struct spi_master *master;
365 struct CONTROLLER *c;
367 master = spi_alloc_master(dev, sizeof *c);
371 c = class_get_devdata(&master->cdev);
373 The driver will initialize the fields of that spi_master, including the
374 bus number (maybe the same as the platform device ID) and three methods
375 used to interact with the SPI core and SPI protocol drivers. It will
376 also initialize its own internal state.
378 master->setup(struct spi_device *spi)
379 This sets up the device clock rate, SPI mode, and word sizes.
380 Drivers may change the defaults provided by board_info, and then
381 call spi_setup(spi) to invoke this routine. It may sleep.
383 master->transfer(struct spi_device *spi, struct spi_message *message)
384 This must not sleep. Its responsibility is arrange that the
385 transfer happens and its complete() callback is issued; the two
386 will normally happen later, after other transfers complete.
388 master->cleanup(struct spi_device *spi)
389 Your controller driver may use spi_device.controller_state to hold
390 state it dynamically associates with that device. If you do that,
391 be sure to provide the cleanup() method to free that state.
393 The bulk of the driver will be managing the I/O queue fed by transfer().
395 That queue could be purely conceptual. For example, a driver used only
396 for low-frequency sensor acess might be fine using synchronous PIO.
398 But the queue will probably be very real, using message->queue, PIO,
399 often DMA (especially if the root filesystem is in SPI flash), and
400 execution contexts like IRQ handlers, tasklets, or workqueues (such
401 as keventd). Your driver can be as fancy, or as simple, as you need.
406 Contributors to Linux-SPI discussions include (in alphabetical order,