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1 | Overview of Linux kernel SPI support |
2 | ==================================== | |
3 | ||
ffbbdd21 | 4 | 02-Feb-2012 |
8ae12a0d DB |
5 | |
6 | What is SPI? | |
7 | ------------ | |
b885244e DB |
8 | The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial |
9 | link used to connect microcontrollers to sensors, memory, and peripherals. | |
43d4f961 DB |
10 | It's a simple "de facto" standard, not complicated enough to acquire a |
11 | standardization body. SPI uses a master/slave configuration. | |
8ae12a0d | 12 | |
33e34dc6 | 13 | The three signal wires hold a clock (SCK, often on the order of 10 MHz), |
8ae12a0d DB |
14 | and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In, |
15 | Slave Out" (MISO) signals. (Other names are also used.) There are four | |
16 | clocking modes through which data is exchanged; mode-0 and mode-3 are most | |
b885244e | 17 | commonly used. Each clock cycle shifts data out and data in; the clock |
43d4f961 DB |
18 | doesn't cycle except when there is a data bit to shift. Not all data bits |
19 | are used though; not every protocol uses those full duplex capabilities. | |
8ae12a0d | 20 | |
43d4f961 | 21 | SPI masters use a fourth "chip select" line to activate a given SPI slave |
8ae12a0d | 22 | device, so those three signal wires may be connected to several chips |
43d4f961 DB |
23 | in parallel. All SPI slaves support chipselects; they are usually active |
24 | low signals, labeled nCSx for slave 'x' (e.g. nCS0). Some devices have | |
8ae12a0d DB |
25 | other signals, often including an interrupt to the master. |
26 | ||
43d4f961 | 27 | Unlike serial busses like USB or SMBus, even low level protocols for |
8ae12a0d | 28 | SPI slave functions are usually not interoperable between vendors |
33e34dc6 | 29 | (except for commodities like SPI memory chips). |
8ae12a0d DB |
30 | |
31 | - SPI may be used for request/response style device protocols, as with | |
32 | touchscreen sensors and memory chips. | |
33 | ||
34 | - It may also be used to stream data in either direction (half duplex), | |
35 | or both of them at the same time (full duplex). | |
36 | ||
0c64bc1b | 37 | - Some devices may use eight bit words. Others may use different word |
8ae12a0d DB |
38 | lengths, such as streams of 12-bit or 20-bit digital samples. |
39 | ||
43d4f961 DB |
40 | - Words are usually sent with their most significant bit (MSB) first, |
41 | but sometimes the least significant bit (LSB) goes first instead. | |
42 | ||
43 | - Sometimes SPI is used to daisy-chain devices, like shift registers. | |
44 | ||
8ae12a0d DB |
45 | In the same way, SPI slaves will only rarely support any kind of automatic |
46 | discovery/enumeration protocol. The tree of slave devices accessible from | |
47 | a given SPI master will normally be set up manually, with configuration | |
48 | tables. | |
49 | ||
50 | SPI is only one of the names used by such four-wire protocols, and | |
51 | most controllers have no problem handling "MicroWire" (think of it as | |
52 | half-duplex SPI, for request/response protocols), SSP ("Synchronous | |
53 | Serial Protocol"), PSP ("Programmable Serial Protocol"), and other | |
54 | related protocols. | |
55 | ||
43d4f961 DB |
56 | Some chips eliminate a signal line by combining MOSI and MISO, and |
57 | limiting themselves to half-duplex at the hardware level. In fact | |
58 | some SPI chips have this signal mode as a strapping option. These | |
59 | can be accessed using the same programming interface as SPI, but of | |
60 | course they won't handle full duplex transfers. You may find such | |
61 | chips described as using "three wire" signaling: SCK, data, nCSx. | |
62 | (That data line is sometimes called MOMI or SISO.) | |
63 | ||
8ae12a0d | 64 | Microcontrollers often support both master and slave sides of the SPI |
aa2ea911 GU |
65 | protocol. This document (and Linux) supports both the master and slave |
66 | sides of SPI interactions. | |
8ae12a0d DB |
67 | |
68 | ||
69 | Who uses it? On what kinds of systems? | |
70 | --------------------------------------- | |
71 | Linux developers using SPI are probably writing device drivers for embedded | |
72 | systems boards. SPI is used to control external chips, and it is also a | |
73 | protocol supported by every MMC or SD memory card. (The older "DataFlash" | |
74 | cards, predating MMC cards but using the same connectors and card shape, | |
75 | support only SPI.) Some PC hardware uses SPI flash for BIOS code. | |
76 | ||
77 | SPI slave chips range from digital/analog converters used for analog | |
78 | sensors and codecs, to memory, to peripherals like USB controllers | |
79 | or Ethernet adapters; and more. | |
80 | ||
81 | Most systems using SPI will integrate a few devices on a mainboard. | |
82 | Some provide SPI links on expansion connectors; in cases where no | |
83 | dedicated SPI controller exists, GPIO pins can be used to create a | |
84 | low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI | |
85 | controller; the reasons to use SPI focus on low cost and simple operation, | |
86 | and if dynamic reconfiguration is important, USB will often be a more | |
87 | appropriate low-pincount peripheral bus. | |
88 | ||
89 | Many microcontrollers that can run Linux integrate one or more I/O | |
90 | interfaces with SPI modes. Given SPI support, they could use MMC or SD | |
91 | cards without needing a special purpose MMC/SD/SDIO controller. | |
92 | ||
93 | ||
43d4f961 DB |
94 | I'm confused. What are these four SPI "clock modes"? |
95 | ----------------------------------------------------- | |
96 | It's easy to be confused here, and the vendor documentation you'll | |
97 | find isn't necessarily helpful. The four modes combine two mode bits: | |
98 | ||
99 | - CPOL indicates the initial clock polarity. CPOL=0 means the | |
100 | clock starts low, so the first (leading) edge is rising, and | |
101 | the second (trailing) edge is falling. CPOL=1 means the clock | |
102 | starts high, so the first (leading) edge is falling. | |
103 | ||
104 | - CPHA indicates the clock phase used to sample data; CPHA=0 says | |
105 | sample on the leading edge, CPHA=1 means the trailing edge. | |
106 | ||
107 | Since the signal needs to stablize before it's sampled, CPHA=0 | |
108 | implies that its data is written half a clock before the first | |
109 | clock edge. The chipselect may have made it become available. | |
110 | ||
111 | Chip specs won't always say "uses SPI mode X" in as many words, | |
112 | but their timing diagrams will make the CPOL and CPHA modes clear. | |
113 | ||
114 | In the SPI mode number, CPOL is the high order bit and CPHA is the | |
115 | low order bit. So when a chip's timing diagram shows the clock | |
116 | starting low (CPOL=0) and data stabilized for sampling during the | |
117 | trailing clock edge (CPHA=1), that's SPI mode 1. | |
118 | ||
6395bee7 DB |
119 | Note that the clock mode is relevant as soon as the chipselect goes |
120 | active. So the master must set the clock to inactive before selecting | |
121 | a slave, and the slave can tell the chosen polarity by sampling the | |
122 | clock level when its select line goes active. That's why many devices | |
123 | support for example both modes 0 and 3: they don't care about polarity, | |
0c64bc1b | 124 | and always clock data in/out on rising clock edges. |
6395bee7 | 125 | |
43d4f961 | 126 | |
8ae12a0d DB |
127 | How do these driver programming interfaces work? |
128 | ------------------------------------------------ | |
129 | The <linux/spi/spi.h> header file includes kerneldoc, as does the | |
33e34dc6 DB |
130 | main source code, and you should certainly read that chapter of the |
131 | kernel API document. This is just an overview, so you get the big | |
132 | picture before those details. | |
8ae12a0d | 133 | |
b885244e DB |
134 | SPI requests always go into I/O queues. Requests for a given SPI device |
135 | are always executed in FIFO order, and complete asynchronously through | |
136 | completion callbacks. There are also some simple synchronous wrappers | |
137 | for those calls, including ones for common transaction types like writing | |
138 | a command and then reading its response. | |
139 | ||
8ae12a0d DB |
140 | There are two types of SPI driver, here called: |
141 | ||
0c64bc1b | 142 | Controller drivers ... controllers may be built into System-On-Chip |
8ae12a0d DB |
143 | processors, and often support both Master and Slave roles. |
144 | These drivers touch hardware registers and may use DMA. | |
b885244e | 145 | Or they can be PIO bitbangers, needing just GPIO pins. |
8ae12a0d DB |
146 | |
147 | Protocol drivers ... these pass messages through the controller | |
148 | driver to communicate with a Slave or Master device on the | |
149 | other side of an SPI link. | |
150 | ||
151 | So for example one protocol driver might talk to the MTD layer to export | |
152 | data to filesystems stored on SPI flash like DataFlash; and others might | |
153 | control audio interfaces, present touchscreen sensors as input interfaces, | |
154 | or monitor temperature and voltage levels during industrial processing. | |
155 | And those might all be sharing the same controller driver. | |
156 | ||
aa2ea911 GU |
157 | A "struct spi_device" encapsulates the controller-side interface between |
158 | those two types of drivers. | |
8ae12a0d DB |
159 | |
160 | There is a minimal core of SPI programming interfaces, focussing on | |
33e34dc6 | 161 | using the driver model to connect controller and protocol drivers using |
8ae12a0d DB |
162 | device tables provided by board specific initialization code. SPI |
163 | shows up in sysfs in several locations: | |
164 | ||
49dce689 TJ |
165 | /sys/devices/.../CTLR ... physical node for a given SPI controller |
166 | ||
33e34dc6 | 167 | /sys/devices/.../CTLR/spiB.C ... spi_device on bus "B", |
8ae12a0d DB |
168 | chipselect C, accessed through CTLR. |
169 | ||
49dce689 TJ |
170 | /sys/bus/spi/devices/spiB.C ... symlink to that physical |
171 | .../CTLR/spiB.C device | |
172 | ||
7111763d DB |
173 | /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver |
174 | that should be used with this device (for hotplug/coldplug) | |
175 | ||
8ae12a0d DB |
176 | /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices |
177 | ||
49dce689 | 178 | /sys/class/spi_master/spiB ... symlink (or actual device node) to |
aa2ea911 GU |
179 | a logical node which could hold class related state for the SPI |
180 | master controller managing bus "B". All spiB.* devices share one | |
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181 | physical SPI bus segment, with SCLK, MOSI, and MISO. |
182 | ||
aa2ea911 GU |
183 | /sys/devices/.../CTLR/slave ... virtual file for (un)registering the |
184 | slave device for an SPI slave controller. | |
185 | Writing the driver name of an SPI slave handler to this file | |
186 | registers the slave device; writing "(null)" unregisters the slave | |
187 | device. | |
188 | Reading from this file shows the name of the slave device ("(null)" | |
189 | if not registered). | |
190 | ||
191 | /sys/class/spi_slave/spiB ... symlink (or actual device node) to | |
192 | a logical node which could hold class related state for the SPI | |
193 | slave controller on bus "B". When registered, a single spiB.* | |
194 | device is present here, possible sharing the physical SPI bus | |
195 | segment with other SPI slave devices. | |
196 | ||
49dce689 TJ |
197 | Note that the actual location of the controller's class state depends |
198 | on whether you enabled CONFIG_SYSFS_DEPRECATED or not. At this time, | |
199 | the only class-specific state is the bus number ("B" in "spiB"), so | |
200 | those /sys/class entries are only useful to quickly identify busses. | |
201 | ||
8ae12a0d DB |
202 | |
203 | How does board-specific init code declare SPI devices? | |
204 | ------------------------------------------------------ | |
205 | Linux needs several kinds of information to properly configure SPI devices. | |
206 | That information is normally provided by board-specific code, even for | |
207 | chips that do support some of automated discovery/enumeration. | |
208 | ||
209 | DECLARE CONTROLLERS | |
210 | ||
211 | The first kind of information is a list of what SPI controllers exist. | |
212 | For System-on-Chip (SOC) based boards, these will usually be platform | |
213 | devices, and the controller may need some platform_data in order to | |
214 | operate properly. The "struct platform_device" will include resources | |
215 | like the physical address of the controller's first register and its IRQ. | |
216 | ||
217 | Platforms will often abstract the "register SPI controller" operation, | |
218 | maybe coupling it with code to initialize pin configurations, so that | |
219 | the arch/.../mach-*/board-*.c files for several boards can all share the | |
220 | same basic controller setup code. This is because most SOCs have several | |
221 | SPI-capable controllers, and only the ones actually usable on a given | |
222 | board should normally be set up and registered. | |
223 | ||
224 | So for example arch/.../mach-*/board-*.c files might have code like: | |
225 | ||
a09e64fb | 226 | #include <mach/spi.h> /* for mysoc_spi_data */ |
8ae12a0d DB |
227 | |
228 | /* if your mach-* infrastructure doesn't support kernels that can | |
229 | * run on multiple boards, pdata wouldn't benefit from "__init". | |
230 | */ | |
61679efe | 231 | static struct mysoc_spi_data pdata __initdata = { ... }; |
8ae12a0d DB |
232 | |
233 | static __init board_init(void) | |
234 | { | |
235 | ... | |
236 | /* this board only uses SPI controller #2 */ | |
237 | mysoc_register_spi(2, &pdata); | |
238 | ... | |
239 | } | |
240 | ||
241 | And SOC-specific utility code might look something like: | |
242 | ||
a09e64fb | 243 | #include <mach/spi.h> |
8ae12a0d DB |
244 | |
245 | static struct platform_device spi2 = { ... }; | |
246 | ||
247 | void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata) | |
248 | { | |
249 | struct mysoc_spi_data *pdata2; | |
250 | ||
251 | pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL); | |
252 | *pdata2 = pdata; | |
253 | ... | |
254 | if (n == 2) { | |
255 | spi2->dev.platform_data = pdata2; | |
256 | register_platform_device(&spi2); | |
257 | ||
258 | /* also: set up pin modes so the spi2 signals are | |
259 | * visible on the relevant pins ... bootloaders on | |
260 | * production boards may already have done this, but | |
261 | * developer boards will often need Linux to do it. | |
262 | */ | |
263 | } | |
264 | ... | |
265 | } | |
266 | ||
267 | Notice how the platform_data for boards may be different, even if the | |
268 | same SOC controller is used. For example, on one board SPI might use | |
269 | an external clock, where another derives the SPI clock from current | |
270 | settings of some master clock. | |
271 | ||
272 | ||
273 | DECLARE SLAVE DEVICES | |
274 | ||
275 | The second kind of information is a list of what SPI slave devices exist | |
276 | on the target board, often with some board-specific data needed for the | |
277 | driver to work correctly. | |
278 | ||
279 | Normally your arch/.../mach-*/board-*.c files would provide a small table | |
280 | listing the SPI devices on each board. (This would typically be only a | |
281 | small handful.) That might look like: | |
282 | ||
283 | static struct ads7846_platform_data ads_info = { | |
284 | .vref_delay_usecs = 100, | |
285 | .x_plate_ohms = 580, | |
286 | .y_plate_ohms = 410, | |
287 | }; | |
288 | ||
289 | static struct spi_board_info spi_board_info[] __initdata = { | |
290 | { | |
291 | .modalias = "ads7846", | |
292 | .platform_data = &ads_info, | |
293 | .mode = SPI_MODE_0, | |
294 | .irq = GPIO_IRQ(31), | |
295 | .max_speed_hz = 120000 /* max sample rate at 3V */ * 16, | |
296 | .bus_num = 1, | |
297 | .chip_select = 0, | |
298 | }, | |
299 | }; | |
300 | ||
301 | Again, notice how board-specific information is provided; each chip may need | |
302 | several types. This example shows generic constraints like the fastest SPI | |
303 | clock to allow (a function of board voltage in this case) or how an IRQ pin | |
304 | is wired, plus chip-specific constraints like an important delay that's | |
305 | changed by the capacitance at one pin. | |
306 | ||
307 | (There's also "controller_data", information that may be useful to the | |
308 | controller driver. An example would be peripheral-specific DMA tuning | |
309 | data or chipselect callbacks. This is stored in spi_device later.) | |
310 | ||
311 | The board_info should provide enough information to let the system work | |
312 | without the chip's driver being loaded. The most troublesome aspect of | |
313 | that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since | |
314 | sharing a bus with a device that interprets chipselect "backwards" is | |
33e34dc6 | 315 | not possible until the infrastructure knows how to deselect it. |
8ae12a0d DB |
316 | |
317 | Then your board initialization code would register that table with the SPI | |
318 | infrastructure, so that it's available later when the SPI master controller | |
319 | driver is registered: | |
320 | ||
321 | spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info)); | |
322 | ||
323 | Like with other static board-specific setup, you won't unregister those. | |
324 | ||
7111763d DB |
325 | The widely used "card" style computers bundle memory, cpu, and little else |
326 | onto a card that's maybe just thirty square centimeters. On such systems, | |
327 | your arch/.../mach-.../board-*.c file would primarily provide information | |
328 | about the devices on the mainboard into which such a card is plugged. That | |
329 | certainly includes SPI devices hooked up through the card connectors! | |
330 | ||
8ae12a0d DB |
331 | |
332 | NON-STATIC CONFIGURATIONS | |
333 | ||
334 | Developer boards often play by different rules than product boards, and one | |
335 | example is the potential need to hotplug SPI devices and/or controllers. | |
336 | ||
670e9f34 | 337 | For those cases you might need to use spi_busnum_to_master() to look |
8ae12a0d DB |
338 | up the spi bus master, and will likely need spi_new_device() to provide the |
339 | board info based on the board that was hotplugged. Of course, you'd later | |
340 | call at least spi_unregister_device() when that board is removed. | |
341 | ||
7111763d | 342 | When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those |
33e34dc6 DB |
343 | configurations will also be dynamic. Fortunately, such devices all support |
344 | basic device identification probes, so they should hotplug normally. | |
7111763d | 345 | |
8ae12a0d DB |
346 | |
347 | How do I write an "SPI Protocol Driver"? | |
348 | ---------------------------------------- | |
33e34dc6 DB |
349 | Most SPI drivers are currently kernel drivers, but there's also support |
350 | for userspace drivers. Here we talk only about kernel drivers. | |
8ae12a0d | 351 | |
b885244e DB |
352 | SPI protocol drivers somewhat resemble platform device drivers: |
353 | ||
354 | static struct spi_driver CHIP_driver = { | |
355 | .driver = { | |
356 | .name = "CHIP", | |
b885244e | 357 | .owner = THIS_MODULE, |
47875e81 | 358 | .pm = &CHIP_pm_ops, |
b885244e | 359 | }, |
8ae12a0d | 360 | |
8ae12a0d | 361 | .probe = CHIP_probe, |
63a29f74 | 362 | .remove = CHIP_remove, |
8ae12a0d DB |
363 | }; |
364 | ||
9ed7ef52 | 365 | The driver core will automatically attempt to bind this driver to any SPI |
8ae12a0d | 366 | device whose board_info gave a modalias of "CHIP". Your probe() code |
49dce689 TJ |
367 | might look like this unless you're creating a device which is managing |
368 | a bus (appearing under /sys/class/spi_master). | |
8ae12a0d | 369 | |
63a29f74 | 370 | static int CHIP_probe(struct spi_device *spi) |
8ae12a0d | 371 | { |
8ae12a0d | 372 | struct CHIP *chip; |
b885244e DB |
373 | struct CHIP_platform_data *pdata; |
374 | ||
375 | /* assuming the driver requires board-specific data: */ | |
376 | pdata = &spi->dev.platform_data; | |
377 | if (!pdata) | |
378 | return -ENODEV; | |
8ae12a0d DB |
379 | |
380 | /* get memory for driver's per-chip state */ | |
381 | chip = kzalloc(sizeof *chip, GFP_KERNEL); | |
382 | if (!chip) | |
383 | return -ENOMEM; | |
9b40ff4d | 384 | spi_set_drvdata(spi, chip); |
8ae12a0d DB |
385 | |
386 | ... etc | |
387 | return 0; | |
388 | } | |
389 | ||
390 | As soon as it enters probe(), the driver may issue I/O requests to | |
391 | the SPI device using "struct spi_message". When remove() returns, | |
33e34dc6 DB |
392 | or after probe() fails, the driver guarantees that it won't submit |
393 | any more such messages. | |
8ae12a0d | 394 | |
670e9f34 | 395 | - An spi_message is a sequence of protocol operations, executed |
8ae12a0d DB |
396 | as one atomic sequence. SPI driver controls include: |
397 | ||
398 | + when bidirectional reads and writes start ... by how its | |
399 | sequence of spi_transfer requests is arranged; | |
400 | ||
6395bee7 DB |
401 | + which I/O buffers are used ... each spi_transfer wraps a |
402 | buffer for each transfer direction, supporting full duplex | |
403 | (two pointers, maybe the same one in both cases) and half | |
404 | duplex (one pointer is NULL) transfers; | |
405 | ||
8ae12a0d | 406 | + optionally defining short delays after transfers ... using |
6395bee7 DB |
407 | the spi_transfer.delay_usecs setting (this delay can be the |
408 | only protocol effect, if the buffer length is zero); | |
8ae12a0d DB |
409 | |
410 | + whether the chipselect becomes inactive after a transfer and | |
411 | any delay ... by using the spi_transfer.cs_change flag; | |
412 | ||
413 | + hinting whether the next message is likely to go to this same | |
414 | device ... using the spi_transfer.cs_change flag on the last | |
415 | transfer in that atomic group, and potentially saving costs | |
416 | for chip deselect and select operations. | |
417 | ||
418 | - Follow standard kernel rules, and provide DMA-safe buffers in | |
419 | your messages. That way controller drivers using DMA aren't forced | |
420 | to make extra copies unless the hardware requires it (e.g. working | |
421 | around hardware errata that force the use of bounce buffering). | |
422 | ||
423 | If standard dma_map_single() handling of these buffers is inappropriate, | |
424 | you can use spi_message.is_dma_mapped to tell the controller driver | |
425 | that you've already provided the relevant DMA addresses. | |
426 | ||
427 | - The basic I/O primitive is spi_async(). Async requests may be | |
428 | issued in any context (irq handler, task, etc) and completion | |
429 | is reported using a callback provided with the message. | |
b885244e DB |
430 | After any detected error, the chip is deselected and processing |
431 | of that spi_message is aborted. | |
8ae12a0d DB |
432 | |
433 | - There are also synchronous wrappers like spi_sync(), and wrappers | |
434 | like spi_read(), spi_write(), and spi_write_then_read(). These | |
435 | may be issued only in contexts that may sleep, and they're all | |
436 | clean (and small, and "optional") layers over spi_async(). | |
437 | ||
438 | - The spi_write_then_read() call, and convenience wrappers around | |
439 | it, should only be used with small amounts of data where the | |
440 | cost of an extra copy may be ignored. It's designed to support | |
441 | common RPC-style requests, such as writing an eight bit command | |
442 | and reading a sixteen bit response -- spi_w8r16() being one its | |
443 | wrappers, doing exactly that. | |
444 | ||
445 | Some drivers may need to modify spi_device characteristics like the | |
446 | transfer mode, wordsize, or clock rate. This is done with spi_setup(), | |
447 | which would normally be called from probe() before the first I/O is | |
33e34dc6 DB |
448 | done to the device. However, that can also be called at any time |
449 | that no message is pending for that device. | |
8ae12a0d DB |
450 | |
451 | While "spi_device" would be the bottom boundary of the driver, the | |
452 | upper boundaries might include sysfs (especially for sensor readings), | |
453 | the input layer, ALSA, networking, MTD, the character device framework, | |
454 | or other Linux subsystems. | |
455 | ||
0c868461 DB |
456 | Note that there are two types of memory your driver must manage as part |
457 | of interacting with SPI devices. | |
458 | ||
459 | - I/O buffers use the usual Linux rules, and must be DMA-safe. | |
460 | You'd normally allocate them from the heap or free page pool. | |
461 | Don't use the stack, or anything that's declared "static". | |
462 | ||
463 | - The spi_message and spi_transfer metadata used to glue those | |
464 | I/O buffers into a group of protocol transactions. These can | |
465 | be allocated anywhere it's convenient, including as part of | |
466 | other allocate-once driver data structures. Zero-init these. | |
467 | ||
468 | If you like, spi_message_alloc() and spi_message_free() convenience | |
469 | routines are available to allocate and zero-initialize an spi_message | |
470 | with several transfers. | |
471 | ||
8ae12a0d DB |
472 | |
473 | How do I write an "SPI Master Controller Driver"? | |
474 | ------------------------------------------------- | |
475 | An SPI controller will probably be registered on the platform_bus; write | |
476 | a driver to bind to the device, whichever bus is involved. | |
477 | ||
478 | The main task of this type of driver is to provide an "spi_master". | |
49dce689 | 479 | Use spi_alloc_master() to allocate the master, and spi_master_get_devdata() |
8ae12a0d DB |
480 | to get the driver-private data allocated for that device. |
481 | ||
482 | struct spi_master *master; | |
483 | struct CONTROLLER *c; | |
484 | ||
485 | master = spi_alloc_master(dev, sizeof *c); | |
486 | if (!master) | |
487 | return -ENODEV; | |
488 | ||
49dce689 | 489 | c = spi_master_get_devdata(master); |
8ae12a0d DB |
490 | |
491 | The driver will initialize the fields of that spi_master, including the | |
492 | bus number (maybe the same as the platform device ID) and three methods | |
493 | used to interact with the SPI core and SPI protocol drivers. It will | |
a020ed75 DB |
494 | also initialize its own internal state. (See below about bus numbering |
495 | and those methods.) | |
496 | ||
497 | After you initialize the spi_master, then use spi_register_master() to | |
ffbbdd21 LW |
498 | publish it to the rest of the system. At that time, device nodes for the |
499 | controller and any predeclared spi devices will be made available, and | |
500 | the driver model core will take care of binding them to drivers. | |
a020ed75 DB |
501 | |
502 | If you need to remove your SPI controller driver, spi_unregister_master() | |
503 | will reverse the effect of spi_register_master(). | |
504 | ||
505 | ||
506 | BUS NUMBERING | |
507 | ||
508 | Bus numbering is important, since that's how Linux identifies a given | |
509 | SPI bus (shared SCK, MOSI, MISO). Valid bus numbers start at zero. On | |
510 | SOC systems, the bus numbers should match the numbers defined by the chip | |
511 | manufacturer. For example, hardware controller SPI2 would be bus number 2, | |
512 | and spi_board_info for devices connected to it would use that number. | |
513 | ||
514 | If you don't have such hardware-assigned bus number, and for some reason | |
515 | you can't just assign them, then provide a negative bus number. That will | |
516 | then be replaced by a dynamically assigned number. You'd then need to treat | |
517 | this as a non-static configuration (see above). | |
518 | ||
519 | ||
520 | SPI MASTER METHODS | |
8ae12a0d DB |
521 | |
522 | master->setup(struct spi_device *spi) | |
523 | This sets up the device clock rate, SPI mode, and word sizes. | |
524 | Drivers may change the defaults provided by board_info, and then | |
525 | call spi_setup(spi) to invoke this routine. It may sleep. | |
6e538aaf | 526 | |
33e34dc6 DB |
527 | Unless each SPI slave has its own configuration registers, don't |
528 | change them right away ... otherwise drivers could corrupt I/O | |
529 | that's in progress for other SPI devices. | |
8ae12a0d | 530 | |
6e538aaf DB |
531 | ** BUG ALERT: for some reason the first version of |
532 | ** many spi_master drivers seems to get this wrong. | |
533 | ** When you code setup(), ASSUME that the controller | |
534 | ** is actively processing transfers for another device. | |
535 | ||
8ae12a0d DB |
536 | master->cleanup(struct spi_device *spi) |
537 | Your controller driver may use spi_device.controller_state to hold | |
538 | state it dynamically associates with that device. If you do that, | |
539 | be sure to provide the cleanup() method to free that state. | |
540 | ||
ffbbdd21 LW |
541 | master->prepare_transfer_hardware(struct spi_master *master) |
542 | This will be called by the queue mechanism to signal to the driver | |
543 | that a message is coming in soon, so the subsystem requests the | |
544 | driver to prepare the transfer hardware by issuing this call. | |
545 | This may sleep. | |
546 | ||
547 | master->unprepare_transfer_hardware(struct spi_master *master) | |
548 | This will be called by the queue mechanism to signal to the driver | |
549 | that there are no more messages pending in the queue and it may | |
550 | relax the hardware (e.g. by power management calls). This may sleep. | |
551 | ||
552 | master->transfer_one_message(struct spi_master *master, | |
553 | struct spi_message *mesg) | |
554 | The subsystem calls the driver to transfer a single message while | |
555 | queuing transfers that arrive in the meantime. When the driver is | |
556 | finished with this message, it must call | |
557 | spi_finalize_current_message() so the subsystem can issue the next | |
e9305331 | 558 | message. This may sleep. |
ffbbdd21 | 559 | |
18cc0adb BS |
560 | master->transfer_one(struct spi_master *master, struct spi_device *spi, |
561 | struct spi_transfer *transfer) | |
562 | The subsystem calls the driver to transfer a single transfer while | |
563 | queuing transfers that arrive in the meantime. When the driver is | |
564 | finished with this transfer, it must call | |
565 | spi_finalize_current_transfer() so the subsystem can issue the next | |
566 | transfer. This may sleep. Note: transfer_one and transfer_one_message | |
567 | are mutually exclusive; when both are set, the generic subsystem does | |
568 | not call your transfer_one callback. | |
569 | ||
570 | Return values: | |
571 | negative errno: error | |
572 | 0: transfer is finished | |
573 | 1: transfer is still in progress | |
574 | ||
ffbbdd21 LW |
575 | DEPRECATED METHODS |
576 | ||
577 | master->transfer(struct spi_device *spi, struct spi_message *message) | |
0c64bc1b | 578 | This must not sleep. Its responsibility is to arrange that the |
ffbbdd21 LW |
579 | transfer happens and its complete() callback is issued. The two |
580 | will normally happen later, after other transfers complete, and | |
581 | if the controller is idle it will need to be kickstarted. This | |
582 | method is not used on queued controllers and must be NULL if | |
583 | transfer_one_message() and (un)prepare_transfer_hardware() are | |
584 | implemented. | |
585 | ||
a020ed75 DB |
586 | |
587 | SPI MESSAGE QUEUE | |
588 | ||
ffbbdd21 LW |
589 | If you are happy with the standard queueing mechanism provided by the |
590 | SPI subsystem, just implement the queued methods specified above. Using | |
591 | the message queue has the upside of centralizing a lot of code and | |
592 | providing pure process-context execution of methods. The message queue | |
593 | can also be elevated to realtime priority on high-priority SPI traffic. | |
594 | ||
595 | Unless the queueing mechanism in the SPI subsystem is selected, the bulk | |
596 | of the driver will be managing the I/O queue fed by the now deprecated | |
597 | function transfer(). | |
8ae12a0d DB |
598 | |
599 | That queue could be purely conceptual. For example, a driver used only | |
af901ca1 | 600 | for low-frequency sensor access might be fine using synchronous PIO. |
8ae12a0d DB |
601 | |
602 | But the queue will probably be very real, using message->queue, PIO, | |
603 | often DMA (especially if the root filesystem is in SPI flash), and | |
604 | execution contexts like IRQ handlers, tasklets, or workqueues (such | |
605 | as keventd). Your driver can be as fancy, or as simple, as you need. | |
a020ed75 DB |
606 | Such a transfer() method would normally just add the message to a |
607 | queue, and then start some asynchronous transfer engine (unless it's | |
608 | already running). | |
8ae12a0d DB |
609 | |
610 | ||
611 | THANKS TO | |
612 | --------- | |
613 | Contributors to Linux-SPI discussions include (in alphabetical order, | |
614 | by last name): | |
615 | ||
2be01d29 | 616 | Mark Brown |
8ae12a0d DB |
617 | David Brownell |
618 | Russell King | |
2be01d29 | 619 | Grant Likely |
8ae12a0d DB |
620 | Dmitry Pervushin |
621 | Stephen Street | |
622 | Mark Underwood | |
623 | Andrew Victor | |
ffbbdd21 | 624 | Linus Walleij |
2be01d29 | 625 | Vitaly Wool |