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1 | Overview of Linux kernel SPI support |
2 | ==================================== | |
3 | ||
b885244e | 4 | 02-Dec-2005 |
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5 | |
6 | What is SPI? | |
7 | ------------ | |
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8 | The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial |
9 | link used to connect microcontrollers to sensors, memory, and peripherals. | |
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10 | |
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 | |
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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. | |
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17 | |
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. | |
22 | ||
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). | |
26 | ||
27 | - SPI may be used for request/response style device protocols, as with | |
28 | touchscreen sensors and memory chips. | |
29 | ||
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). | |
32 | ||
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. | |
35 | ||
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 | |
39 | tables. | |
40 | ||
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 | |
45 | related protocols. | |
46 | ||
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. | |
50 | ||
51 | ||
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. | |
59 | ||
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. | |
63 | ||
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. | |
71 | ||
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. | |
75 | ||
76 | ||
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. | |
82 | ||
b885244e DB |
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. | |
88 | ||
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89 | There are two types of SPI driver, here called: |
90 | ||
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. | |
b885244e | 94 | Or they can be PIO bitbangers, needing just GPIO pins. |
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95 | |
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. | |
99 | ||
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. | |
105 | ||
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. | |
109 | ||
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: | |
114 | ||
115 | /sys/devices/.../CTLR/spiB.C ... spi_device for on bus "B", | |
116 | chipselect C, accessed through CTLR. | |
117 | ||
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118 | /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver |
119 | that should be used with this device (for hotplug/coldplug) | |
120 | ||
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121 | /sys/bus/spi/devices/spiB.C ... symlink to the physical |
122 | spiB-C device | |
123 | ||
124 | /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices | |
125 | ||
126 | /sys/class/spi_master/spiB ... class device for the controller | |
127 | managing bus "B". All the spiB.* devices share the same | |
128 | physical SPI bus segment, with SCLK, MOSI, and MISO. | |
129 | ||
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130 | |
131 | How does board-specific init code declare SPI devices? | |
132 | ------------------------------------------------------ | |
133 | Linux needs several kinds of information to properly configure SPI devices. | |
134 | That information is normally provided by board-specific code, even for | |
135 | chips that do support some of automated discovery/enumeration. | |
136 | ||
137 | DECLARE CONTROLLERS | |
138 | ||
139 | The first kind of information is a list of what SPI controllers exist. | |
140 | For System-on-Chip (SOC) based boards, these will usually be platform | |
141 | devices, and the controller may need some platform_data in order to | |
142 | operate properly. The "struct platform_device" will include resources | |
143 | like the physical address of the controller's first register and its IRQ. | |
144 | ||
145 | Platforms will often abstract the "register SPI controller" operation, | |
146 | maybe coupling it with code to initialize pin configurations, so that | |
147 | the arch/.../mach-*/board-*.c files for several boards can all share the | |
148 | same basic controller setup code. This is because most SOCs have several | |
149 | SPI-capable controllers, and only the ones actually usable on a given | |
150 | board should normally be set up and registered. | |
151 | ||
152 | So for example arch/.../mach-*/board-*.c files might have code like: | |
153 | ||
154 | #include <asm/arch/spi.h> /* for mysoc_spi_data */ | |
155 | ||
156 | /* if your mach-* infrastructure doesn't support kernels that can | |
157 | * run on multiple boards, pdata wouldn't benefit from "__init". | |
158 | */ | |
159 | static struct mysoc_spi_data __init pdata = { ... }; | |
160 | ||
161 | static __init board_init(void) | |
162 | { | |
163 | ... | |
164 | /* this board only uses SPI controller #2 */ | |
165 | mysoc_register_spi(2, &pdata); | |
166 | ... | |
167 | } | |
168 | ||
169 | And SOC-specific utility code might look something like: | |
170 | ||
171 | #include <asm/arch/spi.h> | |
172 | ||
173 | static struct platform_device spi2 = { ... }; | |
174 | ||
175 | void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata) | |
176 | { | |
177 | struct mysoc_spi_data *pdata2; | |
178 | ||
179 | pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL); | |
180 | *pdata2 = pdata; | |
181 | ... | |
182 | if (n == 2) { | |
183 | spi2->dev.platform_data = pdata2; | |
184 | register_platform_device(&spi2); | |
185 | ||
186 | /* also: set up pin modes so the spi2 signals are | |
187 | * visible on the relevant pins ... bootloaders on | |
188 | * production boards may already have done this, but | |
189 | * developer boards will often need Linux to do it. | |
190 | */ | |
191 | } | |
192 | ... | |
193 | } | |
194 | ||
195 | Notice how the platform_data for boards may be different, even if the | |
196 | same SOC controller is used. For example, on one board SPI might use | |
197 | an external clock, where another derives the SPI clock from current | |
198 | settings of some master clock. | |
199 | ||
200 | ||
201 | DECLARE SLAVE DEVICES | |
202 | ||
203 | The second kind of information is a list of what SPI slave devices exist | |
204 | on the target board, often with some board-specific data needed for the | |
205 | driver to work correctly. | |
206 | ||
207 | Normally your arch/.../mach-*/board-*.c files would provide a small table | |
208 | listing the SPI devices on each board. (This would typically be only a | |
209 | small handful.) That might look like: | |
210 | ||
211 | static struct ads7846_platform_data ads_info = { | |
212 | .vref_delay_usecs = 100, | |
213 | .x_plate_ohms = 580, | |
214 | .y_plate_ohms = 410, | |
215 | }; | |
216 | ||
217 | static struct spi_board_info spi_board_info[] __initdata = { | |
218 | { | |
219 | .modalias = "ads7846", | |
220 | .platform_data = &ads_info, | |
221 | .mode = SPI_MODE_0, | |
222 | .irq = GPIO_IRQ(31), | |
223 | .max_speed_hz = 120000 /* max sample rate at 3V */ * 16, | |
224 | .bus_num = 1, | |
225 | .chip_select = 0, | |
226 | }, | |
227 | }; | |
228 | ||
229 | Again, notice how board-specific information is provided; each chip may need | |
230 | several types. This example shows generic constraints like the fastest SPI | |
231 | clock to allow (a function of board voltage in this case) or how an IRQ pin | |
232 | is wired, plus chip-specific constraints like an important delay that's | |
233 | changed by the capacitance at one pin. | |
234 | ||
235 | (There's also "controller_data", information that may be useful to the | |
236 | controller driver. An example would be peripheral-specific DMA tuning | |
237 | data or chipselect callbacks. This is stored in spi_device later.) | |
238 | ||
239 | The board_info should provide enough information to let the system work | |
240 | without the chip's driver being loaded. The most troublesome aspect of | |
241 | that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since | |
242 | sharing a bus with a device that interprets chipselect "backwards" is | |
243 | not possible. | |
244 | ||
245 | Then your board initialization code would register that table with the SPI | |
246 | infrastructure, so that it's available later when the SPI master controller | |
247 | driver is registered: | |
248 | ||
249 | spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info)); | |
250 | ||
251 | Like with other static board-specific setup, you won't unregister those. | |
252 | ||
7111763d DB |
253 | The widely used "card" style computers bundle memory, cpu, and little else |
254 | onto a card that's maybe just thirty square centimeters. On such systems, | |
255 | your arch/.../mach-.../board-*.c file would primarily provide information | |
256 | about the devices on the mainboard into which such a card is plugged. That | |
257 | certainly includes SPI devices hooked up through the card connectors! | |
258 | ||
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259 | |
260 | NON-STATIC CONFIGURATIONS | |
261 | ||
262 | Developer boards often play by different rules than product boards, and one | |
263 | example is the potential need to hotplug SPI devices and/or controllers. | |
264 | ||
265 | For those cases you might need to use use spi_busnum_to_master() to look | |
266 | up the spi bus master, and will likely need spi_new_device() to provide the | |
267 | board info based on the board that was hotplugged. Of course, you'd later | |
268 | call at least spi_unregister_device() when that board is removed. | |
269 | ||
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270 | When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those |
271 | configurations will also be dynamic. Fortunately, those devices all support | |
272 | basic device identification probes, so that support should hotplug normally. | |
273 | ||
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274 | |
275 | How do I write an "SPI Protocol Driver"? | |
276 | ---------------------------------------- | |
277 | All SPI drivers are currently kernel drivers. A userspace driver API | |
278 | would just be another kernel driver, probably offering some lowlevel | |
279 | access through aio_read(), aio_write(), and ioctl() calls and using the | |
280 | standard userspace sysfs mechanisms to bind to a given SPI device. | |
281 | ||
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282 | SPI protocol drivers somewhat resemble platform device drivers: |
283 | ||
284 | static struct spi_driver CHIP_driver = { | |
285 | .driver = { | |
286 | .name = "CHIP", | |
287 | .bus = &spi_bus_type, | |
288 | .owner = THIS_MODULE, | |
289 | }, | |
8ae12a0d | 290 | |
8ae12a0d | 291 | .probe = CHIP_probe, |
b885244e | 292 | .remove = __devexit_p(CHIP_remove), |
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293 | .suspend = CHIP_suspend, |
294 | .resume = CHIP_resume, | |
295 | }; | |
296 | ||
b885244e | 297 | The driver core will autmatically attempt to bind this driver to any SPI |
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298 | device whose board_info gave a modalias of "CHIP". Your probe() code |
299 | might look like this unless you're creating a class_device: | |
300 | ||
b885244e | 301 | static int __devinit CHIP_probe(struct spi_device *spi) |
8ae12a0d | 302 | { |
8ae12a0d | 303 | struct CHIP *chip; |
b885244e DB |
304 | struct CHIP_platform_data *pdata; |
305 | ||
306 | /* assuming the driver requires board-specific data: */ | |
307 | pdata = &spi->dev.platform_data; | |
308 | if (!pdata) | |
309 | return -ENODEV; | |
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310 | |
311 | /* get memory for driver's per-chip state */ | |
312 | chip = kzalloc(sizeof *chip, GFP_KERNEL); | |
313 | if (!chip) | |
314 | return -ENOMEM; | |
b885244e | 315 | dev_set_drvdata(&spi->dev, chip); |
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316 | |
317 | ... etc | |
318 | return 0; | |
319 | } | |
320 | ||
321 | As soon as it enters probe(), the driver may issue I/O requests to | |
322 | the SPI device using "struct spi_message". When remove() returns, | |
323 | the driver guarantees that it won't submit any more such messages. | |
324 | ||
325 | - An spi_message is a sequence of of protocol operations, executed | |
326 | as one atomic sequence. SPI driver controls include: | |
327 | ||
328 | + when bidirectional reads and writes start ... by how its | |
329 | sequence of spi_transfer requests is arranged; | |
330 | ||
331 | + optionally defining short delays after transfers ... using | |
332 | the spi_transfer.delay_usecs setting; | |
333 | ||
334 | + whether the chipselect becomes inactive after a transfer and | |
335 | any delay ... by using the spi_transfer.cs_change flag; | |
336 | ||
337 | + hinting whether the next message is likely to go to this same | |
338 | device ... using the spi_transfer.cs_change flag on the last | |
339 | transfer in that atomic group, and potentially saving costs | |
340 | for chip deselect and select operations. | |
341 | ||
342 | - Follow standard kernel rules, and provide DMA-safe buffers in | |
343 | your messages. That way controller drivers using DMA aren't forced | |
344 | to make extra copies unless the hardware requires it (e.g. working | |
345 | around hardware errata that force the use of bounce buffering). | |
346 | ||
347 | If standard dma_map_single() handling of these buffers is inappropriate, | |
348 | you can use spi_message.is_dma_mapped to tell the controller driver | |
349 | that you've already provided the relevant DMA addresses. | |
350 | ||
351 | - The basic I/O primitive is spi_async(). Async requests may be | |
352 | issued in any context (irq handler, task, etc) and completion | |
353 | is reported using a callback provided with the message. | |
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354 | After any detected error, the chip is deselected and processing |
355 | of that spi_message is aborted. | |
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356 | |
357 | - There are also synchronous wrappers like spi_sync(), and wrappers | |
358 | like spi_read(), spi_write(), and spi_write_then_read(). These | |
359 | may be issued only in contexts that may sleep, and they're all | |
360 | clean (and small, and "optional") layers over spi_async(). | |
361 | ||
362 | - The spi_write_then_read() call, and convenience wrappers around | |
363 | it, should only be used with small amounts of data where the | |
364 | cost of an extra copy may be ignored. It's designed to support | |
365 | common RPC-style requests, such as writing an eight bit command | |
366 | and reading a sixteen bit response -- spi_w8r16() being one its | |
367 | wrappers, doing exactly that. | |
368 | ||
369 | Some drivers may need to modify spi_device characteristics like the | |
370 | transfer mode, wordsize, or clock rate. This is done with spi_setup(), | |
371 | which would normally be called from probe() before the first I/O is | |
372 | done to the device. | |
373 | ||
374 | While "spi_device" would be the bottom boundary of the driver, the | |
375 | upper boundaries might include sysfs (especially for sensor readings), | |
376 | the input layer, ALSA, networking, MTD, the character device framework, | |
377 | or other Linux subsystems. | |
378 | ||
0c868461 DB |
379 | Note that there are two types of memory your driver must manage as part |
380 | of interacting with SPI devices. | |
381 | ||
382 | - I/O buffers use the usual Linux rules, and must be DMA-safe. | |
383 | You'd normally allocate them from the heap or free page pool. | |
384 | Don't use the stack, or anything that's declared "static". | |
385 | ||
386 | - The spi_message and spi_transfer metadata used to glue those | |
387 | I/O buffers into a group of protocol transactions. These can | |
388 | be allocated anywhere it's convenient, including as part of | |
389 | other allocate-once driver data structures. Zero-init these. | |
390 | ||
391 | If you like, spi_message_alloc() and spi_message_free() convenience | |
392 | routines are available to allocate and zero-initialize an spi_message | |
393 | with several transfers. | |
394 | ||
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395 | |
396 | How do I write an "SPI Master Controller Driver"? | |
397 | ------------------------------------------------- | |
398 | An SPI controller will probably be registered on the platform_bus; write | |
399 | a driver to bind to the device, whichever bus is involved. | |
400 | ||
401 | The main task of this type of driver is to provide an "spi_master". | |
402 | Use spi_alloc_master() to allocate the master, and class_get_devdata() | |
403 | to get the driver-private data allocated for that device. | |
404 | ||
405 | struct spi_master *master; | |
406 | struct CONTROLLER *c; | |
407 | ||
408 | master = spi_alloc_master(dev, sizeof *c); | |
409 | if (!master) | |
410 | return -ENODEV; | |
411 | ||
412 | c = class_get_devdata(&master->cdev); | |
413 | ||
414 | The driver will initialize the fields of that spi_master, including the | |
415 | bus number (maybe the same as the platform device ID) and three methods | |
416 | used to interact with the SPI core and SPI protocol drivers. It will | |
a020ed75 DB |
417 | also initialize its own internal state. (See below about bus numbering |
418 | and those methods.) | |
419 | ||
420 | After you initialize the spi_master, then use spi_register_master() to | |
421 | publish it to the rest of the system. At that time, device nodes for | |
422 | the controller and any predeclared spi devices will be made available, | |
423 | and the driver model core will take care of binding them to drivers. | |
424 | ||
425 | If you need to remove your SPI controller driver, spi_unregister_master() | |
426 | will reverse the effect of spi_register_master(). | |
427 | ||
428 | ||
429 | BUS NUMBERING | |
430 | ||
431 | Bus numbering is important, since that's how Linux identifies a given | |
432 | SPI bus (shared SCK, MOSI, MISO). Valid bus numbers start at zero. On | |
433 | SOC systems, the bus numbers should match the numbers defined by the chip | |
434 | manufacturer. For example, hardware controller SPI2 would be bus number 2, | |
435 | and spi_board_info for devices connected to it would use that number. | |
436 | ||
437 | If you don't have such hardware-assigned bus number, and for some reason | |
438 | you can't just assign them, then provide a negative bus number. That will | |
439 | then be replaced by a dynamically assigned number. You'd then need to treat | |
440 | this as a non-static configuration (see above). | |
441 | ||
442 | ||
443 | SPI MASTER METHODS | |
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444 | |
445 | master->setup(struct spi_device *spi) | |
446 | This sets up the device clock rate, SPI mode, and word sizes. | |
447 | Drivers may change the defaults provided by board_info, and then | |
448 | call spi_setup(spi) to invoke this routine. It may sleep. | |
449 | ||
450 | master->transfer(struct spi_device *spi, struct spi_message *message) | |
451 | This must not sleep. Its responsibility is arrange that the | |
452 | transfer happens and its complete() callback is issued; the two | |
453 | will normally happen later, after other transfers complete. | |
454 | ||
455 | master->cleanup(struct spi_device *spi) | |
456 | Your controller driver may use spi_device.controller_state to hold | |
457 | state it dynamically associates with that device. If you do that, | |
458 | be sure to provide the cleanup() method to free that state. | |
459 | ||
a020ed75 DB |
460 | |
461 | SPI MESSAGE QUEUE | |
462 | ||
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463 | The bulk of the driver will be managing the I/O queue fed by transfer(). |
464 | ||
465 | That queue could be purely conceptual. For example, a driver used only | |
466 | for low-frequency sensor acess might be fine using synchronous PIO. | |
467 | ||
468 | But the queue will probably be very real, using message->queue, PIO, | |
469 | often DMA (especially if the root filesystem is in SPI flash), and | |
470 | execution contexts like IRQ handlers, tasklets, or workqueues (such | |
471 | as keventd). Your driver can be as fancy, or as simple, as you need. | |
a020ed75 DB |
472 | Such a transfer() method would normally just add the message to a |
473 | queue, and then start some asynchronous transfer engine (unless it's | |
474 | already running). | |
8ae12a0d DB |
475 | |
476 | ||
477 | THANKS TO | |
478 | --------- | |
479 | Contributors to Linux-SPI discussions include (in alphabetical order, | |
480 | by last name): | |
481 | ||
482 | David Brownell | |
483 | Russell King | |
484 | Dmitry Pervushin | |
485 | Stephen Street | |
486 | Mark Underwood | |
487 | Andrew Victor | |
488 | Vitaly Wool | |
489 |