spi-summary 19 KB

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  1. Overview of Linux kernel SPI support
  2. ====================================
  3. 02-Dec-2005
  4. What is SPI?
  5. ------------
  6. The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
  7. link used to connect microcontrollers to sensors, memory, and peripherals.
  8. The three signal wires hold a clock (SCLK, often on the order of 10 MHz),
  9. and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
  10. Slave Out" (MISO) signals. (Other names are also used.) There are four
  11. clocking modes through which data is exchanged; mode-0 and mode-3 are most
  12. commonly used. Each clock cycle shifts data out and data in; the clock
  13. doesn't cycle except when there is data to shift.
  14. SPI masters may use a "chip select" line to activate a given SPI slave
  15. device, so those three signal wires may be connected to several chips
  16. in parallel. All SPI slaves support chipselects. Some devices have
  17. other signals, often including an interrupt to the master.
  18. Unlike serial busses like USB or SMBUS, even low level protocols for
  19. SPI slave functions are usually not interoperable between vendors
  20. (except for cases like SPI memory chips).
  21. - SPI may be used for request/response style device protocols, as with
  22. touchscreen sensors and memory chips.
  23. - It may also be used to stream data in either direction (half duplex),
  24. or both of them at the same time (full duplex).
  25. - Some devices may use eight bit words. Others may different word
  26. lengths, such as streams of 12-bit or 20-bit digital samples.
  27. In the same way, SPI slaves will only rarely support any kind of automatic
  28. discovery/enumeration protocol. The tree of slave devices accessible from
  29. a given SPI master will normally be set up manually, with configuration
  30. tables.
  31. SPI is only one of the names used by such four-wire protocols, and
  32. most controllers have no problem handling "MicroWire" (think of it as
  33. half-duplex SPI, for request/response protocols), SSP ("Synchronous
  34. Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
  35. related protocols.
  36. Microcontrollers often support both master and slave sides of the SPI
  37. protocol. This document (and Linux) currently only supports the master
  38. side of SPI interactions.
  39. Who uses it? On what kinds of systems?
  40. ---------------------------------------
  41. Linux developers using SPI are probably writing device drivers for embedded
  42. systems boards. SPI is used to control external chips, and it is also a
  43. protocol supported by every MMC or SD memory card. (The older "DataFlash"
  44. cards, predating MMC cards but using the same connectors and card shape,
  45. support only SPI.) Some PC hardware uses SPI flash for BIOS code.
  46. SPI slave chips range from digital/analog converters used for analog
  47. sensors and codecs, to memory, to peripherals like USB controllers
  48. or Ethernet adapters; and more.
  49. Most systems using SPI will integrate a few devices on a mainboard.
  50. Some provide SPI links on expansion connectors; in cases where no
  51. dedicated SPI controller exists, GPIO pins can be used to create a
  52. low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI
  53. controller; the reasons to use SPI focus on low cost and simple operation,
  54. and if dynamic reconfiguration is important, USB will often be a more
  55. appropriate low-pincount peripheral bus.
  56. Many microcontrollers that can run Linux integrate one or more I/O
  57. interfaces with SPI modes. Given SPI support, they could use MMC or SD
  58. cards without needing a special purpose MMC/SD/SDIO controller.
  59. How do these driver programming interfaces work?
  60. ------------------------------------------------
  61. The <linux/spi/spi.h> header file includes kerneldoc, as does the
  62. main source code, and you should certainly read that. This is just
  63. an overview, so you get the big picture before the details.
  64. SPI requests always go into I/O queues. Requests for a given SPI device
  65. are always executed in FIFO order, and complete asynchronously through
  66. completion callbacks. There are also some simple synchronous wrappers
  67. for those calls, including ones for common transaction types like writing
  68. a command and then reading its response.
  69. There are two types of SPI driver, here called:
  70. Controller drivers ... these are often built in to System-On-Chip
  71. processors, and often support both Master and Slave roles.
  72. These drivers touch hardware registers and may use DMA.
  73. Or they can be PIO bitbangers, needing just GPIO pins.
  74. Protocol drivers ... these pass messages through the controller
  75. driver to communicate with a Slave or Master device on the
  76. other side of an SPI link.
  77. So for example one protocol driver might talk to the MTD layer to export
  78. data to filesystems stored on SPI flash like DataFlash; and others might
  79. control audio interfaces, present touchscreen sensors as input interfaces,
  80. or monitor temperature and voltage levels during industrial processing.
  81. And those might all be sharing the same controller driver.
  82. A "struct spi_device" encapsulates the master-side interface between
  83. those two types of driver. At this writing, Linux has no slave side
  84. programming interface.
  85. There is a minimal core of SPI programming interfaces, focussing on
  86. using driver model to connect controller and protocol drivers using
  87. device tables provided by board specific initialization code. SPI
  88. shows up in sysfs in several locations:
  89. /sys/devices/.../CTLR/spiB.C ... spi_device for on bus "B",
  90. chipselect C, accessed through CTLR.
  91. /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver
  92. that should be used with this device (for hotplug/coldplug)
  93. /sys/bus/spi/devices/spiB.C ... symlink to the physical
  94. spiB-C device
  95. /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
  96. /sys/class/spi_master/spiB ... class device for the controller
  97. managing bus "B". All the spiB.* devices share the same
  98. physical SPI bus segment, with SCLK, MOSI, and MISO.
  99. How does board-specific init code declare SPI devices?
  100. ------------------------------------------------------
  101. Linux needs several kinds of information to properly configure SPI devices.
  102. That information is normally provided by board-specific code, even for
  103. chips that do support some of automated discovery/enumeration.
  104. DECLARE CONTROLLERS
  105. The first kind of information is a list of what SPI controllers exist.
  106. For System-on-Chip (SOC) based boards, these will usually be platform
  107. devices, and the controller may need some platform_data in order to
  108. operate properly. The "struct platform_device" will include resources
  109. like the physical address of the controller's first register and its IRQ.
  110. Platforms will often abstract the "register SPI controller" operation,
  111. maybe coupling it with code to initialize pin configurations, so that
  112. the arch/.../mach-*/board-*.c files for several boards can all share the
  113. same basic controller setup code. This is because most SOCs have several
  114. SPI-capable controllers, and only the ones actually usable on a given
  115. board should normally be set up and registered.
  116. So for example arch/.../mach-*/board-*.c files might have code like:
  117. #include <asm/arch/spi.h> /* for mysoc_spi_data */
  118. /* if your mach-* infrastructure doesn't support kernels that can
  119. * run on multiple boards, pdata wouldn't benefit from "__init".
  120. */
  121. static struct mysoc_spi_data __init pdata = { ... };
  122. static __init board_init(void)
  123. {
  124. ...
  125. /* this board only uses SPI controller #2 */
  126. mysoc_register_spi(2, &pdata);
  127. ...
  128. }
  129. And SOC-specific utility code might look something like:
  130. #include <asm/arch/spi.h>
  131. static struct platform_device spi2 = { ... };
  132. void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
  133. {
  134. struct mysoc_spi_data *pdata2;
  135. pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
  136. *pdata2 = pdata;
  137. ...
  138. if (n == 2) {
  139. spi2->dev.platform_data = pdata2;
  140. register_platform_device(&spi2);
  141. /* also: set up pin modes so the spi2 signals are
  142. * visible on the relevant pins ... bootloaders on
  143. * production boards may already have done this, but
  144. * developer boards will often need Linux to do it.
  145. */
  146. }
  147. ...
  148. }
  149. Notice how the platform_data for boards may be different, even if the
  150. same SOC controller is used. For example, on one board SPI might use
  151. an external clock, where another derives the SPI clock from current
  152. settings of some master clock.
  153. DECLARE SLAVE DEVICES
  154. The second kind of information is a list of what SPI slave devices exist
  155. on the target board, often with some board-specific data needed for the
  156. driver to work correctly.
  157. Normally your arch/.../mach-*/board-*.c files would provide a small table
  158. listing the SPI devices on each board. (This would typically be only a
  159. small handful.) That might look like:
  160. static struct ads7846_platform_data ads_info = {
  161. .vref_delay_usecs = 100,
  162. .x_plate_ohms = 580,
  163. .y_plate_ohms = 410,
  164. };
  165. static struct spi_board_info spi_board_info[] __initdata = {
  166. {
  167. .modalias = "ads7846",
  168. .platform_data = &ads_info,
  169. .mode = SPI_MODE_0,
  170. .irq = GPIO_IRQ(31),
  171. .max_speed_hz = 120000 /* max sample rate at 3V */ * 16,
  172. .bus_num = 1,
  173. .chip_select = 0,
  174. },
  175. };
  176. Again, notice how board-specific information is provided; each chip may need
  177. several types. This example shows generic constraints like the fastest SPI
  178. clock to allow (a function of board voltage in this case) or how an IRQ pin
  179. is wired, plus chip-specific constraints like an important delay that's
  180. changed by the capacitance at one pin.
  181. (There's also "controller_data", information that may be useful to the
  182. controller driver. An example would be peripheral-specific DMA tuning
  183. data or chipselect callbacks. This is stored in spi_device later.)
  184. The board_info should provide enough information to let the system work
  185. without the chip's driver being loaded. The most troublesome aspect of
  186. that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
  187. sharing a bus with a device that interprets chipselect "backwards" is
  188. not possible.
  189. Then your board initialization code would register that table with the SPI
  190. infrastructure, so that it's available later when the SPI master controller
  191. driver is registered:
  192. spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
  193. Like with other static board-specific setup, you won't unregister those.
  194. The widely used "card" style computers bundle memory, cpu, and little else
  195. onto a card that's maybe just thirty square centimeters. On such systems,
  196. your arch/.../mach-.../board-*.c file would primarily provide information
  197. about the devices on the mainboard into which such a card is plugged. That
  198. certainly includes SPI devices hooked up through the card connectors!
  199. NON-STATIC CONFIGURATIONS
  200. Developer boards often play by different rules than product boards, and one
  201. example is the potential need to hotplug SPI devices and/or controllers.
  202. For those cases you might need to use spi_busnum_to_master() to look
  203. up the spi bus master, and will likely need spi_new_device() to provide the
  204. board info based on the board that was hotplugged. Of course, you'd later
  205. call at least spi_unregister_device() when that board is removed.
  206. When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those
  207. configurations will also be dynamic. Fortunately, those devices all support
  208. basic device identification probes, so that support should hotplug normally.
  209. How do I write an "SPI Protocol Driver"?
  210. ----------------------------------------
  211. All SPI drivers are currently kernel drivers. A userspace driver API
  212. would just be another kernel driver, probably offering some lowlevel
  213. access through aio_read(), aio_write(), and ioctl() calls and using the
  214. standard userspace sysfs mechanisms to bind to a given SPI device.
  215. SPI protocol drivers somewhat resemble platform device drivers:
  216. static struct spi_driver CHIP_driver = {
  217. .driver = {
  218. .name = "CHIP",
  219. .owner = THIS_MODULE,
  220. },
  221. .probe = CHIP_probe,
  222. .remove = __devexit_p(CHIP_remove),
  223. .suspend = CHIP_suspend,
  224. .resume = CHIP_resume,
  225. };
  226. The driver core will autmatically attempt to bind this driver to any SPI
  227. device whose board_info gave a modalias of "CHIP". Your probe() code
  228. might look like this unless you're creating a class_device:
  229. static int __devinit CHIP_probe(struct spi_device *spi)
  230. {
  231. struct CHIP *chip;
  232. struct CHIP_platform_data *pdata;
  233. /* assuming the driver requires board-specific data: */
  234. pdata = &spi->dev.platform_data;
  235. if (!pdata)
  236. return -ENODEV;
  237. /* get memory for driver's per-chip state */
  238. chip = kzalloc(sizeof *chip, GFP_KERNEL);
  239. if (!chip)
  240. return -ENOMEM;
  241. spi_set_drvdata(spi, chip);
  242. ... etc
  243. return 0;
  244. }
  245. As soon as it enters probe(), the driver may issue I/O requests to
  246. the SPI device using "struct spi_message". When remove() returns,
  247. the driver guarantees that it won't submit any more such messages.
  248. - An spi_message is a sequence of protocol operations, executed
  249. as one atomic sequence. SPI driver controls include:
  250. + when bidirectional reads and writes start ... by how its
  251. sequence of spi_transfer requests is arranged;
  252. + optionally defining short delays after transfers ... using
  253. the spi_transfer.delay_usecs setting;
  254. + whether the chipselect becomes inactive after a transfer and
  255. any delay ... by using the spi_transfer.cs_change flag;
  256. + hinting whether the next message is likely to go to this same
  257. device ... using the spi_transfer.cs_change flag on the last
  258. transfer in that atomic group, and potentially saving costs
  259. for chip deselect and select operations.
  260. - Follow standard kernel rules, and provide DMA-safe buffers in
  261. your messages. That way controller drivers using DMA aren't forced
  262. to make extra copies unless the hardware requires it (e.g. working
  263. around hardware errata that force the use of bounce buffering).
  264. If standard dma_map_single() handling of these buffers is inappropriate,
  265. you can use spi_message.is_dma_mapped to tell the controller driver
  266. that you've already provided the relevant DMA addresses.
  267. - The basic I/O primitive is spi_async(). Async requests may be
  268. issued in any context (irq handler, task, etc) and completion
  269. is reported using a callback provided with the message.
  270. After any detected error, the chip is deselected and processing
  271. of that spi_message is aborted.
  272. - There are also synchronous wrappers like spi_sync(), and wrappers
  273. like spi_read(), spi_write(), and spi_write_then_read(). These
  274. may be issued only in contexts that may sleep, and they're all
  275. clean (and small, and "optional") layers over spi_async().
  276. - The spi_write_then_read() call, and convenience wrappers around
  277. it, should only be used with small amounts of data where the
  278. cost of an extra copy may be ignored. It's designed to support
  279. common RPC-style requests, such as writing an eight bit command
  280. and reading a sixteen bit response -- spi_w8r16() being one its
  281. wrappers, doing exactly that.
  282. Some drivers may need to modify spi_device characteristics like the
  283. transfer mode, wordsize, or clock rate. This is done with spi_setup(),
  284. which would normally be called from probe() before the first I/O is
  285. done to the device.
  286. While "spi_device" would be the bottom boundary of the driver, the
  287. upper boundaries might include sysfs (especially for sensor readings),
  288. the input layer, ALSA, networking, MTD, the character device framework,
  289. or other Linux subsystems.
  290. Note that there are two types of memory your driver must manage as part
  291. of interacting with SPI devices.
  292. - I/O buffers use the usual Linux rules, and must be DMA-safe.
  293. You'd normally allocate them from the heap or free page pool.
  294. Don't use the stack, or anything that's declared "static".
  295. - The spi_message and spi_transfer metadata used to glue those
  296. I/O buffers into a group of protocol transactions. These can
  297. be allocated anywhere it's convenient, including as part of
  298. other allocate-once driver data structures. Zero-init these.
  299. If you like, spi_message_alloc() and spi_message_free() convenience
  300. routines are available to allocate and zero-initialize an spi_message
  301. with several transfers.
  302. How do I write an "SPI Master Controller Driver"?
  303. -------------------------------------------------
  304. An SPI controller will probably be registered on the platform_bus; write
  305. a driver to bind to the device, whichever bus is involved.
  306. The main task of this type of driver is to provide an "spi_master".
  307. Use spi_alloc_master() to allocate the master, and class_get_devdata()
  308. to get the driver-private data allocated for that device.
  309. struct spi_master *master;
  310. struct CONTROLLER *c;
  311. master = spi_alloc_master(dev, sizeof *c);
  312. if (!master)
  313. return -ENODEV;
  314. c = class_get_devdata(&master->cdev);
  315. The driver will initialize the fields of that spi_master, including the
  316. bus number (maybe the same as the platform device ID) and three methods
  317. used to interact with the SPI core and SPI protocol drivers. It will
  318. also initialize its own internal state. (See below about bus numbering
  319. and those methods.)
  320. After you initialize the spi_master, then use spi_register_master() to
  321. publish it to the rest of the system. At that time, device nodes for
  322. the controller and any predeclared spi devices will be made available,
  323. and the driver model core will take care of binding them to drivers.
  324. If you need to remove your SPI controller driver, spi_unregister_master()
  325. will reverse the effect of spi_register_master().
  326. BUS NUMBERING
  327. Bus numbering is important, since that's how Linux identifies a given
  328. SPI bus (shared SCK, MOSI, MISO). Valid bus numbers start at zero. On
  329. SOC systems, the bus numbers should match the numbers defined by the chip
  330. manufacturer. For example, hardware controller SPI2 would be bus number 2,
  331. and spi_board_info for devices connected to it would use that number.
  332. If you don't have such hardware-assigned bus number, and for some reason
  333. you can't just assign them, then provide a negative bus number. That will
  334. then be replaced by a dynamically assigned number. You'd then need to treat
  335. this as a non-static configuration (see above).
  336. SPI MASTER METHODS
  337. master->setup(struct spi_device *spi)
  338. This sets up the device clock rate, SPI mode, and word sizes.
  339. Drivers may change the defaults provided by board_info, and then
  340. call spi_setup(spi) to invoke this routine. It may sleep.
  341. master->transfer(struct spi_device *spi, struct spi_message *message)
  342. This must not sleep. Its responsibility is arrange that the
  343. transfer happens and its complete() callback is issued; the two
  344. will normally happen later, after other transfers complete.
  345. master->cleanup(struct spi_device *spi)
  346. Your controller driver may use spi_device.controller_state to hold
  347. state it dynamically associates with that device. If you do that,
  348. be sure to provide the cleanup() method to free that state.
  349. SPI MESSAGE QUEUE
  350. The bulk of the driver will be managing the I/O queue fed by transfer().
  351. That queue could be purely conceptual. For example, a driver used only
  352. for low-frequency sensor acess might be fine using synchronous PIO.
  353. But the queue will probably be very real, using message->queue, PIO,
  354. often DMA (especially if the root filesystem is in SPI flash), and
  355. execution contexts like IRQ handlers, tasklets, or workqueues (such
  356. as keventd). Your driver can be as fancy, or as simple, as you need.
  357. Such a transfer() method would normally just add the message to a
  358. queue, and then start some asynchronous transfer engine (unless it's
  359. already running).
  360. THANKS TO
  361. ---------
  362. Contributors to Linux-SPI discussions include (in alphabetical order,
  363. by last name):
  364. David Brownell
  365. Russell King
  366. Dmitry Pervushin
  367. Stephen Street
  368. Mark Underwood
  369. Andrew Victor
  370. Vitaly Wool