1 ==================================== 1 ====================================
2 Overview of Linux kernel SPI support 2 Overview of Linux kernel SPI support
3 ==================================== 3 ====================================
4 4
5 02-Feb-2012 5 02-Feb-2012
6 6
7 What is SPI? 7 What is SPI?
8 ------------ 8 ------------
9 The "Serial Peripheral Interface" (SPI) is a s 9 The "Serial Peripheral Interface" (SPI) is a synchronous four wire serial
10 link used to connect microcontrollers to senso 10 link used to connect microcontrollers to sensors, memory, and peripherals.
11 It's a simple "de facto" standard, not complic 11 It's a simple "de facto" standard, not complicated enough to acquire a
12 standardization body. SPI uses a host/target !! 12 standardization body. SPI uses a master/slave configuration.
13 13
14 The three signal wires hold a clock (SCK, ofte 14 The three signal wires hold a clock (SCK, often on the order of 10 MHz),
15 and parallel data lines with "Master Out, Slav 15 and parallel data lines with "Master Out, Slave In" (MOSI) or "Master In,
16 Slave Out" (MISO) signals. (Other names are a 16 Slave Out" (MISO) signals. (Other names are also used.) There are four
17 clocking modes through which data is exchanged 17 clocking modes through which data is exchanged; mode-0 and mode-3 are most
18 commonly used. Each clock cycle shifts data o 18 commonly used. Each clock cycle shifts data out and data in; the clock
19 doesn't cycle except when there is a data bit 19 doesn't cycle except when there is a data bit to shift. Not all data bits
20 are used though; not every protocol uses those 20 are used though; not every protocol uses those full duplex capabilities.
21 21
22 SPI hosts use a fourth "chip select" line to a !! 22 SPI masters use a fourth "chip select" line to activate a given SPI slave
23 device, so those three signal wires may be con 23 device, so those three signal wires may be connected to several chips
24 in parallel. All SPI targets support chipsele !! 24 in parallel. All SPI slaves support chipselects; they are usually active
25 low signals, labeled nCSx for target 'x' (e.g. !! 25 low signals, labeled nCSx for slave 'x' (e.g. nCS0). Some devices have
26 other signals, often including an interrupt to !! 26 other signals, often including an interrupt to the master.
27 27
28 Unlike serial busses like USB or SMBus, even l 28 Unlike serial busses like USB or SMBus, even low level protocols for
29 SPI target functions are usually not interoper !! 29 SPI slave functions are usually not interoperable between vendors
30 (except for commodities like SPI memory chips) 30 (except for commodities like SPI memory chips).
31 31
32 - SPI may be used for request/response style 32 - SPI may be used for request/response style device protocols, as with
33 touchscreen sensors and memory chips. 33 touchscreen sensors and memory chips.
34 34
35 - It may also be used to stream data in eith 35 - It may also be used to stream data in either direction (half duplex),
36 or both of them at the same time (full dup 36 or both of them at the same time (full duplex).
37 37
38 - Some devices may use eight bit words. Oth 38 - Some devices may use eight bit words. Others may use different word
39 lengths, such as streams of 12-bit or 20-b 39 lengths, such as streams of 12-bit or 20-bit digital samples.
40 40
41 - Words are usually sent with their most sig 41 - Words are usually sent with their most significant bit (MSB) first,
42 but sometimes the least significant bit (L 42 but sometimes the least significant bit (LSB) goes first instead.
43 43
44 - Sometimes SPI is used to daisy-chain devic 44 - Sometimes SPI is used to daisy-chain devices, like shift registers.
45 45
46 In the same way, SPI targets will only rarely !! 46 In the same way, SPI slaves will only rarely support any kind of automatic
47 discovery/enumeration protocol. The tree of ta !! 47 discovery/enumeration protocol. The tree of slave devices accessible from
48 a given SPI host controller will normally be s !! 48 a given SPI master will normally be set up manually, with configuration
49 configuration tables. !! 49 tables.
50 50
51 SPI is only one of the names used by such four 51 SPI is only one of the names used by such four-wire protocols, and
52 most controllers have no problem handling "Mic 52 most controllers have no problem handling "MicroWire" (think of it as
53 half-duplex SPI, for request/response protocol 53 half-duplex SPI, for request/response protocols), SSP ("Synchronous
54 Serial Protocol"), PSP ("Programmable Serial P 54 Serial Protocol"), PSP ("Programmable Serial Protocol"), and other
55 related protocols. 55 related protocols.
56 56
57 Some chips eliminate a signal line by combinin 57 Some chips eliminate a signal line by combining MOSI and MISO, and
58 limiting themselves to half-duplex at the hard 58 limiting themselves to half-duplex at the hardware level. In fact
59 some SPI chips have this signal mode as a stra 59 some SPI chips have this signal mode as a strapping option. These
60 can be accessed using the same programming int 60 can be accessed using the same programming interface as SPI, but of
61 course they won't handle full duplex transfers 61 course they won't handle full duplex transfers. You may find such
62 chips described as using "three wire" signalin 62 chips described as using "three wire" signaling: SCK, data, nCSx.
63 (That data line is sometimes called MOMI or SI 63 (That data line is sometimes called MOMI or SISO.)
64 64
65 Microcontrollers often support both host and t !! 65 Microcontrollers often support both master and slave sides of the SPI
66 protocol. This document (and Linux) supports !! 66 protocol. This document (and Linux) supports both the master and slave
67 sides of SPI interactions. 67 sides of SPI interactions.
68 68
69 69
70 Who uses it? On what kinds of systems? 70 Who uses it? On what kinds of systems?
71 --------------------------------------- 71 ---------------------------------------
72 Linux developers using SPI are probably writin 72 Linux developers using SPI are probably writing device drivers for embedded
73 systems boards. SPI is used to control extern 73 systems boards. SPI is used to control external chips, and it is also a
74 protocol supported by every MMC or SD memory c 74 protocol supported by every MMC or SD memory card. (The older "DataFlash"
75 cards, predating MMC cards but using the same 75 cards, predating MMC cards but using the same connectors and card shape,
76 support only SPI.) Some PC hardware uses SPI 76 support only SPI.) Some PC hardware uses SPI flash for BIOS code.
77 77
78 SPI target chips range from digital/analog con !! 78 SPI slave chips range from digital/analog converters used for analog
79 sensors and codecs, to memory, to peripherals 79 sensors and codecs, to memory, to peripherals like USB controllers
80 or Ethernet adapters; and more. 80 or Ethernet adapters; and more.
81 81
82 Most systems using SPI will integrate a few de 82 Most systems using SPI will integrate a few devices on a mainboard.
83 Some provide SPI links on expansion connectors 83 Some provide SPI links on expansion connectors; in cases where no
84 dedicated SPI controller exists, GPIO pins can 84 dedicated SPI controller exists, GPIO pins can be used to create a
85 low speed "bitbanging" adapter. Very few syst 85 low speed "bitbanging" adapter. Very few systems will "hotplug" an SPI
86 controller; the reasons to use SPI focus on lo 86 controller; the reasons to use SPI focus on low cost and simple operation,
87 and if dynamic reconfiguration is important, U 87 and if dynamic reconfiguration is important, USB will often be a more
88 appropriate low-pincount peripheral bus. 88 appropriate low-pincount peripheral bus.
89 89
90 Many microcontrollers that can run Linux integ 90 Many microcontrollers that can run Linux integrate one or more I/O
91 interfaces with SPI modes. Given SPI support, 91 interfaces with SPI modes. Given SPI support, they could use MMC or SD
92 cards without needing a special purpose MMC/SD 92 cards without needing a special purpose MMC/SD/SDIO controller.
93 93
94 94
95 I'm confused. What are these four SPI "clock 95 I'm confused. What are these four SPI "clock modes"?
96 ---------------------------------------------- 96 -----------------------------------------------------
97 It's easy to be confused here, and the vendor 97 It's easy to be confused here, and the vendor documentation you'll
98 find isn't necessarily helpful. The four mode 98 find isn't necessarily helpful. The four modes combine two mode bits:
99 99
100 - CPOL indicates the initial clock polarity. 100 - CPOL indicates the initial clock polarity. CPOL=0 means the
101 clock starts low, so the first (leading) ed 101 clock starts low, so the first (leading) edge is rising, and
102 the second (trailing) edge is falling. CPO 102 the second (trailing) edge is falling. CPOL=1 means the clock
103 starts high, so the first (leading) edge is 103 starts high, so the first (leading) edge is falling.
104 104
105 - CPHA indicates the clock phase used to samp 105 - CPHA indicates the clock phase used to sample data; CPHA=0 says
106 sample on the leading edge, CPHA=1 means th 106 sample on the leading edge, CPHA=1 means the trailing edge.
107 107
108 Since the signal needs to stabilize before 108 Since the signal needs to stabilize before it's sampled, CPHA=0
109 implies that its data is written half a clo 109 implies that its data is written half a clock before the first
110 clock edge. The chipselect may have made i 110 clock edge. The chipselect may have made it become available.
111 111
112 Chip specs won't always say "uses SPI mode X" 112 Chip specs won't always say "uses SPI mode X" in as many words,
113 but their timing diagrams will make the CPOL a 113 but their timing diagrams will make the CPOL and CPHA modes clear.
114 114
115 In the SPI mode number, CPOL is the high order 115 In the SPI mode number, CPOL is the high order bit and CPHA is the
116 low order bit. So when a chip's timing diagra 116 low order bit. So when a chip's timing diagram shows the clock
117 starting low (CPOL=0) and data stabilized for 117 starting low (CPOL=0) and data stabilized for sampling during the
118 trailing clock edge (CPHA=1), that's SPI mode 118 trailing clock edge (CPHA=1), that's SPI mode 1.
119 119
120 Note that the clock mode is relevant as soon a 120 Note that the clock mode is relevant as soon as the chipselect goes
121 active. So the host must set the clock to ina !! 121 active. So the master must set the clock to inactive before selecting
122 a target, and the target can tell the chosen p !! 122 a slave, and the slave can tell the chosen polarity by sampling the
123 clock level when its select line goes active. 123 clock level when its select line goes active. That's why many devices
124 support for example both modes 0 and 3: they 124 support for example both modes 0 and 3: they don't care about polarity,
125 and always clock data in/out on rising clock e 125 and always clock data in/out on rising clock edges.
126 126
127 127
128 How do these driver programming interfaces wor 128 How do these driver programming interfaces work?
129 ---------------------------------------------- 129 ------------------------------------------------
130 The <linux/spi/spi.h> header file includes ker 130 The <linux/spi/spi.h> header file includes kerneldoc, as does the
131 main source code, and you should certainly rea 131 main source code, and you should certainly read that chapter of the
132 kernel API document. This is just an overview 132 kernel API document. This is just an overview, so you get the big
133 picture before those details. 133 picture before those details.
134 134
135 SPI requests always go into I/O queues. Reque 135 SPI requests always go into I/O queues. Requests for a given SPI device
136 are always executed in FIFO order, and complet 136 are always executed in FIFO order, and complete asynchronously through
137 completion callbacks. There are also some sim 137 completion callbacks. There are also some simple synchronous wrappers
138 for those calls, including ones for common tra 138 for those calls, including ones for common transaction types like writing
139 a command and then reading its response. 139 a command and then reading its response.
140 140
141 There are two types of SPI driver, here called 141 There are two types of SPI driver, here called:
142 142
143 Controller drivers ... 143 Controller drivers ...
144 controllers may be built into System-O 144 controllers may be built into System-On-Chip
145 processors, and often support both Con !! 145 processors, and often support both Master and Slave roles.
146 These drivers touch hardware registers 146 These drivers touch hardware registers and may use DMA.
147 Or they can be PIO bitbangers, needing 147 Or they can be PIO bitbangers, needing just GPIO pins.
148 148
149 Protocol drivers ... 149 Protocol drivers ...
150 these pass messages through the contro 150 these pass messages through the controller
151 driver to communicate with a target or !! 151 driver to communicate with a Slave or Master device on the
152 other side of an SPI link. 152 other side of an SPI link.
153 153
154 So for example one protocol driver might talk 154 So for example one protocol driver might talk to the MTD layer to export
155 data to filesystems stored on SPI flash like D 155 data to filesystems stored on SPI flash like DataFlash; and others might
156 control audio interfaces, present touchscreen 156 control audio interfaces, present touchscreen sensors as input interfaces,
157 or monitor temperature and voltage levels duri 157 or monitor temperature and voltage levels during industrial processing.
158 And those might all be sharing the same contro 158 And those might all be sharing the same controller driver.
159 159
160 A "struct spi_device" encapsulates the control 160 A "struct spi_device" encapsulates the controller-side interface between
161 those two types of drivers. 161 those two types of drivers.
162 162
163 There is a minimal core of SPI programming int 163 There is a minimal core of SPI programming interfaces, focussing on
164 using the driver model to connect controller a 164 using the driver model to connect controller and protocol drivers using
165 device tables provided by board specific initi 165 device tables provided by board specific initialization code. SPI
166 shows up in sysfs in several locations:: 166 shows up in sysfs in several locations::
167 167
168 /sys/devices/.../CTLR ... physical node for 168 /sys/devices/.../CTLR ... physical node for a given SPI controller
169 169
170 /sys/devices/.../CTLR/spiB.C ... spi_device 170 /sys/devices/.../CTLR/spiB.C ... spi_device on bus "B",
171 chipselect C, accessed through CTLR. 171 chipselect C, accessed through CTLR.
172 172
173 /sys/bus/spi/devices/spiB.C ... symlink to 173 /sys/bus/spi/devices/spiB.C ... symlink to that physical
174 .../CTLR/spiB.C device 174 .../CTLR/spiB.C device
175 175
176 /sys/devices/.../CTLR/spiB.C/modalias ... i 176 /sys/devices/.../CTLR/spiB.C/modalias ... identifies the driver
177 that should be used with this device ( 177 that should be used with this device (for hotplug/coldplug)
178 178
179 /sys/bus/spi/drivers/D ... driver for one o 179 /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
180 180
181 /sys/class/spi_master/spiB ... symlink to a !! 181 /sys/class/spi_master/spiB ... symlink (or actual device node) to
182 class related state for the SPI host c !! 182 a logical node which could hold class related state for the SPI
183 All spiB.* devices share one physical !! 183 master controller managing bus "B". All spiB.* devices share one
184 MOSI, and MISO. !! 184 physical SPI bus segment, with SCLK, MOSI, and MISO.
185 185
186 /sys/devices/.../CTLR/slave ... virtual fil 186 /sys/devices/.../CTLR/slave ... virtual file for (un)registering the
187 target device for an SPI target contro !! 187 slave device for an SPI slave controller.
188 Writing the driver name of an SPI targ !! 188 Writing the driver name of an SPI slave handler to this file
189 registers the target device; writing " !! 189 registers the slave device; writing "(null)" unregisters the slave
190 device. 190 device.
191 Reading from this file shows the name !! 191 Reading from this file shows the name of the slave device ("(null)"
192 if not registered). 192 if not registered).
193 193
194 /sys/class/spi_slave/spiB ... symlink to a !! 194 /sys/class/spi_slave/spiB ... symlink (or actual device node) to
195 class related state for the SPI target !! 195 a logical node which could hold class related state for the SPI
196 registered, a single spiB.* device is !! 196 slave controller on bus "B". When registered, a single spiB.*
197 the physical SPI bus segment with othe !! 197 device is present here, possible sharing the physical SPI bus
198 !! 198 segment with other SPI slave devices.
199 At this time, the only class-specific state is !! 199
200 so those /sys/class entries are only useful to !! 200 Note that the actual location of the controller's class state depends
>> 201 on whether you enabled CONFIG_SYSFS_DEPRECATED or not. At this time,
>> 202 the only class-specific state is the bus number ("B" in "spiB"), so
>> 203 those /sys/class entries are only useful to quickly identify busses.
201 204
202 205
203 How does board-specific init code declare SPI 206 How does board-specific init code declare SPI devices?
204 ---------------------------------------------- 207 ------------------------------------------------------
205 Linux needs several kinds of information to pr 208 Linux needs several kinds of information to properly configure SPI devices.
206 That information is normally provided by board 209 That information is normally provided by board-specific code, even for
207 chips that do support some of automated discov 210 chips that do support some of automated discovery/enumeration.
208 211
209 Declare Controllers 212 Declare Controllers
210 ^^^^^^^^^^^^^^^^^^^ 213 ^^^^^^^^^^^^^^^^^^^
211 214
212 The first kind of information is a list of wha 215 The first kind of information is a list of what SPI controllers exist.
213 For System-on-Chip (SOC) based boards, these w 216 For System-on-Chip (SOC) based boards, these will usually be platform
214 devices, and the controller may need some plat 217 devices, and the controller may need some platform_data in order to
215 operate properly. The "struct platform_device 218 operate properly. The "struct platform_device" will include resources
216 like the physical address of the controller's 219 like the physical address of the controller's first register and its IRQ.
217 220
218 Platforms will often abstract the "register SP 221 Platforms will often abstract the "register SPI controller" operation,
219 maybe coupling it with code to initialize pin 222 maybe coupling it with code to initialize pin configurations, so that
220 the arch/.../mach-*/board-*.c files for severa 223 the arch/.../mach-*/board-*.c files for several boards can all share the
221 same basic controller setup code. This is bec 224 same basic controller setup code. This is because most SOCs have several
222 SPI-capable controllers, and only the ones act 225 SPI-capable controllers, and only the ones actually usable on a given
223 board should normally be set up and registered 226 board should normally be set up and registered.
224 227
225 So for example arch/.../mach-*/board-*.c files 228 So for example arch/.../mach-*/board-*.c files might have code like::
226 229
227 #include <mach/spi.h> /* for mysoc_s 230 #include <mach/spi.h> /* for mysoc_spi_data */
228 231
229 /* if your mach-* infrastructure doesn 232 /* if your mach-* infrastructure doesn't support kernels that can
230 * run on multiple boards, pdata would 233 * run on multiple boards, pdata wouldn't benefit from "__init".
231 */ 234 */
232 static struct mysoc_spi_data pdata __i 235 static struct mysoc_spi_data pdata __initdata = { ... };
233 236
234 static __init board_init(void) 237 static __init board_init(void)
235 { 238 {
236 ... 239 ...
237 /* this board only uses SPI co 240 /* this board only uses SPI controller #2 */
238 mysoc_register_spi(2, &pdata); 241 mysoc_register_spi(2, &pdata);
239 ... 242 ...
240 } 243 }
241 244
242 And SOC-specific utility code might look somet 245 And SOC-specific utility code might look something like::
243 246
244 #include <mach/spi.h> 247 #include <mach/spi.h>
245 248
246 static struct platform_device spi2 = { 249 static struct platform_device spi2 = { ... };
247 250
248 void mysoc_register_spi(unsigned n, st 251 void mysoc_register_spi(unsigned n, struct mysoc_spi_data *pdata)
249 { 252 {
250 struct mysoc_spi_data *pdata2; 253 struct mysoc_spi_data *pdata2;
251 254
252 pdata2 = kmalloc(sizeof *pdata 255 pdata2 = kmalloc(sizeof *pdata2, GFP_KERNEL);
253 *pdata2 = pdata; 256 *pdata2 = pdata;
254 ... 257 ...
255 if (n == 2) { 258 if (n == 2) {
256 spi2->dev.platform_dat 259 spi2->dev.platform_data = pdata2;
257 register_platform_devi 260 register_platform_device(&spi2);
258 261
259 /* also: set up pin mo 262 /* also: set up pin modes so the spi2 signals are
260 * visible on the rele 263 * visible on the relevant pins ... bootloaders on
261 * production boards m 264 * production boards may already have done this, but
262 * developer boards wi 265 * developer boards will often need Linux to do it.
263 */ 266 */
264 } 267 }
265 ... 268 ...
266 } 269 }
267 270
268 Notice how the platform_data for boards may be 271 Notice how the platform_data for boards may be different, even if the
269 same SOC controller is used. For example, on 272 same SOC controller is used. For example, on one board SPI might use
270 an external clock, where another derives the S 273 an external clock, where another derives the SPI clock from current
271 settings of some master clock. 274 settings of some master clock.
272 275
273 Declare target Devices !! 276 Declare Slave Devices
274 ^^^^^^^^^^^^^^^^^^^^^^ !! 277 ^^^^^^^^^^^^^^^^^^^^^
275 278
276 The second kind of information is a list of wh !! 279 The second kind of information is a list of what SPI slave devices exist
277 on the target board, often with some board-spe 280 on the target board, often with some board-specific data needed for the
278 driver to work correctly. 281 driver to work correctly.
279 282
280 Normally your arch/.../mach-*/board-*.c files 283 Normally your arch/.../mach-*/board-*.c files would provide a small table
281 listing the SPI devices on each board. (This 284 listing the SPI devices on each board. (This would typically be only a
282 small handful.) That might look like:: 285 small handful.) That might look like::
283 286
284 static struct ads7846_platform_data ad 287 static struct ads7846_platform_data ads_info = {
285 .vref_delay_usecs = 100, 288 .vref_delay_usecs = 100,
286 .x_plate_ohms = 580, 289 .x_plate_ohms = 580,
287 .y_plate_ohms = 410, 290 .y_plate_ohms = 410,
288 }; 291 };
289 292
290 static struct spi_board_info spi_board 293 static struct spi_board_info spi_board_info[] __initdata = {
291 { 294 {
292 .modalias = "ads7846", 295 .modalias = "ads7846",
293 .platform_data = &ads_info, 296 .platform_data = &ads_info,
294 .mode = SPI_MODE_0, 297 .mode = SPI_MODE_0,
295 .irq = GPIO_IRQ(31) 298 .irq = GPIO_IRQ(31),
296 .max_speed_hz = 120000 /* ma 299 .max_speed_hz = 120000 /* max sample rate at 3V */ * 16,
297 .bus_num = 1, 300 .bus_num = 1,
298 .chip_select = 0, 301 .chip_select = 0,
299 }, 302 },
300 }; 303 };
301 304
302 Again, notice how board-specific information i 305 Again, notice how board-specific information is provided; each chip may need
303 several types. This example shows generic con 306 several types. This example shows generic constraints like the fastest SPI
304 clock to allow (a function of board voltage in 307 clock to allow (a function of board voltage in this case) or how an IRQ pin
305 is wired, plus chip-specific constraints like 308 is wired, plus chip-specific constraints like an important delay that's
306 changed by the capacitance at one pin. 309 changed by the capacitance at one pin.
307 310
308 (There's also "controller_data", information t 311 (There's also "controller_data", information that may be useful to the
309 controller driver. An example would be periph 312 controller driver. An example would be peripheral-specific DMA tuning
310 data or chipselect callbacks. This is stored 313 data or chipselect callbacks. This is stored in spi_device later.)
311 314
312 The board_info should provide enough informati 315 The board_info should provide enough information to let the system work
313 without the chip's driver being loaded. The m 316 without the chip's driver being loaded. The most troublesome aspect of
314 that is likely the SPI_CS_HIGH bit in the spi_ 317 that is likely the SPI_CS_HIGH bit in the spi_device.mode field, since
315 sharing a bus with a device that interprets ch 318 sharing a bus with a device that interprets chipselect "backwards" is
316 not possible until the infrastructure knows ho 319 not possible until the infrastructure knows how to deselect it.
317 320
318 Then your board initialization code would regi 321 Then your board initialization code would register that table with the SPI
319 infrastructure, so that it's available later w !! 322 infrastructure, so that it's available later when the SPI master controller
320 driver is registered:: 323 driver is registered::
321 324
322 spi_register_board_info(spi_board_info 325 spi_register_board_info(spi_board_info, ARRAY_SIZE(spi_board_info));
323 326
324 Like with other static board-specific setup, y 327 Like with other static board-specific setup, you won't unregister those.
325 328
326 The widely used "card" style computers bundle 329 The widely used "card" style computers bundle memory, cpu, and little else
327 onto a card that's maybe just thirty square ce 330 onto a card that's maybe just thirty square centimeters. On such systems,
328 your ``arch/.../mach-.../board-*.c`` file woul 331 your ``arch/.../mach-.../board-*.c`` file would primarily provide information
329 about the devices on the mainboard into which 332 about the devices on the mainboard into which such a card is plugged. That
330 certainly includes SPI devices hooked up throu 333 certainly includes SPI devices hooked up through the card connectors!
331 334
332 335
333 Non-static Configurations 336 Non-static Configurations
334 ^^^^^^^^^^^^^^^^^^^^^^^^^ 337 ^^^^^^^^^^^^^^^^^^^^^^^^^
335 338
336 When Linux includes support for MMC/SD/SDIO/Da 339 When Linux includes support for MMC/SD/SDIO/DataFlash cards through SPI, those
337 configurations will also be dynamic. Fortunat 340 configurations will also be dynamic. Fortunately, such devices all support
338 basic device identification probes, so they sh 341 basic device identification probes, so they should hotplug normally.
339 342
340 343
341 How do I write an "SPI Protocol Driver"? 344 How do I write an "SPI Protocol Driver"?
342 ---------------------------------------- 345 ----------------------------------------
343 Most SPI drivers are currently kernel drivers, 346 Most SPI drivers are currently kernel drivers, but there's also support
344 for userspace drivers. Here we talk only abou 347 for userspace drivers. Here we talk only about kernel drivers.
345 348
346 SPI protocol drivers somewhat resemble platfor 349 SPI protocol drivers somewhat resemble platform device drivers::
347 350
348 static struct spi_driver CHIP_driver = 351 static struct spi_driver CHIP_driver = {
349 .driver = { 352 .driver = {
350 .name = "CHI 353 .name = "CHIP",
>> 354 .owner = THIS_MODULE,
351 .pm = &CHI 355 .pm = &CHIP_pm_ops,
352 }, 356 },
353 357
354 .probe = CHIP_probe, 358 .probe = CHIP_probe,
355 .remove = CHIP_remove, 359 .remove = CHIP_remove,
356 }; 360 };
357 361
358 The driver core will automatically attempt to 362 The driver core will automatically attempt to bind this driver to any SPI
359 device whose board_info gave a modalias of "CH 363 device whose board_info gave a modalias of "CHIP". Your probe() code
360 might look like this unless you're creating a 364 might look like this unless you're creating a device which is managing
361 a bus (appearing under /sys/class/spi_master). 365 a bus (appearing under /sys/class/spi_master).
362 366
363 :: 367 ::
364 368
365 static int CHIP_probe(struct spi_devic 369 static int CHIP_probe(struct spi_device *spi)
366 { 370 {
367 struct CHIP 371 struct CHIP *chip;
368 struct CHIP_platform_data 372 struct CHIP_platform_data *pdata;
369 373
370 /* assuming the driver require 374 /* assuming the driver requires board-specific data: */
371 pdata = &spi->dev.platform_dat 375 pdata = &spi->dev.platform_data;
372 if (!pdata) 376 if (!pdata)
373 return -ENODEV; 377 return -ENODEV;
374 378
375 /* get memory for driver's per 379 /* get memory for driver's per-chip state */
376 chip = kzalloc(sizeof *chip, G 380 chip = kzalloc(sizeof *chip, GFP_KERNEL);
377 if (!chip) 381 if (!chip)
378 return -ENOMEM; 382 return -ENOMEM;
379 spi_set_drvdata(spi, chip); 383 spi_set_drvdata(spi, chip);
380 384
381 ... etc 385 ... etc
382 return 0; 386 return 0;
383 } 387 }
384 388
385 As soon as it enters probe(), the driver may i 389 As soon as it enters probe(), the driver may issue I/O requests to
386 the SPI device using "struct spi_message". Wh 390 the SPI device using "struct spi_message". When remove() returns,
387 or after probe() fails, the driver guarantees 391 or after probe() fails, the driver guarantees that it won't submit
388 any more such messages. 392 any more such messages.
389 393
390 - An spi_message is a sequence of protocol o 394 - An spi_message is a sequence of protocol operations, executed
391 as one atomic sequence. SPI driver contro 395 as one atomic sequence. SPI driver controls include:
392 396
393 + when bidirectional reads and writes st 397 + when bidirectional reads and writes start ... by how its
394 sequence of spi_transfer requests is a 398 sequence of spi_transfer requests is arranged;
395 399
396 + which I/O buffers are used ... each sp 400 + which I/O buffers are used ... each spi_transfer wraps a
397 buffer for each transfer direction, su 401 buffer for each transfer direction, supporting full duplex
398 (two pointers, maybe the same one in b 402 (two pointers, maybe the same one in both cases) and half
399 duplex (one pointer is NULL) transfers 403 duplex (one pointer is NULL) transfers;
400 404
401 + optionally defining short delays after 405 + optionally defining short delays after transfers ... using
402 the spi_transfer.delay.value setting ( 406 the spi_transfer.delay.value setting (this delay can be the
403 only protocol effect, if the buffer le 407 only protocol effect, if the buffer length is zero) ...
404 when specifying this delay the default 408 when specifying this delay the default spi_transfer.delay.unit
405 is microseconds, however this can be a 409 is microseconds, however this can be adjusted to clock cycles
406 or nanoseconds if needed; 410 or nanoseconds if needed;
407 411
408 + whether the chipselect becomes inactiv 412 + whether the chipselect becomes inactive after a transfer and
409 any delay ... by using the spi_transfe 413 any delay ... by using the spi_transfer.cs_change flag;
410 414
411 + hinting whether the next message is li 415 + hinting whether the next message is likely to go to this same
412 device ... using the spi_transfer.cs_c 416 device ... using the spi_transfer.cs_change flag on the last
413 transfer in that atomic group, and pot 417 transfer in that atomic group, and potentially saving costs
414 for chip deselect and select operation 418 for chip deselect and select operations.
415 419
416 - Follow standard kernel rules, and provide 420 - Follow standard kernel rules, and provide DMA-safe buffers in
417 your messages. That way controller driver 421 your messages. That way controller drivers using DMA aren't forced
418 to make extra copies unless the hardware r 422 to make extra copies unless the hardware requires it (e.g. working
419 around hardware errata that force the use 423 around hardware errata that force the use of bounce buffering).
420 424
>> 425 If standard dma_map_single() handling of these buffers is inappropriate,
>> 426 you can use spi_message.is_dma_mapped to tell the controller driver
>> 427 that you've already provided the relevant DMA addresses.
>> 428
421 - The basic I/O primitive is spi_async(). A 429 - The basic I/O primitive is spi_async(). Async requests may be
422 issued in any context (irq handler, task, 430 issued in any context (irq handler, task, etc) and completion
423 is reported using a callback provided with 431 is reported using a callback provided with the message.
424 After any detected error, the chip is dese 432 After any detected error, the chip is deselected and processing
425 of that spi_message is aborted. 433 of that spi_message is aborted.
426 434
427 - There are also synchronous wrappers like s 435 - There are also synchronous wrappers like spi_sync(), and wrappers
428 like spi_read(), spi_write(), and spi_writ 436 like spi_read(), spi_write(), and spi_write_then_read(). These
429 may be issued only in contexts that may sl 437 may be issued only in contexts that may sleep, and they're all
430 clean (and small, and "optional") layers o 438 clean (and small, and "optional") layers over spi_async().
431 439
432 - The spi_write_then_read() call, and conven 440 - The spi_write_then_read() call, and convenience wrappers around
433 it, should only be used with small amounts 441 it, should only be used with small amounts of data where the
434 cost of an extra copy may be ignored. It' 442 cost of an extra copy may be ignored. It's designed to support
435 common RPC-style requests, such as writing 443 common RPC-style requests, such as writing an eight bit command
436 and reading a sixteen bit response -- spi_ 444 and reading a sixteen bit response -- spi_w8r16() being one its
437 wrappers, doing exactly that. 445 wrappers, doing exactly that.
438 446
439 Some drivers may need to modify spi_device cha 447 Some drivers may need to modify spi_device characteristics like the
440 transfer mode, wordsize, or clock rate. This 448 transfer mode, wordsize, or clock rate. This is done with spi_setup(),
441 which would normally be called from probe() be 449 which would normally be called from probe() before the first I/O is
442 done to the device. However, that can also be 450 done to the device. However, that can also be called at any time
443 that no message is pending for that device. 451 that no message is pending for that device.
444 452
445 While "spi_device" would be the bottom boundar 453 While "spi_device" would be the bottom boundary of the driver, the
446 upper boundaries might include sysfs (especial 454 upper boundaries might include sysfs (especially for sensor readings),
447 the input layer, ALSA, networking, MTD, the ch 455 the input layer, ALSA, networking, MTD, the character device framework,
448 or other Linux subsystems. 456 or other Linux subsystems.
449 457
450 Note that there are two types of memory your d 458 Note that there are two types of memory your driver must manage as part
451 of interacting with SPI devices. 459 of interacting with SPI devices.
452 460
453 - I/O buffers use the usual Linux rules, and 461 - I/O buffers use the usual Linux rules, and must be DMA-safe.
454 You'd normally allocate them from the heap 462 You'd normally allocate them from the heap or free page pool.
455 Don't use the stack, or anything that's de 463 Don't use the stack, or anything that's declared "static".
456 464
457 - The spi_message and spi_transfer metadata 465 - The spi_message and spi_transfer metadata used to glue those
458 I/O buffers into a group of protocol trans 466 I/O buffers into a group of protocol transactions. These can
459 be allocated anywhere it's convenient, inc 467 be allocated anywhere it's convenient, including as part of
460 other allocate-once driver data structures 468 other allocate-once driver data structures. Zero-init these.
461 469
462 If you like, spi_message_alloc() and spi_messa 470 If you like, spi_message_alloc() and spi_message_free() convenience
463 routines are available to allocate and zero-in 471 routines are available to allocate and zero-initialize an spi_message
464 with several transfers. 472 with several transfers.
465 473
466 474
467 How do I write an "SPI Controller Driver"? !! 475 How do I write an "SPI Master Controller Driver"?
468 ---------------------------------------------- 476 -------------------------------------------------
469 An SPI controller will probably be registered 477 An SPI controller will probably be registered on the platform_bus; write
470 a driver to bind to the device, whichever bus 478 a driver to bind to the device, whichever bus is involved.
471 479
472 The main task of this type of driver is to pro !! 480 The main task of this type of driver is to provide an "spi_master".
473 Use spi_alloc_host() to allocate the host cont !! 481 Use spi_alloc_master() to allocate the master, and spi_master_get_devdata()
474 spi_controller_get_devdata() to get the driver !! 482 to get the driver-private data allocated for that device.
475 device. <<
476 483
477 :: 484 ::
478 485
479 struct spi_controller *ctlr; !! 486 struct spi_master *master;
480 struct CONTROLLER *c; 487 struct CONTROLLER *c;
481 488
482 ctlr = spi_alloc_host(dev, sizeof *c); !! 489 master = spi_alloc_master(dev, sizeof *c);
483 if (!ctlr) !! 490 if (!master)
484 return -ENODEV; 491 return -ENODEV;
485 492
486 c = spi_controller_get_devdata(ctlr); !! 493 c = spi_master_get_devdata(master);
487 494
488 The driver will initialize the fields of that !! 495 The driver will initialize the fields of that spi_master, including the
489 number (maybe the same as the platform device !! 496 bus number (maybe the same as the platform device ID) and three methods
490 interact with the SPI core and SPI protocol dr !! 497 used to interact with the SPI core and SPI protocol drivers. It will
491 its own internal state. (See below about bus !! 498 also initialize its own internal state. (See below about bus numbering
>> 499 and those methods.)
492 500
493 After you initialize the spi_controller, then !! 501 After you initialize the spi_master, then use spi_register_master() to
494 publish it to the rest of the system. At that 502 publish it to the rest of the system. At that time, device nodes for the
495 controller and any predeclared spi devices wil 503 controller and any predeclared spi devices will be made available, and
496 the driver model core will take care of bindin 504 the driver model core will take care of binding them to drivers.
497 505
498 If you need to remove your SPI controller driv !! 506 If you need to remove your SPI controller driver, spi_unregister_master()
499 will reverse the effect of spi_register_contro !! 507 will reverse the effect of spi_register_master().
500 508
501 509
502 Bus Numbering 510 Bus Numbering
503 ^^^^^^^^^^^^^ 511 ^^^^^^^^^^^^^
504 512
505 Bus numbering is important, since that's how L 513 Bus numbering is important, since that's how Linux identifies a given
506 SPI bus (shared SCK, MOSI, MISO). Valid bus n 514 SPI bus (shared SCK, MOSI, MISO). Valid bus numbers start at zero. On
507 SOC systems, the bus numbers should match the 515 SOC systems, the bus numbers should match the numbers defined by the chip
508 manufacturer. For example, hardware controlle 516 manufacturer. For example, hardware controller SPI2 would be bus number 2,
509 and spi_board_info for devices connected to it 517 and spi_board_info for devices connected to it would use that number.
510 518
511 If you don't have such hardware-assigned bus n 519 If you don't have such hardware-assigned bus number, and for some reason
512 you can't just assign them, then provide a neg 520 you can't just assign them, then provide a negative bus number. That will
513 then be replaced by a dynamically assigned num 521 then be replaced by a dynamically assigned number. You'd then need to treat
514 this as a non-static configuration (see above) 522 this as a non-static configuration (see above).
515 523
516 524
517 SPI Host Controller Methods !! 525 SPI Master Methods
518 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ !! 526 ^^^^^^^^^^^^^^^^^^
519 527
520 ``ctlr->setup(struct spi_device *spi)`` !! 528 ``master->setup(struct spi_device *spi)``
521 This sets up the device clock rate, SP 529 This sets up the device clock rate, SPI mode, and word sizes.
522 Drivers may change the defaults provid 530 Drivers may change the defaults provided by board_info, and then
523 call spi_setup(spi) to invoke this rou 531 call spi_setup(spi) to invoke this routine. It may sleep.
524 532
525 Unless each SPI target has its own con !! 533 Unless each SPI slave has its own configuration registers, don't
526 change them right away ... otherwise d 534 change them right away ... otherwise drivers could corrupt I/O
527 that's in progress for other SPI devic 535 that's in progress for other SPI devices.
528 536
529 .. note:: 537 .. note::
530 538
531 BUG ALERT: for some reason th 539 BUG ALERT: for some reason the first version of
532 many spi_controller drivers se !! 540 many spi_master drivers seems to get this wrong.
533 When you code setup(), ASSUME 541 When you code setup(), ASSUME that the controller
534 is actively processing transfe 542 is actively processing transfers for another device.
535 543
536 ``ctlr->cleanup(struct spi_device *spi)`` !! 544 ``master->cleanup(struct spi_device *spi)``
537 Your controller driver may use spi_dev 545 Your controller driver may use spi_device.controller_state to hold
538 state it dynamically associates with t 546 state it dynamically associates with that device. If you do that,
539 be sure to provide the cleanup() metho 547 be sure to provide the cleanup() method to free that state.
540 548
541 ``ctlr->prepare_transfer_hardware(struct spi_c !! 549 ``master->prepare_transfer_hardware(struct spi_master *master)``
542 This will be called by the queue mecha 550 This will be called by the queue mechanism to signal to the driver
543 that a message is coming in soon, so t 551 that a message is coming in soon, so the subsystem requests the
544 driver to prepare the transfer hardwar 552 driver to prepare the transfer hardware by issuing this call.
545 This may sleep. 553 This may sleep.
546 554
547 ``ctlr->unprepare_transfer_hardware(struct spi !! 555 ``master->unprepare_transfer_hardware(struct spi_master *master)``
548 This will be called by the queue mecha 556 This will be called by the queue mechanism to signal to the driver
549 that there are no more messages pendin 557 that there are no more messages pending in the queue and it may
550 relax the hardware (e.g. by power mana 558 relax the hardware (e.g. by power management calls). This may sleep.
551 559
552 ``ctlr->transfer_one_message(struct spi_contro !! 560 ``master->transfer_one_message(struct spi_master *master, struct spi_message *mesg)``
553 The subsystem calls the driver to tran 561 The subsystem calls the driver to transfer a single message while
554 queuing transfers that arrive in the m 562 queuing transfers that arrive in the meantime. When the driver is
555 finished with this message, it must ca 563 finished with this message, it must call
556 spi_finalize_current_message() so the 564 spi_finalize_current_message() so the subsystem can issue the next
557 message. This may sleep. 565 message. This may sleep.
558 566
559 ``ctrl->transfer_one(struct spi_controller *ct !! 567 ``master->transfer_one(struct spi_master *master, struct spi_device *spi, struct spi_transfer *transfer)``
560 The subsystem calls the driver to tran 568 The subsystem calls the driver to transfer a single transfer while
561 queuing transfers that arrive in the m 569 queuing transfers that arrive in the meantime. When the driver is
562 finished with this transfer, it must c 570 finished with this transfer, it must call
563 spi_finalize_current_transfer() so the 571 spi_finalize_current_transfer() so the subsystem can issue the next
564 transfer. This may sleep. Note: transf 572 transfer. This may sleep. Note: transfer_one and transfer_one_message
565 are mutually exclusive; when both are 573 are mutually exclusive; when both are set, the generic subsystem does
566 not call your transfer_one callback. 574 not call your transfer_one callback.
567 575
568 Return values: 576 Return values:
569 577
570 * negative errno: error 578 * negative errno: error
571 * 0: transfer is finished 579 * 0: transfer is finished
572 * 1: transfer is still in progress 580 * 1: transfer is still in progress
573 581
574 ``ctrl->set_cs_timing(struct spi_device *spi, !! 582 ``master->set_cs_timing(struct spi_device *spi, u8 setup_clk_cycles, u8 hold_clk_cycles, u8 inactive_clk_cycles)``
575 This method allows SPI client drivers !! 583 This method allows SPI client drivers to request SPI master controller
576 for configuring device specific CS set 584 for configuring device specific CS setup, hold and inactive timing
577 requirements. 585 requirements.
578 586
579 Deprecated Methods 587 Deprecated Methods
580 ^^^^^^^^^^^^^^^^^^ 588 ^^^^^^^^^^^^^^^^^^
581 589
582 ``ctrl->transfer(struct spi_device *spi, struc !! 590 ``master->transfer(struct spi_device *spi, struct spi_message *message)``
583 This must not sleep. Its responsibilit 591 This must not sleep. Its responsibility is to arrange that the
584 transfer happens and its complete() ca 592 transfer happens and its complete() callback is issued. The two
585 will normally happen later, after othe 593 will normally happen later, after other transfers complete, and
586 if the controller is idle it will need 594 if the controller is idle it will need to be kickstarted. This
587 method is not used on queued controlle 595 method is not used on queued controllers and must be NULL if
588 transfer_one_message() and (un)prepare 596 transfer_one_message() and (un)prepare_transfer_hardware() are
589 implemented. 597 implemented.
590 598
591 599
592 SPI Message Queue 600 SPI Message Queue
593 ^^^^^^^^^^^^^^^^^ 601 ^^^^^^^^^^^^^^^^^
594 602
595 If you are happy with the standard queueing me 603 If you are happy with the standard queueing mechanism provided by the
596 SPI subsystem, just implement the queued metho 604 SPI subsystem, just implement the queued methods specified above. Using
597 the message queue has the upside of centralizi 605 the message queue has the upside of centralizing a lot of code and
598 providing pure process-context execution of me 606 providing pure process-context execution of methods. The message queue
599 can also be elevated to realtime priority on h 607 can also be elevated to realtime priority on high-priority SPI traffic.
600 608
601 Unless the queueing mechanism in the SPI subsy 609 Unless the queueing mechanism in the SPI subsystem is selected, the bulk
602 of the driver will be managing the I/O queue f 610 of the driver will be managing the I/O queue fed by the now deprecated
603 function transfer(). 611 function transfer().
604 612
605 That queue could be purely conceptual. For ex 613 That queue could be purely conceptual. For example, a driver used only
606 for low-frequency sensor access might be fine 614 for low-frequency sensor access might be fine using synchronous PIO.
607 615
608 But the queue will probably be very real, usin 616 But the queue will probably be very real, using message->queue, PIO,
609 often DMA (especially if the root filesystem i 617 often DMA (especially if the root filesystem is in SPI flash), and
610 execution contexts like IRQ handlers, tasklets 618 execution contexts like IRQ handlers, tasklets, or workqueues (such
611 as keventd). Your driver can be as fancy, or 619 as keventd). Your driver can be as fancy, or as simple, as you need.
612 Such a transfer() method would normally just a 620 Such a transfer() method would normally just add the message to a
613 queue, and then start some asynchronous transf 621 queue, and then start some asynchronous transfer engine (unless it's
614 already running). 622 already running).
615 <<
616 <<
617 Extensions to the SPI protocol <<
618 ------------------------------ <<
619 The fact that SPI doesn't have a formal specif <<
620 manufacturers to implement the SPI protocol in <<
621 cases, SPI protocol implementations from diffe <<
622 each other. For example, in SPI mode 0 (CPOL=0 <<
623 like the following: <<
624 <<
625 :: <<
626 <<
627 nCSx ___ <<
628 \___________________________________ <<
629 • <<
630 • <<
631 SCLK ___ ___ ___ ___ <<
632 _______/ \___/ \___/ \___/ \___ <<
633 • : ; : ; : ; : ; <<
634 • : ; : ; : ; : ; <<
635 MOSI XXX__________ _______ <<
636 0xA5 XXX__/ 1 \_0_____/ 1 \_0_______ <<
637 • ; ; ; ; <<
638 • ; ; ; ; <<
639 MISO XXX__________ _________________ <<
640 0xBA XXX__/ 1 \_____0_/ 1 1 <<
641 <<
642 Legend:: <<
643 <<
644 • marks the start/end of transmission; <<
645 : marks when data is clocked into the periph <<
646 ; marks when data is clocked into the contro <<
647 X marks when line states are not specified. <<
648 <<
649 In some few cases, chips extend the SPI protoc <<
650 that other SPI protocols don't (e.g. data line <<
651 asserted). Those distinct SPI protocols, modes <<
652 by different SPI mode flags. <<
653 <<
654 MOSI idle state configuration <<
655 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ <<
656 <<
657 Common SPI protocol implementations don't spec <<
658 MOSI line when the controller is not clocking <<
659 peripherals that require specific MOSI line st <<
660 out. For example, if the peripheral expects th <<
661 controller is not clocking out data (``SPI_MOS <<
662 SPI mode 0 would look like the following: <<
663 <<
664 :: <<
665 <<
666 nCSx ___ <<
667 \___________________________________ <<
668 • <<
669 • <<
670 SCLK ___ ___ ___ ___ <<
671 _______/ \___/ \___/ \___/ \___ <<
672 • : ; : ; : ; : ; <<
673 • : ; : ; : ; : ; <<
674 MOSI _____ _______ _______ <<
675 0x56 \_0_____/ 1 \_0_____/ 1 \_ <<
676 • ; ; ; ; <<
677 • ; ; ; ; <<
678 MISO XXX__________ _________________ <<
679 0xBA XXX__/ 1 \_____0_/ 1 1 <<
680 <<
681 Legend:: <<
682 <<
683 • marks the start/end of transmission; <<
684 : marks when data is clocked into the periph <<
685 ; marks when data is clocked into the contro <<
686 X marks when line states are not specified. <<
687 <<
688 In this extension to the usual SPI protocol, t <<
689 be kept high when CS is asserted but the contr <<
690 the peripheral and also when CS is not asserte <<
691 <<
692 Peripherals that require this extension must r <<
693 ``SPI_MOSI_IDLE_HIGH`` bit into the mode attri <<
694 spi_device`` and call spi_setup(). Controllers <<
695 should indicate it by setting ``SPI_MOSI_IDLE_ <<
696 of their ``struct spi_controller``. The config <<
697 analogous but uses the ``SPI_MOSI_IDLE_LOW`` m <<
698 623
699 624
700 THANKS TO 625 THANKS TO
701 --------- 626 ---------
702 Contributors to Linux-SPI discussions include 627 Contributors to Linux-SPI discussions include (in alphabetical order,
703 by last name): 628 by last name):
704 629
705 - Mark Brown 630 - Mark Brown
706 - David Brownell 631 - David Brownell
707 - Russell King 632 - Russell King
708 - Grant Likely 633 - Grant Likely
709 - Dmitry Pervushin 634 - Dmitry Pervushin
710 - Stephen Street 635 - Stephen Street
711 - Mark Underwood 636 - Mark Underwood
712 - Andrew Victor 637 - Andrew Victor
713 - Linus Walleij 638 - Linus Walleij
714 - Vitaly Wool 639 - Vitaly Wool
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