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1 GPIO Interfaces
2
3 This provides an overview of GPIO access conventions on Linux.
4
5 These calls use the gpio_* naming prefix. No other calls should use that
6 prefix, or the related __gpio_* prefix.
7
8
9 What is a GPIO?
10 ===============
11 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
12 digital signal. They are provided from many kinds of chip, and are familiar
13 to Linux developers working with embedded and custom hardware. Each GPIO
14 represents a bit connected to a particular pin, or "ball" on Ball Grid Array
15 (BGA) packages. Board schematics show which external hardware connects to
16 which GPIOs. Drivers can be written generically, so that board setup code
17 passes such pin configuration data to drivers.
18
19 System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
20 non-dedicated pin can be configured as a GPIO; and most chips have at least
21 several dozen of them. Programmable logic devices (like FPGAs) can easily
22 provide GPIOs; multifunction chips like power managers, and audio codecs
23 often have a few such pins to help with pin scarcity on SOCs; and there are
24 also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
25 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
26 firmware knowing how they're used).
27
28 The exact capabilities of GPIOs vary between systems. Common options:
29
30 - Output values are writable (high=1, low=0). Some chips also have
31 options about how that value is driven, so that for example only one
32 value might be driven ... supporting "wire-OR" and similar schemes
33 for the other value (notably, "open drain" signaling).
34
35 - Input values are likewise readable (1, 0). Some chips support readback
36 of pins configured as "output", which is very useful in such "wire-OR"
37 cases (to support bidirectional signaling). GPIO controllers may have
38 input de-glitch/debounce logic, sometimes with software controls.
39
40 - Inputs can often be used as IRQ signals, often edge triggered but
41 sometimes level triggered. Such IRQs may be configurable as system
42 wakeup events, to wake the system from a low power state.
43
44 - Usually a GPIO will be configurable as either input or output, as needed
45 by different product boards; single direction ones exist too.
46
47 - Most GPIOs can be accessed while holding spinlocks, but those accessed
48 through a serial bus normally can't. Some systems support both types.
49
50 On a given board each GPIO is used for one specific purpose like monitoring
51 MMC/SD card insertion/removal, detecting card writeprotect status, driving
52 a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
53 watchdog, sensing a switch, and so on.
54
55
56 GPIO conventions
57 ================
58 Note that this is called a "convention" because you don't need to do it this
59 way, and it's no crime if you don't. There **are** cases where portability
60 is not the main issue; GPIOs are often used for the kind of board-specific
61 glue logic that may even change between board revisions, and can't ever be
62 used on a board that's wired differently. Only least-common-denominator
63 functionality can be very portable. Other features are platform-specific,
64 and that can be critical for glue logic.
65
66 Plus, this doesn't require any implementation framework, just an interface.
67 One platform might implement it as simple inline functions accessing chip
68 registers; another might implement it by delegating through abstractions
69 used for several very different kinds of GPIO controller. (There is some
70 optional code supporting such an implementation strategy, described later
71 in this document, but drivers acting as clients to the GPIO interface must
72 not care how it's implemented.)
73
74 That said, if the convention is supported on their platform, drivers should
75 use it when possible. Platforms must declare GENERIC_GPIO support in their
76 Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't
77 work without standard GPIO calls should have Kconfig entries which depend
78 on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as
79 optimized-away stubs, when drivers use the include file:
80
81 #include <linux/gpio.h>
82
83 If you stick to this convention then it'll be easier for other developers to
84 see what your code is doing, and help maintain it.
85
86 Note that these operations include I/O barriers on platforms which need to
87 use them; drivers don't need to add them explicitly.
88
89
90 Identifying GPIOs
91 -----------------
92 GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
93 reserves "negative" numbers for other purposes like marking signals as
94 "not available on this board", or indicating faults. Code that doesn't
95 touch the underlying hardware treats these integers as opaque cookies.
96
97 Platforms define how they use those integers, and usually #define symbols
98 for the GPIO lines so that board-specific setup code directly corresponds
99 to the relevant schematics. In contrast, drivers should only use GPIO
100 numbers passed to them from that setup code, using platform_data to hold
101 board-specific pin configuration data (along with other board specific
102 data they need). That avoids portability problems.
103
104 So for example one platform uses numbers 32-159 for GPIOs; while another
105 uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
106 type of GPIO controller, and on one particular board 80-95 with an FPGA.
107 The numbers need not be contiguous; either of those platforms could also
108 use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
109
110 If you want to initialize a structure with an invalid GPIO number, use
111 some negative number (perhaps "-EINVAL"); that will never be valid. To
112 test if such number from such a structure could reference a GPIO, you
113 may use this predicate:
114
115 int gpio_is_valid(int number);
116
117 A number that's not valid will be rejected by calls which may request
118 or free GPIOs (see below). Other numbers may also be rejected; for
119 example, a number might be valid but temporarily unused on a given board.
120
121 Whether a platform supports multiple GPIO controllers is a platform-specific
122 implementation issue, as are whether that support can leave "holes" in the space
123 of GPIO numbers, and whether new controllers can be added at runtime. Such issues
124 can affect things including whether adjacent GPIO numbers are both valid.
125
126 Using GPIOs
127 -----------
128 The first thing a system should do with a GPIO is allocate it, using
129 the gpio_request() call; see later.
130
131 One of the next things to do with a GPIO, often in board setup code when
132 setting up a platform_device using the GPIO, is mark its direction:
133
134 /* set as input or output, returning 0 or negative errno */
135 int gpio_direction_input(unsigned gpio);
136 int gpio_direction_output(unsigned gpio, int value);
137
138 The return value is zero for success, else a negative errno. It should
139 be checked, since the get/set calls don't have error returns and since
140 misconfiguration is possible. You should normally issue these calls from
141 a task context. However, for spinlock-safe GPIOs it's OK to use them
142 before tasking is enabled, as part of early board setup.
143
144 For output GPIOs, the value provided becomes the initial output value.
145 This helps avoid signal glitching during system startup.
146
147 For compatibility with legacy interfaces to GPIOs, setting the direction
148 of a GPIO implicitly requests that GPIO (see below) if it has not been
149 requested already. That compatibility is being removed from the optional
150 gpiolib framework.
151
152 Setting the direction can fail if the GPIO number is invalid, or when
153 that particular GPIO can't be used in that mode. It's generally a bad
154 idea to rely on boot firmware to have set the direction correctly, since
155 it probably wasn't validated to do more than boot Linux. (Similarly,
156 that board setup code probably needs to multiplex that pin as a GPIO,
157 and configure pullups/pulldowns appropriately.)
158
159
160 Spinlock-Safe GPIO access
161 -------------------------
162 Most GPIO controllers can be accessed with memory read/write instructions.
163 Those don't need to sleep, and can safely be done from inside hard
164 (nonthreaded) IRQ handlers and similar contexts.
165
166 Use the following calls to access such GPIOs,
167 for which gpio_cansleep() will always return false (see below):
168
169 /* GPIO INPUT: return zero or nonzero */
170 int gpio_get_value(unsigned gpio);
171
172 /* GPIO OUTPUT */
173 void gpio_set_value(unsigned gpio, int value);
174
175 The values are boolean, zero for low, nonzero for high. When reading the
176 value of an output pin, the value returned should be what's seen on the
177 pin ... that won't always match the specified output value, because of
178 issues including open-drain signaling and output latencies.
179
180 The get/set calls have no error returns because "invalid GPIO" should have
181 been reported earlier from gpio_direction_*(). However, note that not all
182 platforms can read the value of output pins; those that can't should always
183 return zero. Also, using these calls for GPIOs that can't safely be accessed
184 without sleeping (see below) is an error.
185
186 Platform-specific implementations are encouraged to optimize the two
187 calls to access the GPIO value in cases where the GPIO number (and for
188 output, value) are constant. It's normal for them to need only a couple
189 of instructions in such cases (reading or writing a hardware register),
190 and not to need spinlocks. Such optimized calls can make bitbanging
191 applications a lot more efficient (in both space and time) than spending
192 dozens of instructions on subroutine calls.
193
194
195 GPIO access that may sleep
196 --------------------------
197 Some GPIO controllers must be accessed using message based busses like I2C
198 or SPI. Commands to read or write those GPIO values require waiting to
199 get to the head of a queue to transmit a command and get its response.
200 This requires sleeping, which can't be done from inside IRQ handlers.
201
202 Platforms that support this type of GPIO distinguish them from other GPIOs
203 by returning nonzero from this call (which requires a valid GPIO number,
204 which should have been previously allocated with gpio_request):
205
206 int gpio_cansleep(unsigned gpio);
207
208 To access such GPIOs, a different set of accessors is defined:
209
210 /* GPIO INPUT: return zero or nonzero, might sleep */
211 int gpio_get_value_cansleep(unsigned gpio);
212
213 /* GPIO OUTPUT, might sleep */
214 void gpio_set_value_cansleep(unsigned gpio, int value);
215
216
217 Accessing such GPIOs requires a context which may sleep, for example
218 a threaded IRQ handler, and those accessors must be used instead of
219 spinlock-safe accessors without the cansleep() name suffix.
220
221 Other than the fact that these accessors might sleep, and will work
222 on GPIOs that can't be accessed from hardIRQ handlers, these calls act
223 the same as the spinlock-safe calls.
224
225 ** IN ADDITION ** calls to setup and configure such GPIOs must be made
226 from contexts which may sleep, since they may need to access the GPIO
227 controller chip too: (These setup calls are usually made from board
228 setup or driver probe/teardown code, so this is an easy constraint.)
229
230 gpio_direction_input()
231 gpio_direction_output()
232 gpio_request()
233
234 ## gpio_request_one()
235 ## gpio_request_array()
236 ## gpio_free_array()
237
238 gpio_free()
239 gpio_set_debounce()
240
241
242
243 Claiming and Releasing GPIOs
244 ----------------------------
245 To help catch system configuration errors, two calls are defined.
246
247 /* request GPIO, returning 0 or negative errno.
248 * non-null labels may be useful for diagnostics.
249 */
250 int gpio_request(unsigned gpio, const char *label);
251
252 /* release previously-claimed GPIO */
253 void gpio_free(unsigned gpio);
254
255 Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
256 GPIOs that have already been claimed with that call. The return value of
257 gpio_request() must be checked. You should normally issue these calls from
258 a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
259 before tasking is enabled, as part of early board setup.
260
261 These calls serve two basic purposes. One is marking the signals which
262 are actually in use as GPIOs, for better diagnostics; systems may have
263 several hundred potential GPIOs, but often only a dozen are used on any
264 given board. Another is to catch conflicts, identifying errors when
265 (a) two or more drivers wrongly think they have exclusive use of that
266 signal, or (b) something wrongly believes it's safe to remove drivers
267 needed to manage a signal that's in active use. That is, requesting a
268 GPIO can serve as a kind of lock.
269
270 Some platforms may also use knowledge about what GPIOs are active for
271 power management, such as by powering down unused chip sectors and, more
272 easily, gating off unused clocks.
273
274 For GPIOs that use pins known to the pinctrl subsystem, that subsystem should
275 be informed of their use; a gpiolib driver's .request() operation may call
276 pinctrl_request_gpio(), and a gpiolib driver's .free() operation may call
277 pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio()
278 to succeed concurrently with a pin or pingroup being "owned" by a device for
279 pin multiplexing.
280
281 Any programming of pin multiplexing hardware that is needed to route the
282 GPIO signal to the appropriate pin should occur within a GPIO driver's
283 .direction_input() or .direction_output() operations, and occur after any
284 setup of an output GPIO's value. This allows a glitch-free migration from a
285 pin's special function to GPIO. This is sometimes required when using a GPIO
286 to implement a workaround on signals typically driven by a non-GPIO HW block.
287
288 Some platforms allow some or all GPIO signals to be routed to different pins.
289 Similarly, other aspects of the GPIO or pin may need to be configured, such as
290 pullup/pulldown. Platform software should arrange that any such details are
291 configured prior to gpio_request() being called for those GPIOs, e.g. using
292 the pinctrl subsystem's mapping table, so that GPIO users need not be aware
293 of these details.
294
295 Also note that it's your responsibility to have stopped using a GPIO
296 before you free it.
297
298 Considering in most cases GPIOs are actually configured right after they
299 are claimed, three additional calls are defined:
300
301 /* request a single GPIO, with initial configuration specified by
302 * 'flags', identical to gpio_request() wrt other arguments and
303 * return value
304 */
305 int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
306
307 /* request multiple GPIOs in a single call
308 */
309 int gpio_request_array(struct gpio *array, size_t num);
310
311 /* release multiple GPIOs in a single call
312 */
313 void gpio_free_array(struct gpio *array, size_t num);
314
315 where 'flags' is currently defined to specify the following properties:
316
317 * GPIOF_DIR_IN - to configure direction as input
318 * GPIOF_DIR_OUT - to configure direction as output
319
320 * GPIOF_INIT_LOW - as output, set initial level to LOW
321 * GPIOF_INIT_HIGH - as output, set initial level to HIGH
322 * GPIOF_OPEN_DRAIN - gpio pin is open drain type.
323 * GPIOF_OPEN_SOURCE - gpio pin is open source type.
324
325 * GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction
326 * GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction
327
328 since GPIOF_INIT_* are only valid when configured as output, so group valid
329 combinations as:
330
331 * GPIOF_IN - configure as input
332 * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
333 * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
334
335 When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is
336 open drain type. Such pins will not be driven to 1 in output mode. It is
337 require to connect pull-up on such pins. By enabling this flag, gpio lib will
338 make the direction to input when it is asked to set value of 1 in output mode
339 to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
340
341 When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is
342 open source type. Such pins will not be driven to 0 in output mode. It is
343 require to connect pull-down on such pin. By enabling this flag, gpio lib will
344 make the direction to input when it is asked to set value of 0 in output mode
345 to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
346
347 In the future, these flags can be extended to support more properties.
348
349 Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
350 introduced to encapsulate all three fields as:
351
352 struct gpio {
353 unsigned gpio;
354 unsigned long flags;
355 const char *label;
356 };
357
358 A typical example of usage:
359
360 static struct gpio leds_gpios[] = {
361 { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
362 { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
363 { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
364 { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
365 { ... },
366 };
367
368 err = gpio_request_one(31, GPIOF_IN, "Reset Button");
369 if (err)
370 ...
371
372 err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
373 if (err)
374 ...
375
376 gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
377
378
379 GPIOs mapped to IRQs
380 --------------------
381 GPIO numbers are unsigned integers; so are IRQ numbers. These make up
382 two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
383 map between them using calls like:
384
385 /* map GPIO numbers to IRQ numbers */
386 int gpio_to_irq(unsigned gpio);
387
388 /* map IRQ numbers to GPIO numbers (avoid using this) */
389 int irq_to_gpio(unsigned irq);
390
391 Those return either the corresponding number in the other namespace, or
392 else a negative errno code if the mapping can't be done. (For example,
393 some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
394 number that wasn't set up as an input using gpio_direction_input(), or
395 to use an IRQ number that didn't originally come from gpio_to_irq().
396
397 These two mapping calls are expected to cost on the order of a single
398 addition or subtraction. They're not allowed to sleep.
399
400 Non-error values returned from gpio_to_irq() can be passed to request_irq()
401 or free_irq(). They will often be stored into IRQ resources for platform
402 devices, by the board-specific initialization code. Note that IRQ trigger
403 options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
404 system wakeup capabilities.
405
406 Non-error values returned from irq_to_gpio() would most commonly be used
407 with gpio_get_value(), for example to initialize or update driver state
408 when the IRQ is edge-triggered. Note that some platforms don't support
409 this reverse mapping, so you should avoid using it.
410
411
412 Emulating Open Drain Signals
413 ----------------------------
414 Sometimes shared signals need to use "open drain" signaling, where only the
415 low signal level is actually driven. (That term applies to CMOS transistors;
416 "open collector" is used for TTL.) A pullup resistor causes the high signal
417 level. This is sometimes called a "wire-AND"; or more practically, from the
418 negative logic (low=true) perspective this is a "wire-OR".
419
420 One common example of an open drain signal is a shared active-low IRQ line.
421 Also, bidirectional data bus signals sometimes use open drain signals.
422
423 Some GPIO controllers directly support open drain outputs; many don't. When
424 you need open drain signaling but your hardware doesn't directly support it,
425 there's a common idiom you can use to emulate it with any GPIO pin that can
426 be used as either an input or an output:
427
428 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
429 and overrides the pullup.
430
431 HIGH: gpio_direction_input(gpio) ... this turns off the output,
432 so the pullup (or some other device) controls the signal.
433
434 If you are "driving" the signal high but gpio_get_value(gpio) reports a low
435 value (after the appropriate rise time passes), you know some other component
436 is driving the shared signal low. That's not necessarily an error. As one
437 common example, that's how I2C clocks are stretched: a slave that needs a
438 slower clock delays the rising edge of SCK, and the I2C master adjusts its
439 signaling rate accordingly.
440
441
442 What do these conventions omit?
443 ===============================
444 One of the biggest things these conventions omit is pin multiplexing, since
445 this is highly chip-specific and nonportable. One platform might not need
446 explicit multiplexing; another might have just two options for use of any
447 given pin; another might have eight options per pin; another might be able
448 to route a given GPIO to any one of several pins. (Yes, those examples all
449 come from systems that run Linux today.)
450
451 Related to multiplexing is configuration and enabling of the pullups or
452 pulldowns integrated on some platforms. Not all platforms support them,
453 or support them in the same way; and any given board might use external
454 pullups (or pulldowns) so that the on-chip ones should not be used.
455 (When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
456 Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
457 platform-specific issue, as are models like (not) having a one-to-one
458 correspondence between configurable pins and GPIOs.
459
460 There are other system-specific mechanisms that are not specified here,
461 like the aforementioned options for input de-glitching and wire-OR output.
462 Hardware may support reading or writing GPIOs in gangs, but that's usually
463 configuration dependent: for GPIOs sharing the same bank. (GPIOs are
464 commonly grouped in banks of 16 or 32, with a given SOC having several such
465 banks.) Some systems can trigger IRQs from output GPIOs, or read values
466 from pins not managed as GPIOs. Code relying on such mechanisms will
467 necessarily be nonportable.
468
469 Dynamic definition of GPIOs is not currently standard; for example, as
470 a side effect of configuring an add-on board with some GPIO expanders.
471
472
473 GPIO implementor's framework (OPTIONAL)
474 =======================================
475 As noted earlier, there is an optional implementation framework making it
476 easier for platforms to support different kinds of GPIO controller using
477 the same programming interface. This framework is called "gpiolib".
478
479 As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
480 will be found there. That will list all the controllers registered through
481 this framework, and the state of the GPIOs currently in use.
482
483
484 Controller Drivers: gpio_chip
485 -----------------------------
486 In this framework each GPIO controller is packaged as a "struct gpio_chip"
487 with information common to each controller of that type:
488
489 - methods to establish GPIO direction
490 - methods used to access GPIO values
491 - flag saying whether calls to its methods may sleep
492 - optional debugfs dump method (showing extra state like pullup config)
493 - label for diagnostics
494
495 There is also per-instance data, which may come from device.platform_data:
496 the number of its first GPIO, and how many GPIOs it exposes.
497
498 The code implementing a gpio_chip should support multiple instances of the
499 controller, possibly using the driver model. That code will configure each
500 gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
501 rare; use gpiochip_remove() when it is unavoidable.
502
503 Most often a gpio_chip is part of an instance-specific structure with state
504 not exposed by the GPIO interfaces, such as addressing, power management,
505 and more. Chips such as codecs will have complex non-GPIO state.
506
507 Any debugfs dump method should normally ignore signals which haven't been
508 requested as GPIOs. They can use gpiochip_is_requested(), which returns
509 either NULL or the label associated with that GPIO when it was requested.
510
511
512 Platform Support
513 ----------------
514 To support this framework, a platform's Kconfig will "select" either
515 ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
516 and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
517 three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
518
519 It may also provide a custom value for ARCH_NR_GPIOS, so that it better
520 reflects the number of GPIOs in actual use on that platform, without
521 wasting static table space. (It should count both built-in/SoC GPIOs and
522 also ones on GPIO expanders.
523
524 ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
525 into the kernel on that architecture.
526
527 ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user
528 can enable it and build it into the kernel optionally.
529
530 If neither of these options are selected, the platform does not support
531 GPIOs through GPIO-lib and the code cannot be enabled by the user.
532
533 Trivial implementations of those functions can directly use framework
534 code, which always dispatches through the gpio_chip:
535
536 #define gpio_get_value __gpio_get_value
537 #define gpio_set_value __gpio_set_value
538 #define gpio_cansleep __gpio_cansleep
539
540 Fancier implementations could instead define those as inline functions with
541 logic optimizing access to specific SOC-based GPIOs. For example, if the
542 referenced GPIO is the constant "12", getting or setting its value could
543 cost as little as two or three instructions, never sleeping. When such an
544 optimization is not possible those calls must delegate to the framework
545 code, costing at least a few dozen instructions. For bitbanged I/O, such
546 instruction savings can be significant.
547
548 For SOCs, platform-specific code defines and registers gpio_chip instances
549 for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
550 match chip vendor documentation, and directly match board schematics. They
551 may well start at zero and go up to a platform-specific limit. Such GPIOs
552 are normally integrated into platform initialization to make them always be
553 available, from arch_initcall() or earlier; they can often serve as IRQs.
554
555
556 Board Support
557 -------------
558 For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
559 function devices, FPGAs or CPLDs -- most often board-specific code handles
560 registering controller devices and ensures that their drivers know what GPIO
561 numbers to use with gpiochip_add(). Their numbers often start right after
562 platform-specific GPIOs.
563
564 For example, board setup code could create structures identifying the range
565 of GPIOs that chip will expose, and passes them to each GPIO expander chip
566 using platform_data. Then the chip driver's probe() routine could pass that
567 data to gpiochip_add().
568
569 Initialization order can be important. For example, when a device relies on
570 an I2C-based GPIO, its probe() routine should only be called after that GPIO
571 becomes available. That may mean the device should not be registered until
572 calls for that GPIO can work. One way to address such dependencies is for
573 such gpio_chip controllers to provide setup() and teardown() callbacks to
574 board specific code; those board specific callbacks would register devices
575 once all the necessary resources are available, and remove them later when
576 the GPIO controller device becomes unavailable.
577
578
579 Sysfs Interface for Userspace (OPTIONAL)
580 ========================================
581 Platforms which use the "gpiolib" implementors framework may choose to
582 configure a sysfs user interface to GPIOs. This is different from the
583 debugfs interface, since it provides control over GPIO direction and
584 value instead of just showing a gpio state summary. Plus, it could be
585 present on production systems without debugging support.
586
587 Given appropriate hardware documentation for the system, userspace could
588 know for example that GPIO #23 controls the write protect line used to
589 protect boot loader segments in flash memory. System upgrade procedures
590 may need to temporarily remove that protection, first importing a GPIO,
591 then changing its output state, then updating the code before re-enabling
592 the write protection. In normal use, GPIO #23 would never be touched,
593 and the kernel would have no need to know about it.
594
595 Again depending on appropriate hardware documentation, on some systems
596 userspace GPIO can be used to determine system configuration data that
597 standard kernels won't know about. And for some tasks, simple userspace
598 GPIO drivers could be all that the system really needs.
599
600 Note that standard kernel drivers exist for common "LEDs and Buttons"
601 GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
602 instead of talking directly to the GPIOs; they integrate with kernel
603 frameworks better than your userspace code could.
604
605
606 Paths in Sysfs
607 --------------
608 There are three kinds of entry in /sys/class/gpio:
609
610 - Control interfaces used to get userspace control over GPIOs;
611
612 - GPIOs themselves; and
613
614 - GPIO controllers ("gpio_chip" instances).
615
616 That's in addition to standard files including the "device" symlink.
617
618 The control interfaces are write-only:
619
620 /sys/class/gpio/
621
622 "export" ... Userspace may ask the kernel to export control of
623 a GPIO to userspace by writing its number to this file.
624
625 Example: "echo 19 > export" will create a "gpio19" node
626 for GPIO #19, if that's not requested by kernel code.
627
628 "unexport" ... Reverses the effect of exporting to userspace.
629
630 Example: "echo 19 > unexport" will remove a "gpio19"
631 node exported using the "export" file.
632
633 GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
634 and have the following read/write attributes:
635
636 /sys/class/gpio/gpioN/
637
638 "direction" ... reads as either "in" or "out". This value may
639 normally be written. Writing as "out" defaults to
640 initializing the value as low. To ensure glitch free
641 operation, values "low" and "high" may be written to
642 configure the GPIO as an output with that initial value.
643
644 Note that this attribute *will not exist* if the kernel
645 doesn't support changing the direction of a GPIO, or
646 it was exported by kernel code that didn't explicitly
647 allow userspace to reconfigure this GPIO's direction.
648
649 "value" ... reads as either 0 (low) or 1 (high). If the GPIO
650 is configured as an output, this value may be written;
651 any nonzero value is treated as high.
652
653 If the pin can be configured as interrupt-generating interrupt
654 and if it has been configured to generate interrupts (see the
655 description of "edge"), you can poll(2) on that file and
656 poll(2) will return whenever the interrupt was triggered. If
657 you use poll(2), set the events POLLPRI and POLLERR. If you
658 use select(2), set the file descriptor in exceptfds. After
659 poll(2) returns, either lseek(2) to the beginning of the sysfs
660 file and read the new value or close the file and re-open it
661 to read the value.
662
663 "edge" ... reads as either "none", "rising", "falling", or
664 "both". Write these strings to select the signal edge(s)
665 that will make poll(2) on the "value" file return.
666
667 This file exists only if the pin can be configured as an
668 interrupt generating input pin.
669
670 "active_low" ... reads as either 0 (false) or 1 (true). Write
671 any nonzero value to invert the value attribute both
672 for reading and writing. Existing and subsequent
673 poll(2) support configuration via the edge attribute
674 for "rising" and "falling" edges will follow this
675 setting.
676
677 GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
678 controller implementing GPIOs starting at #42) and have the following
679 read-only attributes:
680
681 /sys/class/gpio/gpiochipN/
682
683 "base" ... same as N, the first GPIO managed by this chip
684
685 "label" ... provided for diagnostics (not always unique)
686
687 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
688
689 Board documentation should in most cases cover what GPIOs are used for
690 what purposes. However, those numbers are not always stable; GPIOs on
691 a daughtercard might be different depending on the base board being used,
692 or other cards in the stack. In such cases, you may need to use the
693 gpiochip nodes (possibly in conjunction with schematics) to determine
694 the correct GPIO number to use for a given signal.
695
696
697 Exporting from Kernel code
698 --------------------------
699 Kernel code can explicitly manage exports of GPIOs which have already been
700 requested using gpio_request():
701
702 /* export the GPIO to userspace */
703 int gpio_export(unsigned gpio, bool direction_may_change);
704
705 /* reverse gpio_export() */
706 void gpio_unexport();
707
708 /* create a sysfs link to an exported GPIO node */
709 int gpio_export_link(struct device *dev, const char *name,
710 unsigned gpio)
711
712 /* change the polarity of a GPIO node in sysfs */
713 int gpio_sysfs_set_active_low(unsigned gpio, int value);
714
715 After a kernel driver requests a GPIO, it may only be made available in
716 the sysfs interface by gpio_export(). The driver can control whether the
717 signal direction may change. This helps drivers prevent userspace code
718 from accidentally clobbering important system state.
719
720 This explicit exporting can help with debugging (by making some kinds
721 of experiments easier), or can provide an always-there interface that's
722 suitable for documenting as part of a board support package.
723
724 After the GPIO has been exported, gpio_export_link() allows creating
725 symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
726 use this to provide the interface under their own device in sysfs with
727 a descriptive name.
728
729 Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
730 differences between boards from user space. This only affects the
731 sysfs interface. Polarity change can be done both before and after
732 gpio_export(), and previously enabled poll(2) support for either
733 rising or falling edge will be reconfigured to follow this setting.