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David Brownell4c203862007-02-12 00:53:11 -08001GPIO Interfaces
2
3This provides an overview of GPIO access conventions on Linux.
4
David Brownell7560fa62008-03-04 14:28:27 -08005These calls use the gpio_* naming prefix. No other calls should use that
6prefix, or the related __gpio_* prefix.
7
David Brownell4c203862007-02-12 00:53:11 -08008
9What is a GPIO?
10===============
11A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
12digital signal. They are provided from many kinds of chip, and are familiar
13to Linux developers working with embedded and custom hardware. Each GPIO
14represents a bit connected to a particular pin, or "ball" on Ball Grid Array
15(BGA) packages. Board schematics show which external hardware connects to
16which GPIOs. Drivers can be written generically, so that board setup code
17passes such pin configuration data to drivers.
18
19System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
20non-dedicated pin can be configured as a GPIO; and most chips have at least
21several dozen of them. Programmable logic devices (like FPGAs) can easily
22provide GPIOs; multifunction chips like power managers, and audio codecs
23often have a few such pins to help with pin scarcity on SOCs; and there are
24also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
25Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
26firmware knowing how they're used).
27
28The 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
David Brownell1668be72007-04-11 23:28:42 -070033 for the other value (notably, "open drain" signaling).
David Brownell4c203862007-02-12 00:53:11 -080034
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
David Brownell7c2db752008-02-04 22:28:21 -080038 input de-glitch/debounce logic, sometimes with software controls.
David Brownell4c203862007-02-12 00:53:11 -080039
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
50On a given board each GPIO is used for one specific purpose like monitoring
51MMC/SD card insertion/removal, detecting card writeprotect status, driving
52a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
53watchdog, sensing a switch, and so on.
54
55
56GPIO conventions
57================
58Note that this is called a "convention" because you don't need to do it this
59way, and it's no crime if you don't. There **are** cases where portability
60is not the main issue; GPIOs are often used for the kind of board-specific
61glue logic that may even change between board revisions, and can't ever be
62used on a board that's wired differently. Only least-common-denominator
63functionality can be very portable. Other features are platform-specific,
64and that can be critical for glue logic.
65
David Brownell7c2db752008-02-04 22:28:21 -080066Plus, this doesn't require any implementation framework, just an interface.
David Brownell4c203862007-02-12 00:53:11 -080067One platform might implement it as simple inline functions accessing chip
68registers; another might implement it by delegating through abstractions
David Brownell7c2db752008-02-04 22:28:21 -080069used for several very different kinds of GPIO controller. (There is some
70optional code supporting such an implementation strategy, described later
71in this document, but drivers acting as clients to the GPIO interface must
72not care how it's implemented.)
David Brownell4c203862007-02-12 00:53:11 -080073
74That said, if the convention is supported on their platform, drivers should
David Brownell7560fa62008-03-04 14:28:27 -080075use it when possible. Platforms must declare GENERIC_GPIO support in their
76Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't
77work without standard GPIO calls should have Kconfig entries which depend
78on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as
79optimized-away stubs, when drivers use the include file:
David Brownell4c203862007-02-12 00:53:11 -080080
David Brownell7560fa62008-03-04 14:28:27 -080081 #include <linux/gpio.h>
David Brownell4c203862007-02-12 00:53:11 -080082
83If you stick to this convention then it'll be easier for other developers to
84see what your code is doing, and help maintain it.
85
David Brownella0a99832007-07-19 01:47:52 -070086Note that these operations include I/O barriers on platforms which need to
87use them; drivers don't need to add them explicitly.
88
David Brownell4c203862007-02-12 00:53:11 -080089
90Identifying GPIOs
91-----------------
92GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
93reserves "negative" numbers for other purposes like marking signals as
David Brownellf5de6112007-02-16 01:27:14 -080094"not available on this board", or indicating faults. Code that doesn't
95touch the underlying hardware treats these integers as opaque cookies.
David Brownell4c203862007-02-12 00:53:11 -080096
97Platforms define how they use those integers, and usually #define symbols
98for the GPIO lines so that board-specific setup code directly corresponds
99to the relevant schematics. In contrast, drivers should only use GPIO
100numbers passed to them from that setup code, using platform_data to hold
101board-specific pin configuration data (along with other board specific
102data they need). That avoids portability problems.
103
104So for example one platform uses numbers 32-159 for GPIOs; while another
105uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
106type of GPIO controller, and on one particular board 80-95 with an FPGA.
107The numbers need not be contiguous; either of those platforms could also
108use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
109
Guennadi Liakhovetskie6de1802008-04-28 02:14:46 -0700110If you want to initialize a structure with an invalid GPIO number, use
111some negative number (perhaps "-EINVAL"); that will never be valid. To
112test if a number could reference a GPIO, you may use this predicate:
113
114 int gpio_is_valid(int number);
115
116A number that's not valid will be rejected by calls which may request
117or free GPIOs (see below). Other numbers may also be rejected; for
118example, a number might be valid but unused on a given board.
119
David Brownell4c203862007-02-12 00:53:11 -0800120Whether a platform supports multiple GPIO controllers is currently a
121platform-specific implementation issue.
122
123
124Using GPIOs
125-----------
126One of the first things to do with a GPIO, often in board setup code when
127setting up a platform_device using the GPIO, is mark its direction:
128
129 /* set as input or output, returning 0 or negative errno */
130 int gpio_direction_input(unsigned gpio);
David Brownell28735a72007-03-16 13:38:14 -0800131 int gpio_direction_output(unsigned gpio, int value);
David Brownell4c203862007-02-12 00:53:11 -0800132
133The return value is zero for success, else a negative errno. It should
134be checked, since the get/set calls don't have error returns and since
David Brownell83c65902007-05-16 22:11:13 -0700135misconfiguration is possible. You should normally issue these calls from
136a task context. However, for spinlock-safe GPIOs it's OK to use them
137before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800138
David Brownell28735a72007-03-16 13:38:14 -0800139For output GPIOs, the value provided becomes the initial output value.
140This helps avoid signal glitching during system startup.
141
David Brownell7c2db752008-02-04 22:28:21 -0800142For compatibility with legacy interfaces to GPIOs, setting the direction
143of a GPIO implicitly requests that GPIO (see below) if it has not been
144requested already. That compatibility may be removed in the future;
145explicitly requesting GPIOs is strongly preferred.
146
David Brownell4c203862007-02-12 00:53:11 -0800147Setting the direction can fail if the GPIO number is invalid, or when
148that particular GPIO can't be used in that mode. It's generally a bad
149idea to rely on boot firmware to have set the direction correctly, since
150it probably wasn't validated to do more than boot Linux. (Similarly,
151that board setup code probably needs to multiplex that pin as a GPIO,
152and configure pullups/pulldowns appropriately.)
153
154
155Spinlock-Safe GPIO access
156-------------------------
157Most GPIO controllers can be accessed with memory read/write instructions.
158That doesn't need to sleep, and can safely be done from inside IRQ handlers.
David Brownell7c2db752008-02-04 22:28:21 -0800159(That includes hardirq contexts on RT kernels.)
David Brownell4c203862007-02-12 00:53:11 -0800160
161Use these calls to access such GPIOs:
162
163 /* GPIO INPUT: return zero or nonzero */
164 int gpio_get_value(unsigned gpio);
165
166 /* GPIO OUTPUT */
167 void gpio_set_value(unsigned gpio, int value);
168
169The values are boolean, zero for low, nonzero for high. When reading the
170value of an output pin, the value returned should be what's seen on the
171pin ... that won't always match the specified output value, because of
David Brownell7c2db752008-02-04 22:28:21 -0800172issues including open-drain signaling and output latencies.
David Brownell4c203862007-02-12 00:53:11 -0800173
174The get/set calls have no error returns because "invalid GPIO" should have
David Brownellbe1ff382007-07-23 18:43:57 -0700175been reported earlier from gpio_direction_*(). However, note that not all
David Brownell4c203862007-02-12 00:53:11 -0800176platforms can read the value of output pins; those that can't should always
David Brownellf5de6112007-02-16 01:27:14 -0800177return zero. Also, using these calls for GPIOs that can't safely be accessed
178without sleeping (see below) is an error.
David Brownell4c203862007-02-12 00:53:11 -0800179
David Brownellf5de6112007-02-16 01:27:14 -0800180Platform-specific implementations are encouraged to optimize the two
David Brownell4c203862007-02-12 00:53:11 -0800181calls to access the GPIO value in cases where the GPIO number (and for
182output, value) are constant. It's normal for them to need only a couple
183of instructions in such cases (reading or writing a hardware register),
184and not to need spinlocks. Such optimized calls can make bitbanging
185applications a lot more efficient (in both space and time) than spending
186dozens of instructions on subroutine calls.
187
188
189GPIO access that may sleep
190--------------------------
191Some GPIO controllers must be accessed using message based busses like I2C
192or SPI. Commands to read or write those GPIO values require waiting to
193get to the head of a queue to transmit a command and get its response.
194This requires sleeping, which can't be done from inside IRQ handlers.
195
196Platforms that support this type of GPIO distinguish them from other GPIOs
David Brownell7c2db752008-02-04 22:28:21 -0800197by returning nonzero from this call (which requires a valid GPIO number,
198either explicitly or implicitly requested):
David Brownell4c203862007-02-12 00:53:11 -0800199
200 int gpio_cansleep(unsigned gpio);
201
202To access such GPIOs, a different set of accessors is defined:
203
204 /* GPIO INPUT: return zero or nonzero, might sleep */
205 int gpio_get_value_cansleep(unsigned gpio);
206
207 /* GPIO OUTPUT, might sleep */
208 void gpio_set_value_cansleep(unsigned gpio, int value);
209
210Other than the fact that these calls might sleep, and will not be ignored
211for GPIOs that can't be accessed from IRQ handlers, these calls act the
212same as the spinlock-safe calls.
213
214
215Claiming and Releasing GPIOs (OPTIONAL)
216---------------------------------------
217To help catch system configuration errors, two calls are defined.
218However, many platforms don't currently support this mechanism.
219
220 /* request GPIO, returning 0 or negative errno.
221 * non-null labels may be useful for diagnostics.
222 */
223 int gpio_request(unsigned gpio, const char *label);
224
225 /* release previously-claimed GPIO */
226 void gpio_free(unsigned gpio);
227
228Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
229GPIOs that have already been claimed with that call. The return value of
David Brownell83c65902007-05-16 22:11:13 -0700230gpio_request() must be checked. You should normally issue these calls from
231a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
232before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800233
234These calls serve two basic purposes. One is marking the signals which
235are actually in use as GPIOs, for better diagnostics; systems may have
236several hundred potential GPIOs, but often only a dozen are used on any
David Brownell7c2db752008-02-04 22:28:21 -0800237given board. Another is to catch conflicts, identifying errors when
238(a) two or more drivers wrongly think they have exclusive use of that
239signal, or (b) something wrongly believes it's safe to remove drivers
240needed to manage a signal that's in active use. That is, requesting a
241GPIO can serve as a kind of lock.
David Brownell4c203862007-02-12 00:53:11 -0800242
David Brownell35e8bb52008-10-15 22:03:16 -0700243Some platforms may also use knowledge about what GPIOs are active for
244power management, such as by powering down unused chip sectors and, more
245easily, gating off unused clocks.
246
David Brownell4c203862007-02-12 00:53:11 -0800247These two calls are optional because not not all current Linux platforms
248offer such functionality in their GPIO support; a valid implementation
249could return success for all gpio_request() calls. Unlike the other calls,
250the state they represent doesn't normally match anything from a hardware
251register; it's just a software bitmap which clearly is not necessary for
252correct operation of hardware or (bug free) drivers.
253
254Note that requesting a GPIO does NOT cause it to be configured in any
255way; it just marks that GPIO as in use. Separate code must handle any
256pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
257
David Brownell7c2db752008-02-04 22:28:21 -0800258Also note that it's your responsibility to have stopped using a GPIO
259before you free it.
260
David Brownell4c203862007-02-12 00:53:11 -0800261
262GPIOs mapped to IRQs
263--------------------
264GPIO numbers are unsigned integers; so are IRQ numbers. These make up
265two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
266map between them using calls like:
267
268 /* map GPIO numbers to IRQ numbers */
269 int gpio_to_irq(unsigned gpio);
270
David Brownell0f6d5042008-10-15 22:03:14 -0700271 /* map IRQ numbers to GPIO numbers (avoid using this) */
David Brownell4c203862007-02-12 00:53:11 -0800272 int irq_to_gpio(unsigned irq);
273
274Those return either the corresponding number in the other namespace, or
275else a negative errno code if the mapping can't be done. (For example,
David Brownell7c2db752008-02-04 22:28:21 -0800276some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
David Brownellbe1ff382007-07-23 18:43:57 -0700277number that wasn't set up as an input using gpio_direction_input(), or
David Brownell4c203862007-02-12 00:53:11 -0800278to use an IRQ number that didn't originally come from gpio_to_irq().
279
280These two mapping calls are expected to cost on the order of a single
281addition or subtraction. They're not allowed to sleep.
282
283Non-error values returned from gpio_to_irq() can be passed to request_irq()
284or free_irq(). They will often be stored into IRQ resources for platform
285devices, by the board-specific initialization code. Note that IRQ trigger
286options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
287system wakeup capabilities.
288
289Non-error values returned from irq_to_gpio() would most commonly be used
David Brownellf5de6112007-02-16 01:27:14 -0800290with gpio_get_value(), for example to initialize or update driver state
David Brownell0f6d5042008-10-15 22:03:14 -0700291when the IRQ is edge-triggered. Note that some platforms don't support
292this reverse mapping, so you should avoid using it.
David Brownell4c203862007-02-12 00:53:11 -0800293
294
David Brownell1668be72007-04-11 23:28:42 -0700295Emulating Open Drain Signals
296----------------------------
297Sometimes shared signals need to use "open drain" signaling, where only the
298low signal level is actually driven. (That term applies to CMOS transistors;
299"open collector" is used for TTL.) A pullup resistor causes the high signal
300level. This is sometimes called a "wire-AND"; or more practically, from the
301negative logic (low=true) perspective this is a "wire-OR".
302
303One common example of an open drain signal is a shared active-low IRQ line.
304Also, bidirectional data bus signals sometimes use open drain signals.
305
306Some GPIO controllers directly support open drain outputs; many don't. When
307you need open drain signaling but your hardware doesn't directly support it,
308there's a common idiom you can use to emulate it with any GPIO pin that can
309be used as either an input or an output:
310
311 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
312 and overrides the pullup.
313
314 HIGH: gpio_direction_input(gpio) ... this turns off the output,
315 so the pullup (or some other device) controls the signal.
316
317If you are "driving" the signal high but gpio_get_value(gpio) reports a low
318value (after the appropriate rise time passes), you know some other component
319is driving the shared signal low. That's not necessarily an error. As one
320common example, that's how I2C clocks are stretched: a slave that needs a
321slower clock delays the rising edge of SCK, and the I2C master adjusts its
322signaling rate accordingly.
323
David Brownell4c203862007-02-12 00:53:11 -0800324
325What do these conventions omit?
326===============================
327One of the biggest things these conventions omit is pin multiplexing, since
328this is highly chip-specific and nonportable. One platform might not need
329explicit multiplexing; another might have just two options for use of any
330given pin; another might have eight options per pin; another might be able
331to route a given GPIO to any one of several pins. (Yes, those examples all
332come from systems that run Linux today.)
333
334Related to multiplexing is configuration and enabling of the pullups or
335pulldowns integrated on some platforms. Not all platforms support them,
336or support them in the same way; and any given board might use external
337pullups (or pulldowns) so that the on-chip ones should not be used.
David Brownell7c2db752008-02-04 22:28:21 -0800338(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
David Brownell7560fa62008-03-04 14:28:27 -0800339Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
340platform-specific issue, as are models like (not) having a one-to-one
341correspondence between configurable pins and GPIOs.
David Brownell4c203862007-02-12 00:53:11 -0800342
343There are other system-specific mechanisms that are not specified here,
344like the aforementioned options for input de-glitching and wire-OR output.
345Hardware may support reading or writing GPIOs in gangs, but that's usually
David Brownellf5de6112007-02-16 01:27:14 -0800346configuration dependent: for GPIOs sharing the same bank. (GPIOs are
David Brownell4c203862007-02-12 00:53:11 -0800347commonly grouped in banks of 16 or 32, with a given SOC having several such
David Brownell7c2db752008-02-04 22:28:21 -0800348banks.) Some systems can trigger IRQs from output GPIOs, or read values
349from pins not managed as GPIOs. Code relying on such mechanisms will
350necessarily be nonportable.
David Brownell4c203862007-02-12 00:53:11 -0800351
David Brownell7c2db752008-02-04 22:28:21 -0800352Dynamic definition of GPIOs is not currently standard; for example, as
David Brownell4c203862007-02-12 00:53:11 -0800353a side effect of configuring an add-on board with some GPIO expanders.
354
David Brownell7c2db752008-02-04 22:28:21 -0800355
356GPIO implementor's framework (OPTIONAL)
357=======================================
358As noted earlier, there is an optional implementation framework making it
359easier for platforms to support different kinds of GPIO controller using
David Brownelld8f388d2008-07-25 01:46:07 -0700360the same programming interface. This framework is called "gpiolib".
David Brownell7c2db752008-02-04 22:28:21 -0800361
362As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
363will be found there. That will list all the controllers registered through
364this framework, and the state of the GPIOs currently in use.
365
366
367Controller Drivers: gpio_chip
368-----------------------------
369In this framework each GPIO controller is packaged as a "struct gpio_chip"
370with information common to each controller of that type:
371
372 - methods to establish GPIO direction
373 - methods used to access GPIO values
374 - flag saying whether calls to its methods may sleep
375 - optional debugfs dump method (showing extra state like pullup config)
376 - label for diagnostics
377
378There is also per-instance data, which may come from device.platform_data:
379the number of its first GPIO, and how many GPIOs it exposes.
380
381The code implementing a gpio_chip should support multiple instances of the
382controller, possibly using the driver model. That code will configure each
383gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
384rare; use gpiochip_remove() when it is unavoidable.
385
386Most often a gpio_chip is part of an instance-specific structure with state
387not exposed by the GPIO interfaces, such as addressing, power management,
388and more. Chips such as codecs will have complex non-GPIO state,
389
390Any debugfs dump method should normally ignore signals which haven't been
391requested as GPIOs. They can use gpiochip_is_requested(), which returns
392either NULL or the label associated with that GPIO when it was requested.
393
394
395Platform Support
396----------------
Michael Buesch7444a722008-07-25 01:46:11 -0700397To support this framework, a platform's Kconfig will "select" either
398ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
David Brownell7c2db752008-02-04 22:28:21 -0800399and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
400three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
401They may also want to provide a custom value for ARCH_NR_GPIOS.
402
Michael Buesch7444a722008-07-25 01:46:11 -0700403ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled
404into the kernel on that architecture.
405
406ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user
407can enable it and build it into the kernel optionally.
408
409If neither of these options are selected, the platform does not support
410GPIOs through GPIO-lib and the code cannot be enabled by the user.
411
David Brownell7c2db752008-02-04 22:28:21 -0800412Trivial implementations of those functions can directly use framework
413code, which always dispatches through the gpio_chip:
414
415 #define gpio_get_value __gpio_get_value
416 #define gpio_set_value __gpio_set_value
417 #define gpio_cansleep __gpio_cansleep
418
419Fancier implementations could instead define those as inline functions with
420logic optimizing access to specific SOC-based GPIOs. For example, if the
421referenced GPIO is the constant "12", getting or setting its value could
422cost as little as two or three instructions, never sleeping. When such an
423optimization is not possible those calls must delegate to the framework
424code, costing at least a few dozen instructions. For bitbanged I/O, such
425instruction savings can be significant.
426
427For SOCs, platform-specific code defines and registers gpio_chip instances
428for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
429match chip vendor documentation, and directly match board schematics. They
430may well start at zero and go up to a platform-specific limit. Such GPIOs
431are normally integrated into platform initialization to make them always be
432available, from arch_initcall() or earlier; they can often serve as IRQs.
433
434
435Board Support
436-------------
437For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
438function devices, FPGAs or CPLDs -- most often board-specific code handles
439registering controller devices and ensures that their drivers know what GPIO
440numbers to use with gpiochip_add(). Their numbers often start right after
441platform-specific GPIOs.
442
443For example, board setup code could create structures identifying the range
444of GPIOs that chip will expose, and passes them to each GPIO expander chip
445using platform_data. Then the chip driver's probe() routine could pass that
446data to gpiochip_add().
447
448Initialization order can be important. For example, when a device relies on
449an I2C-based GPIO, its probe() routine should only be called after that GPIO
450becomes available. That may mean the device should not be registered until
451calls for that GPIO can work. One way to address such dependencies is for
452such gpio_chip controllers to provide setup() and teardown() callbacks to
453board specific code; those board specific callbacks would register devices
David Brownelld8f388d2008-07-25 01:46:07 -0700454once all the necessary resources are available, and remove them later when
455the GPIO controller device becomes unavailable.
456
457
458Sysfs Interface for Userspace (OPTIONAL)
459========================================
460Platforms which use the "gpiolib" implementors framework may choose to
461configure a sysfs user interface to GPIOs. This is different from the
462debugfs interface, since it provides control over GPIO direction and
463value instead of just showing a gpio state summary. Plus, it could be
464present on production systems without debugging support.
465
466Given approprate hardware documentation for the system, userspace could
467know for example that GPIO #23 controls the write protect line used to
468protect boot loader segments in flash memory. System upgrade procedures
469may need to temporarily remove that protection, first importing a GPIO,
470then changing its output state, then updating the code before re-enabling
471the write protection. In normal use, GPIO #23 would never be touched,
472and the kernel would have no need to know about it.
473
474Again depending on appropriate hardware documentation, on some systems
475userspace GPIO can be used to determine system configuration data that
476standard kernels won't know about. And for some tasks, simple userspace
477GPIO drivers could be all that the system really needs.
478
479Note that standard kernel drivers exist for common "LEDs and Buttons"
480GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
481instead of talking directly to the GPIOs; they integrate with kernel
482frameworks better than your userspace code could.
483
484
485Paths in Sysfs
486--------------
487There are three kinds of entry in /sys/class/gpio:
488
489 - Control interfaces used to get userspace control over GPIOs;
490
491 - GPIOs themselves; and
492
493 - GPIO controllers ("gpio_chip" instances).
494
495That's in addition to standard files including the "device" symlink.
496
497The control interfaces are write-only:
498
499 /sys/class/gpio/
500
501 "export" ... Userspace may ask the kernel to export control of
502 a GPIO to userspace by writing its number to this file.
503
504 Example: "echo 19 > export" will create a "gpio19" node
505 for GPIO #19, if that's not requested by kernel code.
506
507 "unexport" ... Reverses the effect of exporting to userspace.
508
509 Example: "echo 19 > unexport" will remove a "gpio19"
510 node exported using the "export" file.
511
512GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
513and have the following read/write attributes:
514
515 /sys/class/gpio/gpioN/
516
517 "direction" ... reads as either "in" or "out". This value may
518 normally be written. Writing as "out" defaults to
519 initializing the value as low. To ensure glitch free
520 operation, values "low" and "high" may be written to
521 configure the GPIO as an output with that initial value.
522
523 Note that this attribute *will not exist* if the kernel
524 doesn't support changing the direction of a GPIO, or
525 it was exported by kernel code that didn't explicitly
526 allow userspace to reconfigure this GPIO's direction.
527
528 "value" ... reads as either 0 (low) or 1 (high). If the GPIO
529 is configured as an output, this value may be written;
530 any nonzero value is treated as high.
531
532GPIO controllers have paths like /sys/class/gpio/chipchip42/ (for the
533controller implementing GPIOs starting at #42) and have the following
534read-only attributes:
535
536 /sys/class/gpio/gpiochipN/
537
538 "base" ... same as N, the first GPIO managed by this chip
539
540 "label" ... provided for diagnostics (not always unique)
541
542 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
543
544Board documentation should in most cases cover what GPIOs are used for
545what purposes. However, those numbers are not always stable; GPIOs on
546a daughtercard might be different depending on the base board being used,
547or other cards in the stack. In such cases, you may need to use the
548gpiochip nodes (possibly in conjunction with schematics) to determine
549the correct GPIO number to use for a given signal.
550
551
552Exporting from Kernel code
553--------------------------
554Kernel code can explicitly manage exports of GPIOs which have already been
555requested using gpio_request():
556
557 /* export the GPIO to userspace */
558 int gpio_export(unsigned gpio, bool direction_may_change);
559
560 /* reverse gpio_export() */
561 void gpio_unexport();
562
563After a kernel driver requests a GPIO, it may only be made available in
564the sysfs interface by gpio_export(). The driver can control whether the
565signal direction may change. This helps drivers prevent userspace code
566from accidentally clobbering important system state.
567
568This explicit exporting can help with debugging (by making some kinds
569of experiments easier), or can provide an always-there interface that's
570suitable for documenting as part of a board support package.