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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-----------
David Brownell8a0cecf2009-04-02 16:57:06 -0700126The first thing a system should do with a GPIO is allocate it, using
127the gpio_request() call; see later.
128
129One of the next things to do with a GPIO, often in board setup code when
David Brownell4c203862007-02-12 00:53:11 -0800130setting up a platform_device using the GPIO, is mark its direction:
131
132 /* set as input or output, returning 0 or negative errno */
133 int gpio_direction_input(unsigned gpio);
David Brownell28735a72007-03-16 13:38:14 -0800134 int gpio_direction_output(unsigned gpio, int value);
David Brownell4c203862007-02-12 00:53:11 -0800135
136The return value is zero for success, else a negative errno. It should
137be checked, since the get/set calls don't have error returns and since
David Brownell83c65902007-05-16 22:11:13 -0700138misconfiguration is possible. You should normally issue these calls from
139a task context. However, for spinlock-safe GPIOs it's OK to use them
140before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800141
David Brownell28735a72007-03-16 13:38:14 -0800142For output GPIOs, the value provided becomes the initial output value.
143This helps avoid signal glitching during system startup.
144
David Brownell7c2db752008-02-04 22:28:21 -0800145For compatibility with legacy interfaces to GPIOs, setting the direction
146of a GPIO implicitly requests that GPIO (see below) if it has not been
David Brownell8a0cecf2009-04-02 16:57:06 -0700147requested already. That compatibility is being removed from the optional
148gpiolib framework.
David Brownell7c2db752008-02-04 22:28:21 -0800149
David Brownell4c203862007-02-12 00:53:11 -0800150Setting the direction can fail if the GPIO number is invalid, or when
151that particular GPIO can't be used in that mode. It's generally a bad
152idea to rely on boot firmware to have set the direction correctly, since
153it probably wasn't validated to do more than boot Linux. (Similarly,
154that board setup code probably needs to multiplex that pin as a GPIO,
155and configure pullups/pulldowns appropriately.)
156
157
158Spinlock-Safe GPIO access
159-------------------------
160Most GPIO controllers can be accessed with memory read/write instructions.
161That doesn't need to sleep, and can safely be done from inside IRQ handlers.
David Brownell7c2db752008-02-04 22:28:21 -0800162(That includes hardirq contexts on RT kernels.)
David Brownell4c203862007-02-12 00:53:11 -0800163
164Use these calls to access such GPIOs:
165
166 /* GPIO INPUT: return zero or nonzero */
167 int gpio_get_value(unsigned gpio);
168
169 /* GPIO OUTPUT */
170 void gpio_set_value(unsigned gpio, int value);
171
172The values are boolean, zero for low, nonzero for high. When reading the
173value of an output pin, the value returned should be what's seen on the
174pin ... that won't always match the specified output value, because of
David Brownell7c2db752008-02-04 22:28:21 -0800175issues including open-drain signaling and output latencies.
David Brownell4c203862007-02-12 00:53:11 -0800176
177The get/set calls have no error returns because "invalid GPIO" should have
David Brownellbe1ff382007-07-23 18:43:57 -0700178been reported earlier from gpio_direction_*(). However, note that not all
David Brownell4c203862007-02-12 00:53:11 -0800179platforms can read the value of output pins; those that can't should always
David Brownellf5de6112007-02-16 01:27:14 -0800180return zero. Also, using these calls for GPIOs that can't safely be accessed
181without sleeping (see below) is an error.
David Brownell4c203862007-02-12 00:53:11 -0800182
David Brownellf5de6112007-02-16 01:27:14 -0800183Platform-specific implementations are encouraged to optimize the two
David Brownell4c203862007-02-12 00:53:11 -0800184calls to access the GPIO value in cases where the GPIO number (and for
185output, value) are constant. It's normal for them to need only a couple
186of instructions in such cases (reading or writing a hardware register),
187and not to need spinlocks. Such optimized calls can make bitbanging
188applications a lot more efficient (in both space and time) than spending
189dozens of instructions on subroutine calls.
190
191
192GPIO access that may sleep
193--------------------------
194Some GPIO controllers must be accessed using message based busses like I2C
195or SPI. Commands to read or write those GPIO values require waiting to
196get to the head of a queue to transmit a command and get its response.
197This requires sleeping, which can't be done from inside IRQ handlers.
198
199Platforms that support this type of GPIO distinguish them from other GPIOs
David Brownell7c2db752008-02-04 22:28:21 -0800200by returning nonzero from this call (which requires a valid GPIO number,
David Brownell8a0cecf2009-04-02 16:57:06 -0700201which should have been previously allocated with gpio_request):
David Brownell4c203862007-02-12 00:53:11 -0800202
203 int gpio_cansleep(unsigned gpio);
204
205To access such GPIOs, a different set of accessors is defined:
206
207 /* GPIO INPUT: return zero or nonzero, might sleep */
208 int gpio_get_value_cansleep(unsigned gpio);
209
210 /* GPIO OUTPUT, might sleep */
211 void gpio_set_value_cansleep(unsigned gpio, int value);
212
213Other than the fact that these calls might sleep, and will not be ignored
214for GPIOs that can't be accessed from IRQ handlers, these calls act the
215same as the spinlock-safe calls.
216
217
David Brownell8a0cecf2009-04-02 16:57:06 -0700218Claiming and Releasing GPIOs
219----------------------------
David Brownell4c203862007-02-12 00:53:11 -0800220To help catch system configuration errors, two calls are defined.
David Brownell4c203862007-02-12 00:53:11 -0800221
222 /* request GPIO, returning 0 or negative errno.
223 * non-null labels may be useful for diagnostics.
224 */
225 int gpio_request(unsigned gpio, const char *label);
226
227 /* release previously-claimed GPIO */
228 void gpio_free(unsigned gpio);
229
230Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
231GPIOs that have already been claimed with that call. The return value of
David Brownell83c65902007-05-16 22:11:13 -0700232gpio_request() must be checked. You should normally issue these calls from
233a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
234before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800235
236These calls serve two basic purposes. One is marking the signals which
237are actually in use as GPIOs, for better diagnostics; systems may have
238several hundred potential GPIOs, but often only a dozen are used on any
David Brownell7c2db752008-02-04 22:28:21 -0800239given board. Another is to catch conflicts, identifying errors when
240(a) two or more drivers wrongly think they have exclusive use of that
241signal, or (b) something wrongly believes it's safe to remove drivers
242needed to manage a signal that's in active use. That is, requesting a
243GPIO can serve as a kind of lock.
David Brownell4c203862007-02-12 00:53:11 -0800244
David Brownell35e8bb52008-10-15 22:03:16 -0700245Some platforms may also use knowledge about what GPIOs are active for
246power management, such as by powering down unused chip sectors and, more
247easily, gating off unused clocks.
248
David Brownell4c203862007-02-12 00:53:11 -0800249Note that requesting a GPIO does NOT cause it to be configured in any
250way; it just marks that GPIO as in use. Separate code must handle any
251pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
252
David Brownell7c2db752008-02-04 22:28:21 -0800253Also note that it's your responsibility to have stopped using a GPIO
254before you free it.
255
David Brownell4c203862007-02-12 00:53:11 -0800256
257GPIOs mapped to IRQs
258--------------------
259GPIO numbers are unsigned integers; so are IRQ numbers. These make up
260two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
261map between them using calls like:
262
263 /* map GPIO numbers to IRQ numbers */
264 int gpio_to_irq(unsigned gpio);
265
David Brownell0f6d5042008-10-15 22:03:14 -0700266 /* map IRQ numbers to GPIO numbers (avoid using this) */
David Brownell4c203862007-02-12 00:53:11 -0800267 int irq_to_gpio(unsigned irq);
268
269Those return either the corresponding number in the other namespace, or
270else a negative errno code if the mapping can't be done. (For example,
David Brownell7c2db752008-02-04 22:28:21 -0800271some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
David Brownellbe1ff382007-07-23 18:43:57 -0700272number that wasn't set up as an input using gpio_direction_input(), or
David Brownell4c203862007-02-12 00:53:11 -0800273to use an IRQ number that didn't originally come from gpio_to_irq().
274
275These two mapping calls are expected to cost on the order of a single
276addition or subtraction. They're not allowed to sleep.
277
278Non-error values returned from gpio_to_irq() can be passed to request_irq()
279or free_irq(). They will often be stored into IRQ resources for platform
280devices, by the board-specific initialization code. Note that IRQ trigger
281options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
282system wakeup capabilities.
283
284Non-error values returned from irq_to_gpio() would most commonly be used
David Brownellf5de6112007-02-16 01:27:14 -0800285with gpio_get_value(), for example to initialize or update driver state
David Brownell0f6d5042008-10-15 22:03:14 -0700286when the IRQ is edge-triggered. Note that some platforms don't support
287this reverse mapping, so you should avoid using it.
David Brownell4c203862007-02-12 00:53:11 -0800288
289
David Brownell1668be72007-04-11 23:28:42 -0700290Emulating Open Drain Signals
291----------------------------
292Sometimes shared signals need to use "open drain" signaling, where only the
293low signal level is actually driven. (That term applies to CMOS transistors;
294"open collector" is used for TTL.) A pullup resistor causes the high signal
295level. This is sometimes called a "wire-AND"; or more practically, from the
296negative logic (low=true) perspective this is a "wire-OR".
297
298One common example of an open drain signal is a shared active-low IRQ line.
299Also, bidirectional data bus signals sometimes use open drain signals.
300
301Some GPIO controllers directly support open drain outputs; many don't. When
302you need open drain signaling but your hardware doesn't directly support it,
303there's a common idiom you can use to emulate it with any GPIO pin that can
304be used as either an input or an output:
305
306 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
307 and overrides the pullup.
308
309 HIGH: gpio_direction_input(gpio) ... this turns off the output,
310 so the pullup (or some other device) controls the signal.
311
312If you are "driving" the signal high but gpio_get_value(gpio) reports a low
313value (after the appropriate rise time passes), you know some other component
314is driving the shared signal low. That's not necessarily an error. As one
315common example, that's how I2C clocks are stretched: a slave that needs a
316slower clock delays the rising edge of SCK, and the I2C master adjusts its
317signaling rate accordingly.
318
David Brownell4c203862007-02-12 00:53:11 -0800319
320What do these conventions omit?
321===============================
322One of the biggest things these conventions omit is pin multiplexing, since
323this is highly chip-specific and nonportable. One platform might not need
324explicit multiplexing; another might have just two options for use of any
325given pin; another might have eight options per pin; another might be able
326to route a given GPIO to any one of several pins. (Yes, those examples all
327come from systems that run Linux today.)
328
329Related to multiplexing is configuration and enabling of the pullups or
330pulldowns integrated on some platforms. Not all platforms support them,
331or support them in the same way; and any given board might use external
332pullups (or pulldowns) so that the on-chip ones should not be used.
David Brownell7c2db752008-02-04 22:28:21 -0800333(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
David Brownell7560fa62008-03-04 14:28:27 -0800334Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
335platform-specific issue, as are models like (not) having a one-to-one
336correspondence between configurable pins and GPIOs.
David Brownell4c203862007-02-12 00:53:11 -0800337
338There are other system-specific mechanisms that are not specified here,
339like the aforementioned options for input de-glitching and wire-OR output.
340Hardware may support reading or writing GPIOs in gangs, but that's usually
David Brownellf5de6112007-02-16 01:27:14 -0800341configuration dependent: for GPIOs sharing the same bank. (GPIOs are
David Brownell4c203862007-02-12 00:53:11 -0800342commonly grouped in banks of 16 or 32, with a given SOC having several such
David Brownell7c2db752008-02-04 22:28:21 -0800343banks.) Some systems can trigger IRQs from output GPIOs, or read values
344from pins not managed as GPIOs. Code relying on such mechanisms will
345necessarily be nonportable.
David Brownell4c203862007-02-12 00:53:11 -0800346
David Brownell7c2db752008-02-04 22:28:21 -0800347Dynamic definition of GPIOs is not currently standard; for example, as
David Brownell4c203862007-02-12 00:53:11 -0800348a side effect of configuring an add-on board with some GPIO expanders.
349
David Brownell7c2db752008-02-04 22:28:21 -0800350
351GPIO implementor's framework (OPTIONAL)
352=======================================
353As noted earlier, there is an optional implementation framework making it
354easier for platforms to support different kinds of GPIO controller using
David Brownelld8f388d2008-07-25 01:46:07 -0700355the same programming interface. This framework is called "gpiolib".
David Brownell7c2db752008-02-04 22:28:21 -0800356
357As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
358will be found there. That will list all the controllers registered through
359this framework, and the state of the GPIOs currently in use.
360
361
362Controller Drivers: gpio_chip
363-----------------------------
364In this framework each GPIO controller is packaged as a "struct gpio_chip"
365with information common to each controller of that type:
366
367 - methods to establish GPIO direction
368 - methods used to access GPIO values
369 - flag saying whether calls to its methods may sleep
370 - optional debugfs dump method (showing extra state like pullup config)
371 - label for diagnostics
372
373There is also per-instance data, which may come from device.platform_data:
374the number of its first GPIO, and how many GPIOs it exposes.
375
376The code implementing a gpio_chip should support multiple instances of the
377controller, possibly using the driver model. That code will configure each
378gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
379rare; use gpiochip_remove() when it is unavoidable.
380
381Most often a gpio_chip is part of an instance-specific structure with state
382not exposed by the GPIO interfaces, such as addressing, power management,
383and more. Chips such as codecs will have complex non-GPIO state,
384
385Any debugfs dump method should normally ignore signals which haven't been
386requested as GPIOs. They can use gpiochip_is_requested(), which returns
387either NULL or the label associated with that GPIO when it was requested.
388
389
390Platform Support
391----------------
Michael Buesch7444a722008-07-25 01:46:11 -0700392To support this framework, a platform's Kconfig will "select" either
393ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
David Brownell7c2db752008-02-04 22:28:21 -0800394and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
395three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
396They may also want to provide a custom value for ARCH_NR_GPIOS.
397
Michael Buesch7444a722008-07-25 01:46:11 -0700398ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled
399into the kernel on that architecture.
400
401ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user
402can enable it and build it into the kernel optionally.
403
404If neither of these options are selected, the platform does not support
405GPIOs through GPIO-lib and the code cannot be enabled by the user.
406
David Brownell7c2db752008-02-04 22:28:21 -0800407Trivial implementations of those functions can directly use framework
408code, which always dispatches through the gpio_chip:
409
410 #define gpio_get_value __gpio_get_value
411 #define gpio_set_value __gpio_set_value
412 #define gpio_cansleep __gpio_cansleep
413
414Fancier implementations could instead define those as inline functions with
415logic optimizing access to specific SOC-based GPIOs. For example, if the
416referenced GPIO is the constant "12", getting or setting its value could
417cost as little as two or three instructions, never sleeping. When such an
418optimization is not possible those calls must delegate to the framework
419code, costing at least a few dozen instructions. For bitbanged I/O, such
420instruction savings can be significant.
421
422For SOCs, platform-specific code defines and registers gpio_chip instances
423for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
424match chip vendor documentation, and directly match board schematics. They
425may well start at zero and go up to a platform-specific limit. Such GPIOs
426are normally integrated into platform initialization to make them always be
427available, from arch_initcall() or earlier; they can often serve as IRQs.
428
429
430Board Support
431-------------
432For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
433function devices, FPGAs or CPLDs -- most often board-specific code handles
434registering controller devices and ensures that their drivers know what GPIO
435numbers to use with gpiochip_add(). Their numbers often start right after
436platform-specific GPIOs.
437
438For example, board setup code could create structures identifying the range
439of GPIOs that chip will expose, and passes them to each GPIO expander chip
440using platform_data. Then the chip driver's probe() routine could pass that
441data to gpiochip_add().
442
443Initialization order can be important. For example, when a device relies on
444an I2C-based GPIO, its probe() routine should only be called after that GPIO
445becomes available. That may mean the device should not be registered until
446calls for that GPIO can work. One way to address such dependencies is for
447such gpio_chip controllers to provide setup() and teardown() callbacks to
448board specific code; those board specific callbacks would register devices
David Brownelld8f388d2008-07-25 01:46:07 -0700449once all the necessary resources are available, and remove them later when
450the GPIO controller device becomes unavailable.
451
452
453Sysfs Interface for Userspace (OPTIONAL)
454========================================
455Platforms which use the "gpiolib" implementors framework may choose to
456configure a sysfs user interface to GPIOs. This is different from the
457debugfs interface, since it provides control over GPIO direction and
458value instead of just showing a gpio state summary. Plus, it could be
459present on production systems without debugging support.
460
461Given approprate hardware documentation for the system, userspace could
462know for example that GPIO #23 controls the write protect line used to
463protect boot loader segments in flash memory. System upgrade procedures
464may need to temporarily remove that protection, first importing a GPIO,
465then changing its output state, then updating the code before re-enabling
466the write protection. In normal use, GPIO #23 would never be touched,
467and the kernel would have no need to know about it.
468
469Again depending on appropriate hardware documentation, on some systems
470userspace GPIO can be used to determine system configuration data that
471standard kernels won't know about. And for some tasks, simple userspace
472GPIO drivers could be all that the system really needs.
473
474Note that standard kernel drivers exist for common "LEDs and Buttons"
475GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
476instead of talking directly to the GPIOs; they integrate with kernel
477frameworks better than your userspace code could.
478
479
480Paths in Sysfs
481--------------
482There are three kinds of entry in /sys/class/gpio:
483
484 - Control interfaces used to get userspace control over GPIOs;
485
486 - GPIOs themselves; and
487
488 - GPIO controllers ("gpio_chip" instances).
489
490That's in addition to standard files including the "device" symlink.
491
492The control interfaces are write-only:
493
494 /sys/class/gpio/
495
496 "export" ... Userspace may ask the kernel to export control of
497 a GPIO to userspace by writing its number to this file.
498
499 Example: "echo 19 > export" will create a "gpio19" node
500 for GPIO #19, if that's not requested by kernel code.
501
502 "unexport" ... Reverses the effect of exporting to userspace.
503
504 Example: "echo 19 > unexport" will remove a "gpio19"
505 node exported using the "export" file.
506
507GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
508and have the following read/write attributes:
509
510 /sys/class/gpio/gpioN/
511
512 "direction" ... reads as either "in" or "out". This value may
513 normally be written. Writing as "out" defaults to
514 initializing the value as low. To ensure glitch free
515 operation, values "low" and "high" may be written to
516 configure the GPIO as an output with that initial value.
517
518 Note that this attribute *will not exist* if the kernel
519 doesn't support changing the direction of a GPIO, or
520 it was exported by kernel code that didn't explicitly
521 allow userspace to reconfigure this GPIO's direction.
522
523 "value" ... reads as either 0 (low) or 1 (high). If the GPIO
524 is configured as an output, this value may be written;
525 any nonzero value is treated as high.
526
527GPIO controllers have paths like /sys/class/gpio/chipchip42/ (for the
528controller implementing GPIOs starting at #42) and have the following
529read-only attributes:
530
531 /sys/class/gpio/gpiochipN/
532
533 "base" ... same as N, the first GPIO managed by this chip
534
535 "label" ... provided for diagnostics (not always unique)
536
537 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
538
539Board documentation should in most cases cover what GPIOs are used for
540what purposes. However, those numbers are not always stable; GPIOs on
541a daughtercard might be different depending on the base board being used,
542or other cards in the stack. In such cases, you may need to use the
543gpiochip nodes (possibly in conjunction with schematics) to determine
544the correct GPIO number to use for a given signal.
545
546
547Exporting from Kernel code
548--------------------------
549Kernel code can explicitly manage exports of GPIOs which have already been
550requested using gpio_request():
551
552 /* export the GPIO to userspace */
553 int gpio_export(unsigned gpio, bool direction_may_change);
554
555 /* reverse gpio_export() */
556 void gpio_unexport();
557
558After a kernel driver requests a GPIO, it may only be made available in
559the sysfs interface by gpio_export(). The driver can control whether the
560signal direction may change. This helps drivers prevent userspace code
561from accidentally clobbering important system state.
562
563This explicit exporting can help with debugging (by making some kinds
564of experiments easier), or can provide an always-there interface that's
565suitable for documenting as part of a board support package.