1GPIO Interfaces
3This provides an overview of GPIO access conventions on Linux.
5These calls use the gpio_* naming prefix. No other calls should use that
6prefix, or the related __gpio_* prefix.
9What is a GPIO?
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.
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).
28The exact capabilities of GPIOs vary between systems. Common options:
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).
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.
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.
44  - Usually a GPIO will be configurable as either input or output, as needed
45    by different product boards; single direction ones exist too.
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.
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.
56GPIO conventions
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.
66Plus, this doesn't require any implementation framework, just an interface.
67One platform might implement it as simple inline functions accessing chip
68registers; another might implement it by delegating through abstractions
69used 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.)
74That said, if the convention is supported on their platform, drivers should
75use 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:
81    #include <linux/gpio.h>
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.
86Note that these operations include I/O barriers on platforms which need to
87use them; drivers don't need to add them explicitly.
90Identifying GPIOs
92GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
93reserves "negative" numbers for other purposes like marking signals as
94"not available on this board", or indicating faults. Code that doesn't
95touch the underlying hardware treats these integers as opaque cookies.
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.
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.
110If 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 such number from such a structure could reference a GPIO, you
113may use this predicate:
115    int gpio_is_valid(int number);
117A number that's not valid will be rejected by calls which may request
118or free GPIOs (see below). Other numbers may also be rejected; for
119example, a number might be valid but temporarily unused on a given board.
121Whether a platform supports multiple GPIO controllers is a platform-specific
122implementation issue, as are whether that support can leave "holes" in the space
123of GPIO numbers, and whether new controllers can be added at runtime. Such issues
124can affect things including whether adjacent GPIO numbers are both valid.
126Using GPIOs
128The first thing a system should do with a GPIO is allocate it, using
129the gpio_request() call; see later.
131One of the next things to do with a GPIO, often in board setup code when
132setting up a platform_device using the GPIO, is mark its direction:
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);
138The return value is zero for success, else a negative errno. It should
139be checked, since the get/set calls don't have error returns and since
140misconfiguration is possible. You should normally issue these calls from
141a task context. However, for spinlock-safe GPIOs it's OK to use them
142before tasking is enabled, as part of early board setup.
144For output GPIOs, the value provided becomes the initial output value.
145This helps avoid signal glitching during system startup.
147For compatibility with legacy interfaces to GPIOs, setting the direction
148of a GPIO implicitly requests that GPIO (see below) if it has not been
149requested already. That compatibility is being removed from the optional
150gpiolib framework.
152Setting the direction can fail if the GPIO number is invalid, or when
153that particular GPIO can't be used in that mode. It's generally a bad
154idea to rely on boot firmware to have set the direction correctly, since
155it probably wasn't validated to do more than boot Linux. (Similarly,
156that board setup code probably needs to multiplex that pin as a GPIO,
157and configure pullups/pulldowns appropriately.)
160Spinlock-Safe GPIO access
162Most GPIO controllers can be accessed with memory read/write instructions.
163Those don't need to sleep, and can safely be done from inside hard
164(nonthreaded) IRQ handlers and similar contexts.
166Use the following calls to access such GPIOs,
167for which gpio_cansleep() will always return false (see below):
169    /* GPIO INPUT: return zero or nonzero */
170    int gpio_get_value(unsigned gpio);
172    /* GPIO OUTPUT */
173    void gpio_set_value(unsigned gpio, int value);
175The values are boolean, zero for low, nonzero for high. When reading the
176value of an output pin, the value returned should be what's seen on the
177pin ... that won't always match the specified output value, because of
178issues including open-drain signaling and output latencies.
180The get/set calls have no error returns because "invalid GPIO" should have
181been reported earlier from gpio_direction_*(). However, note that not all
182platforms can read the value of output pins; those that can't should always
183return zero. Also, using these calls for GPIOs that can't safely be accessed
184without sleeping (see below) is an error.
186Platform-specific implementations are encouraged to optimize the two
187calls to access the GPIO value in cases where the GPIO number (and for
188output, value) are constant. It's normal for them to need only a couple
189of instructions in such cases (reading or writing a hardware register),
190and not to need spinlocks. Such optimized calls can make bitbanging
191applications a lot more efficient (in both space and time) than spending
192dozens of instructions on subroutine calls.
195GPIO access that may sleep
197Some GPIO controllers must be accessed using message based busses like I2C
198or SPI. Commands to read or write those GPIO values require waiting to
199get to the head of a queue to transmit a command and get its response.
200This requires sleeping, which can't be done from inside IRQ handlers.
202Platforms that support this type of GPIO distinguish them from other GPIOs
203by returning nonzero from this call (which requires a valid GPIO number,
204which should have been previously allocated with gpio_request):
206    int gpio_cansleep(unsigned gpio);
208To access such GPIOs, a different set of accessors is defined:
210    /* GPIO INPUT: return zero or nonzero, might sleep */
211    int gpio_get_value_cansleep(unsigned gpio);
213    /* GPIO OUTPUT, might sleep */
214    void gpio_set_value_cansleep(unsigned gpio, int value);
217Accessing such GPIOs requires a context which may sleep, for example
218a threaded IRQ handler, and those accessors must be used instead of
219spinlock-safe accessors without the cansleep() name suffix.
221Other than the fact that these accessors might sleep, and will work
222on GPIOs that can't be accessed from hardIRQ handlers, these calls act
223the same as the spinlock-safe calls.
225  ** IN ADDITION ** calls to setup and configure such GPIOs must be made
226from contexts which may sleep, since they may need to access the GPIO
227controller chip too: (These setup calls are usually made from board
228setup or driver probe/teardown code, so this is an easy constraint.)
230    gpio_direction_input()
231    gpio_direction_output()
232    gpio_request()
234## gpio_request_one()
235## gpio_request_array()
236## gpio_free_array()
238    gpio_free()
239    gpio_set_debounce()
243Claiming and Releasing GPIOs
245To help catch system configuration errors, two calls are defined.
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);
252    /* release previously-claimed GPIO */
253    void gpio_free(unsigned gpio);
255Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
256GPIOs that have already been claimed with that call. The return value of
257gpio_request() must be checked. You should normally issue these calls from
258a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
259before tasking is enabled, as part of early board setup.
261These calls serve two basic purposes. One is marking the signals which
262are actually in use as GPIOs, for better diagnostics; systems may have
263several hundred potential GPIOs, but often only a dozen are used on any
264given board. Another is to catch conflicts, identifying errors when
265(a) two or more drivers wrongly think they have exclusive use of that
266signal, or (b) something wrongly believes it's safe to remove drivers
267needed to manage a signal that's in active use. That is, requesting a
268GPIO can serve as a kind of lock.
270Some platforms may also use knowledge about what GPIOs are active for
271power management, such as by powering down unused chip sectors and, more
272easily, gating off unused clocks.
274Note that requesting a GPIO does NOT cause it to be configured in any
275way; it just marks that GPIO as in use. Separate code must handle any
276pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
278Also note that it's your responsibility to have stopped using a GPIO
279before you free it.
281Considering in most cases GPIOs are actually configured right after they
282are claimed, three additional calls are defined:
284    /* request a single GPIO, with initial configuration specified by
285     * 'flags', identical to gpio_request() wrt other arguments and
286     * return value
287     */
288    int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
290    /* request multiple GPIOs in a single call
291     */
292    int gpio_request_array(struct gpio *array, size_t num);
294    /* release multiple GPIOs in a single call
295     */
296    void gpio_free_array(struct gpio *array, size_t num);
298where 'flags' is currently defined to specify the following properties:
300    * GPIOF_DIR_IN - to configure direction as input
301    * GPIOF_DIR_OUT - to configure direction as output
303    * GPIOF_INIT_LOW - as output, set initial level to LOW
304    * GPIOF_INIT_HIGH - as output, set initial level to HIGH
306since GPIOF_INIT_* are only valid when configured as output, so group valid
307combinations as:
309    * GPIOF_IN - configure as input
310    * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
311    * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
313In the future, these flags can be extended to support more properties such
314as open-drain status.
316Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
317introduced to encapsulate all three fields as:
319    struct gpio {
320        unsigned gpio;
321        unsigned long flags;
322        const char *label;
323    };
325A typical example of usage:
327    static struct gpio leds_gpios[] = {
328        { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
329        { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
330        { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
331        { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
332        { ... },
333    };
335    err = gpio_request_one(31, GPIOF_IN, "Reset Button");
336    if (err)
337        ...
339    err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
340    if (err)
341        ...
343    gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
346GPIOs mapped to IRQs
348GPIO numbers are unsigned integers; so are IRQ numbers. These make up
349two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
350map between them using calls like:
352    /* map GPIO numbers to IRQ numbers */
353    int gpio_to_irq(unsigned gpio);
355    /* map IRQ numbers to GPIO numbers (avoid using this) */
356    int irq_to_gpio(unsigned irq);
358Those return either the corresponding number in the other namespace, or
359else a negative errno code if the mapping can't be done. (For example,
360some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
361number that wasn't set up as an input using gpio_direction_input(), or
362to use an IRQ number that didn't originally come from gpio_to_irq().
364These two mapping calls are expected to cost on the order of a single
365addition or subtraction. They're not allowed to sleep.
367Non-error values returned from gpio_to_irq() can be passed to request_irq()
368or free_irq(). They will often be stored into IRQ resources for platform
369devices, by the board-specific initialization code. Note that IRQ trigger
370options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
371system wakeup capabilities.
373Non-error values returned from irq_to_gpio() would most commonly be used
374with gpio_get_value(), for example to initialize or update driver state
375when the IRQ is edge-triggered. Note that some platforms don't support
376this reverse mapping, so you should avoid using it.
379Emulating Open Drain Signals
381Sometimes shared signals need to use "open drain" signaling, where only the
382low signal level is actually driven. (That term applies to CMOS transistors;
383"open collector" is used for TTL.) A pullup resistor causes the high signal
384level. This is sometimes called a "wire-AND"; or more practically, from the
385negative logic (low=true) perspective this is a "wire-OR".
387One common example of an open drain signal is a shared active-low IRQ line.
388Also, bidirectional data bus signals sometimes use open drain signals.
390Some GPIO controllers directly support open drain outputs; many don't. When
391you need open drain signaling but your hardware doesn't directly support it,
392there's a common idiom you can use to emulate it with any GPIO pin that can
393be used as either an input or an output:
395 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
396    and overrides the pullup.
398 HIGH: gpio_direction_input(gpio) ... this turns off the output,
399    so the pullup (or some other device) controls the signal.
401If you are "driving" the signal high but gpio_get_value(gpio) reports a low
402value (after the appropriate rise time passes), you know some other component
403is driving the shared signal low. That's not necessarily an error. As one
404common example, that's how I2C clocks are stretched: a slave that needs a
405slower clock delays the rising edge of SCK, and the I2C master adjusts its
406signaling rate accordingly.
409What do these conventions omit?
411One of the biggest things these conventions omit is pin multiplexing, since
412this is highly chip-specific and nonportable. One platform might not need
413explicit multiplexing; another might have just two options for use of any
414given pin; another might have eight options per pin; another might be able
415to route a given GPIO to any one of several pins. (Yes, those examples all
416come from systems that run Linux today.)
418Related to multiplexing is configuration and enabling of the pullups or
419pulldowns integrated on some platforms. Not all platforms support them,
420or support them in the same way; and any given board might use external
421pullups (or pulldowns) so that the on-chip ones should not be used.
422(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
423Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
424platform-specific issue, as are models like (not) having a one-to-one
425correspondence between configurable pins and GPIOs.
427There are other system-specific mechanisms that are not specified here,
428like the aforementioned options for input de-glitching and wire-OR output.
429Hardware may support reading or writing GPIOs in gangs, but that's usually
430configuration dependent: for GPIOs sharing the same bank. (GPIOs are
431commonly grouped in banks of 16 or 32, with a given SOC having several such
432banks.) Some systems can trigger IRQs from output GPIOs, or read values
433from pins not managed as GPIOs. Code relying on such mechanisms will
434necessarily be nonportable.
436Dynamic definition of GPIOs is not currently standard; for example, as
437a side effect of configuring an add-on board with some GPIO expanders.
440GPIO implementor's framework (OPTIONAL)
442As noted earlier, there is an optional implementation framework making it
443easier for platforms to support different kinds of GPIO controller using
444the same programming interface. This framework is called "gpiolib".
446As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
447will be found there. That will list all the controllers registered through
448this framework, and the state of the GPIOs currently in use.
451Controller Drivers: gpio_chip
453In this framework each GPIO controller is packaged as a "struct gpio_chip"
454with information common to each controller of that type:
456 - methods to establish GPIO direction
457 - methods used to access GPIO values
458 - flag saying whether calls to its methods may sleep
459 - optional debugfs dump method (showing extra state like pullup config)
460 - label for diagnostics
462There is also per-instance data, which may come from device.platform_data:
463the number of its first GPIO, and how many GPIOs it exposes.
465The code implementing a gpio_chip should support multiple instances of the
466controller, possibly using the driver model. That code will configure each
467gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
468rare; use gpiochip_remove() when it is unavoidable.
470Most often a gpio_chip is part of an instance-specific structure with state
471not exposed by the GPIO interfaces, such as addressing, power management,
472and more. Chips such as codecs will have complex non-GPIO state.
474Any debugfs dump method should normally ignore signals which haven't been
475requested as GPIOs. They can use gpiochip_is_requested(), which returns
476either NULL or the label associated with that GPIO when it was requested.
479Platform Support
481To support this framework, a platform's Kconfig will "select" either
483and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
484three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
486It may also provide a custom value for ARCH_NR_GPIOS, so that it better
487reflects the number of GPIOs in actual use on that platform, without
488wasting static table space. (It should count both built-in/SoC GPIOs and
489also ones on GPIO expanders.
491ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
492into the kernel on that architecture.
494ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user
495can enable it and build it into the kernel optionally.
497If neither of these options are selected, the platform does not support
498GPIOs through GPIO-lib and the code cannot be enabled by the user.
500Trivial implementations of those functions can directly use framework
501code, which always dispatches through the gpio_chip:
503  #define gpio_get_value __gpio_get_value
504  #define gpio_set_value __gpio_set_value
505  #define gpio_cansleep __gpio_cansleep
507Fancier implementations could instead define those as inline functions with
508logic optimizing access to specific SOC-based GPIOs. For example, if the
509referenced GPIO is the constant "12", getting or setting its value could
510cost as little as two or three instructions, never sleeping. When such an
511optimization is not possible those calls must delegate to the framework
512code, costing at least a few dozen instructions. For bitbanged I/O, such
513instruction savings can be significant.
515For SOCs, platform-specific code defines and registers gpio_chip instances
516for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
517match chip vendor documentation, and directly match board schematics. They
518may well start at zero and go up to a platform-specific limit. Such GPIOs
519are normally integrated into platform initialization to make them always be
520available, from arch_initcall() or earlier; they can often serve as IRQs.
523Board Support
525For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
526function devices, FPGAs or CPLDs -- most often board-specific code handles
527registering controller devices and ensures that their drivers know what GPIO
528numbers to use with gpiochip_add(). Their numbers often start right after
529platform-specific GPIOs.
531For example, board setup code could create structures identifying the range
532of GPIOs that chip will expose, and passes them to each GPIO expander chip
533using platform_data. Then the chip driver's probe() routine could pass that
534data to gpiochip_add().
536Initialization order can be important. For example, when a device relies on
537an I2C-based GPIO, its probe() routine should only be called after that GPIO
538becomes available. That may mean the device should not be registered until
539calls for that GPIO can work. One way to address such dependencies is for
540such gpio_chip controllers to provide setup() and teardown() callbacks to
541board specific code; those board specific callbacks would register devices
542once all the necessary resources are available, and remove them later when
543the GPIO controller device becomes unavailable.
546Sysfs Interface for Userspace (OPTIONAL)
548Platforms which use the "gpiolib" implementors framework may choose to
549configure a sysfs user interface to GPIOs. This is different from the
550debugfs interface, since it provides control over GPIO direction and
551value instead of just showing a gpio state summary. Plus, it could be
552present on production systems without debugging support.
554Given appropriate hardware documentation for the system, userspace could
555know for example that GPIO #23 controls the write protect line used to
556protect boot loader segments in flash memory. System upgrade procedures
557may need to temporarily remove that protection, first importing a GPIO,
558then changing its output state, then updating the code before re-enabling
559the write protection. In normal use, GPIO #23 would never be touched,
560and the kernel would have no need to know about it.
562Again depending on appropriate hardware documentation, on some systems
563userspace GPIO can be used to determine system configuration data that
564standard kernels won't know about. And for some tasks, simple userspace
565GPIO drivers could be all that the system really needs.
567Note that standard kernel drivers exist for common "LEDs and Buttons"
568GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
569instead of talking directly to the GPIOs; they integrate with kernel
570frameworks better than your userspace code could.
573Paths in Sysfs
575There are three kinds of entry in /sys/class/gpio:
577   - Control interfaces used to get userspace control over GPIOs;
579   - GPIOs themselves; and
581   - GPIO controllers ("gpio_chip" instances).
583That's in addition to standard files including the "device" symlink.
585The control interfaces are write-only:
587    /sys/class/gpio/
589        "export" ... Userspace may ask the kernel to export control of
590        a GPIO to userspace by writing its number to this file.
592        Example: "echo 19 > export" will create a "gpio19" node
593        for GPIO #19, if that's not requested by kernel code.
595        "unexport" ... Reverses the effect of exporting to userspace.
597        Example: "echo 19 > unexport" will remove a "gpio19"
598        node exported using the "export" file.
600GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
601and have the following read/write attributes:
603    /sys/class/gpio/gpioN/
605    "direction" ... reads as either "in" or "out". This value may
606        normally be written. Writing as "out" defaults to
607        initializing the value as low. To ensure glitch free
608        operation, values "low" and "high" may be written to
609        configure the GPIO as an output with that initial value.
611        Note that this attribute *will not exist* if the kernel
612        doesn't support changing the direction of a GPIO, or
613        it was exported by kernel code that didn't explicitly
614        allow userspace to reconfigure this GPIO's direction.
616    "value" ... reads as either 0 (low) or 1 (high). If the GPIO
617        is configured as an output, this value may be written;
618        any nonzero value is treated as high.
620        If the pin can be configured as interrupt-generating interrupt
621        and if it has been configured to generate interrupts (see the
622        description of "edge"), you can poll(2) on that file and
623        poll(2) will return whenever the interrupt was triggered. If
624        you use poll(2), set the events POLLPRI and POLLERR. If you
625        use select(2), set the file descriptor in exceptfds. After
626        poll(2) returns, either lseek(2) to the beginning of the sysfs
627        file and read the new value or close the file and re-open it
628        to read the value.
630    "edge" ... reads as either "none", "rising", "falling", or
631        "both". Write these strings to select the signal edge(s)
632        that will make poll(2) on the "value" file return.
634        This file exists only if the pin can be configured as an
635        interrupt generating input pin.
637    "active_low" ... reads as either 0 (false) or 1 (true). Write
638        any nonzero value to invert the value attribute both
639        for reading and writing. Existing and subsequent
640        poll(2) support configuration via the edge attribute
641        for "rising" and "falling" edges will follow this
642        setting.
644GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
645controller implementing GPIOs starting at #42) and have the following
646read-only attributes:
648    /sys/class/gpio/gpiochipN/
650        "base" ... same as N, the first GPIO managed by this chip
652        "label" ... provided for diagnostics (not always unique)
654        "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
656Board documentation should in most cases cover what GPIOs are used for
657what purposes. However, those numbers are not always stable; GPIOs on
658a daughtercard might be different depending on the base board being used,
659or other cards in the stack. In such cases, you may need to use the
660gpiochip nodes (possibly in conjunction with schematics) to determine
661the correct GPIO number to use for a given signal.
664Exporting from Kernel code
666Kernel code can explicitly manage exports of GPIOs which have already been
667requested using gpio_request():
669    /* export the GPIO to userspace */
670    int gpio_export(unsigned gpio, bool direction_may_change);
672    /* reverse gpio_export() */
673    void gpio_unexport();
675    /* create a sysfs link to an exported GPIO node */
676    int gpio_export_link(struct device *dev, const char *name,
677        unsigned gpio)
679    /* change the polarity of a GPIO node in sysfs */
680    int gpio_sysfs_set_active_low(unsigned gpio, int value);
682After a kernel driver requests a GPIO, it may only be made available in
683the sysfs interface by gpio_export(). The driver can control whether the
684signal direction may change. This helps drivers prevent userspace code
685from accidentally clobbering important system state.
687This explicit exporting can help with debugging (by making some kinds
688of experiments easier), or can provide an always-there interface that's
689suitable for documenting as part of a board support package.
691After the GPIO has been exported, gpio_export_link() allows creating
692symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
693use this to provide the interface under their own device in sysfs with
694a descriptive name.
696Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
697differences between boards from user space. This only affects the
698sysfs interface. Polarity change can be done both before and after
699gpio_export(), and previously enabled poll(2) support for either
700rising or falling edge will be reconfigured to follow this setting.

Archive Download this file