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 a number could reference a GPIO, you may use this predicate:
114    int gpio_is_valid(int number);
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.
120Whether a platform supports multiple GPIO controllers is currently a
121platform-specific implementation issue.
124Using GPIOs
126The first thing a system should do with a GPIO is allocate it, using
127the gpio_request() call; see later.
129One of the next things to do with a GPIO, often in board setup code when
130setting up a platform_device using the GPIO, is mark its direction:
132    /* set as input or output, returning 0 or negative errno */
133    int gpio_direction_input(unsigned gpio);
134    int gpio_direction_output(unsigned gpio, int value);
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
138misconfiguration 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.
142For output GPIOs, the value provided becomes the initial output value.
143This helps avoid signal glitching during system startup.
145For compatibility with legacy interfaces to GPIOs, setting the direction
146of a GPIO implicitly requests that GPIO (see below) if it has not been
147requested already. That compatibility is being removed from the optional
148gpiolib framework.
150Setting 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.)
158Spinlock-Safe GPIO access
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.
162(That includes hardirq contexts on RT kernels.)
164Use these calls to access such GPIOs:
166    /* GPIO INPUT: return zero or nonzero */
167    int gpio_get_value(unsigned gpio);
169    /* GPIO OUTPUT */
170    void gpio_set_value(unsigned gpio, int value);
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
175issues including open-drain signaling and output latencies.
177The get/set calls have no error returns because "invalid GPIO" should have
178been reported earlier from gpio_direction_*(). However, note that not all
179platforms can read the value of output pins; those that can't should always
180return zero. Also, using these calls for GPIOs that can't safely be accessed
181without sleeping (see below) is an error.
183Platform-specific implementations are encouraged to optimize the two
184calls 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.
192GPIO access that may sleep
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.
199Platforms that support this type of GPIO distinguish them from other GPIOs
200by returning nonzero from this call (which requires a valid GPIO number,
201which should have been previously allocated with gpio_request):
203    int gpio_cansleep(unsigned gpio);
205To access such GPIOs, a different set of accessors is defined:
207    /* GPIO INPUT: return zero or nonzero, might sleep */
208    int gpio_get_value_cansleep(unsigned gpio);
210    /* GPIO OUTPUT, might sleep */
211    void gpio_set_value_cansleep(unsigned gpio, int value);
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.
218Claiming and Releasing GPIOs
220To help catch system configuration errors, two calls are defined.
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);
227    /* release previously-claimed GPIO */
228    void gpio_free(unsigned gpio);
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
232gpio_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.
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
239given 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.
245Some 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.
249Note 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).
253Also note that it's your responsibility to have stopped using a GPIO
254before you free it.
256Considering in most cases GPIOs are actually configured right after they
257are claimed, three additional calls are defined:
259    /* request a single GPIO, with initial configuration specified by
260     * 'flags', identical to gpio_request() wrt other arguments and
261     * return value
262     */
263    int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
265    /* request multiple GPIOs in a single call
266     */
267    int gpio_request_array(struct gpio *array, size_t num);
269    /* release multiple GPIOs in a single call
270     */
271    void gpio_free_array(struct gpio *array, size_t num);
273where 'flags' is currently defined to specify the following properties:
275    * GPIOF_DIR_IN - to configure direction as input
276    * GPIOF_DIR_OUT - to configure direction as output
278    * GPIOF_INIT_LOW - as output, set initial level to LOW
279    * GPIOF_INIT_HIGH - as output, set initial level to HIGH
281since GPIOF_INIT_* are only valid when configured as output, so group valid
282combinations as:
284    * GPIOF_IN - configure as input
285    * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
286    * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
288In the future, these flags can be extended to support more properties such
289as open-drain status.
291Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
292introduced to encapsulate all three fields as:
294    struct gpio {
295        unsigned gpio;
296        unsigned long flags;
297        const char *label;
298    };
300A typical example of usage:
302    static struct gpio leds_gpios[] = {
303        { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
304        { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
305        { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
306        { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
307        { ... },
308    };
310    err = gpio_request_one(31, GPIOF_IN, "Reset Button");
311    if (err)
312        ...
314    err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
315    if (err)
316        ...
318    gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
321GPIOs mapped to IRQs
323GPIO numbers are unsigned integers; so are IRQ numbers. These make up
324two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
325map between them using calls like:
327    /* map GPIO numbers to IRQ numbers */
328    int gpio_to_irq(unsigned gpio);
330    /* map IRQ numbers to GPIO numbers (avoid using this) */
331    int irq_to_gpio(unsigned irq);
333Those return either the corresponding number in the other namespace, or
334else a negative errno code if the mapping can't be done. (For example,
335some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
336number that wasn't set up as an input using gpio_direction_input(), or
337to use an IRQ number that didn't originally come from gpio_to_irq().
339These two mapping calls are expected to cost on the order of a single
340addition or subtraction. They're not allowed to sleep.
342Non-error values returned from gpio_to_irq() can be passed to request_irq()
343or free_irq(). They will often be stored into IRQ resources for platform
344devices, by the board-specific initialization code. Note that IRQ trigger
345options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
346system wakeup capabilities.
348Non-error values returned from irq_to_gpio() would most commonly be used
349with gpio_get_value(), for example to initialize or update driver state
350when the IRQ is edge-triggered. Note that some platforms don't support
351this reverse mapping, so you should avoid using it.
354Emulating Open Drain Signals
356Sometimes shared signals need to use "open drain" signaling, where only the
357low signal level is actually driven. (That term applies to CMOS transistors;
358"open collector" is used for TTL.) A pullup resistor causes the high signal
359level. This is sometimes called a "wire-AND"; or more practically, from the
360negative logic (low=true) perspective this is a "wire-OR".
362One common example of an open drain signal is a shared active-low IRQ line.
363Also, bidirectional data bus signals sometimes use open drain signals.
365Some GPIO controllers directly support open drain outputs; many don't. When
366you need open drain signaling but your hardware doesn't directly support it,
367there's a common idiom you can use to emulate it with any GPIO pin that can
368be used as either an input or an output:
370 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
371    and overrides the pullup.
373 HIGH: gpio_direction_input(gpio) ... this turns off the output,
374    so the pullup (or some other device) controls the signal.
376If you are "driving" the signal high but gpio_get_value(gpio) reports a low
377value (after the appropriate rise time passes), you know some other component
378is driving the shared signal low. That's not necessarily an error. As one
379common example, that's how I2C clocks are stretched: a slave that needs a
380slower clock delays the rising edge of SCK, and the I2C master adjusts its
381signaling rate accordingly.
384What do these conventions omit?
386One of the biggest things these conventions omit is pin multiplexing, since
387this is highly chip-specific and nonportable. One platform might not need
388explicit multiplexing; another might have just two options for use of any
389given pin; another might have eight options per pin; another might be able
390to route a given GPIO to any one of several pins. (Yes, those examples all
391come from systems that run Linux today.)
393Related to multiplexing is configuration and enabling of the pullups or
394pulldowns integrated on some platforms. Not all platforms support them,
395or support them in the same way; and any given board might use external
396pullups (or pulldowns) so that the on-chip ones should not be used.
397(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
398Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
399platform-specific issue, as are models like (not) having a one-to-one
400correspondence between configurable pins and GPIOs.
402There are other system-specific mechanisms that are not specified here,
403like the aforementioned options for input de-glitching and wire-OR output.
404Hardware may support reading or writing GPIOs in gangs, but that's usually
405configuration dependent: for GPIOs sharing the same bank. (GPIOs are
406commonly grouped in banks of 16 or 32, with a given SOC having several such
407banks.) Some systems can trigger IRQs from output GPIOs, or read values
408from pins not managed as GPIOs. Code relying on such mechanisms will
409necessarily be nonportable.
411Dynamic definition of GPIOs is not currently standard; for example, as
412a side effect of configuring an add-on board with some GPIO expanders.
415GPIO implementor's framework (OPTIONAL)
417As noted earlier, there is an optional implementation framework making it
418easier for platforms to support different kinds of GPIO controller using
419the same programming interface. This framework is called "gpiolib".
421As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
422will be found there. That will list all the controllers registered through
423this framework, and the state of the GPIOs currently in use.
426Controller Drivers: gpio_chip
428In this framework each GPIO controller is packaged as a "struct gpio_chip"
429with information common to each controller of that type:
431 - methods to establish GPIO direction
432 - methods used to access GPIO values
433 - flag saying whether calls to its methods may sleep
434 - optional debugfs dump method (showing extra state like pullup config)
435 - label for diagnostics
437There is also per-instance data, which may come from device.platform_data:
438the number of its first GPIO, and how many GPIOs it exposes.
440The code implementing a gpio_chip should support multiple instances of the
441controller, possibly using the driver model. That code will configure each
442gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
443rare; use gpiochip_remove() when it is unavoidable.
445Most often a gpio_chip is part of an instance-specific structure with state
446not exposed by the GPIO interfaces, such as addressing, power management,
447and more. Chips such as codecs will have complex non-GPIO state.
449Any debugfs dump method should normally ignore signals which haven't been
450requested as GPIOs. They can use gpiochip_is_requested(), which returns
451either NULL or the label associated with that GPIO when it was requested.
454Platform Support
456To support this framework, a platform's Kconfig will "select" either
458and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
459three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
460They may also want to provide a custom value for ARCH_NR_GPIOS.
462ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled
463into the kernel on that architecture.
465ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user
466can enable it and build it into the kernel optionally.
468If neither of these options are selected, the platform does not support
469GPIOs through GPIO-lib and the code cannot be enabled by the user.
471Trivial implementations of those functions can directly use framework
472code, which always dispatches through the gpio_chip:
474  #define gpio_get_value __gpio_get_value
475  #define gpio_set_value __gpio_set_value
476  #define gpio_cansleep __gpio_cansleep
478Fancier implementations could instead define those as inline functions with
479logic optimizing access to specific SOC-based GPIOs. For example, if the
480referenced GPIO is the constant "12", getting or setting its value could
481cost as little as two or three instructions, never sleeping. When such an
482optimization is not possible those calls must delegate to the framework
483code, costing at least a few dozen instructions. For bitbanged I/O, such
484instruction savings can be significant.
486For SOCs, platform-specific code defines and registers gpio_chip instances
487for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
488match chip vendor documentation, and directly match board schematics. They
489may well start at zero and go up to a platform-specific limit. Such GPIOs
490are normally integrated into platform initialization to make them always be
491available, from arch_initcall() or earlier; they can often serve as IRQs.
494Board Support
496For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
497function devices, FPGAs or CPLDs -- most often board-specific code handles
498registering controller devices and ensures that their drivers know what GPIO
499numbers to use with gpiochip_add(). Their numbers often start right after
500platform-specific GPIOs.
502For example, board setup code could create structures identifying the range
503of GPIOs that chip will expose, and passes them to each GPIO expander chip
504using platform_data. Then the chip driver's probe() routine could pass that
505data to gpiochip_add().
507Initialization order can be important. For example, when a device relies on
508an I2C-based GPIO, its probe() routine should only be called after that GPIO
509becomes available. That may mean the device should not be registered until
510calls for that GPIO can work. One way to address such dependencies is for
511such gpio_chip controllers to provide setup() and teardown() callbacks to
512board specific code; those board specific callbacks would register devices
513once all the necessary resources are available, and remove them later when
514the GPIO controller device becomes unavailable.
517Sysfs Interface for Userspace (OPTIONAL)
519Platforms which use the "gpiolib" implementors framework may choose to
520configure a sysfs user interface to GPIOs. This is different from the
521debugfs interface, since it provides control over GPIO direction and
522value instead of just showing a gpio state summary. Plus, it could be
523present on production systems without debugging support.
525Given appropriate hardware documentation for the system, userspace could
526know for example that GPIO #23 controls the write protect line used to
527protect boot loader segments in flash memory. System upgrade procedures
528may need to temporarily remove that protection, first importing a GPIO,
529then changing its output state, then updating the code before re-enabling
530the write protection. In normal use, GPIO #23 would never be touched,
531and the kernel would have no need to know about it.
533Again depending on appropriate hardware documentation, on some systems
534userspace GPIO can be used to determine system configuration data that
535standard kernels won't know about. And for some tasks, simple userspace
536GPIO drivers could be all that the system really needs.
538Note that standard kernel drivers exist for common "LEDs and Buttons"
539GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
540instead of talking directly to the GPIOs; they integrate with kernel
541frameworks better than your userspace code could.
544Paths in Sysfs
546There are three kinds of entry in /sys/class/gpio:
548   - Control interfaces used to get userspace control over GPIOs;
550   - GPIOs themselves; and
552   - GPIO controllers ("gpio_chip" instances).
554That's in addition to standard files including the "device" symlink.
556The control interfaces are write-only:
558    /sys/class/gpio/
560        "export" ... Userspace may ask the kernel to export control of
561        a GPIO to userspace by writing its number to this file.
563        Example: "echo 19 > export" will create a "gpio19" node
564        for GPIO #19, if that's not requested by kernel code.
566        "unexport" ... Reverses the effect of exporting to userspace.
568        Example: "echo 19 > unexport" will remove a "gpio19"
569        node exported using the "export" file.
571GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
572and have the following read/write attributes:
574    /sys/class/gpio/gpioN/
576    "direction" ... reads as either "in" or "out". This value may
577        normally be written. Writing as "out" defaults to
578        initializing the value as low. To ensure glitch free
579        operation, values "low" and "high" may be written to
580        configure the GPIO as an output with that initial value.
582        Note that this attribute *will not exist* if the kernel
583        doesn't support changing the direction of a GPIO, or
584        it was exported by kernel code that didn't explicitly
585        allow userspace to reconfigure this GPIO's direction.
587    "value" ... reads as either 0 (low) or 1 (high). If the GPIO
588        is configured as an output, this value may be written;
589        any nonzero value is treated as high.
591    "edge" ... reads as either "none", "rising", "falling", or
592        "both". Write these strings to select the signal edge(s)
593        that will make poll(2) on the "value" file return.
595        This file exists only if the pin can be configured as an
596        interrupt generating input pin.
598    "active_low" ... reads as either 0 (false) or 1 (true). Write
599        any nonzero value to invert the value attribute both
600        for reading and writing. Existing and subsequent
601        poll(2) support configuration via the edge attribute
602        for "rising" and "falling" edges will follow this
603        setting.
605GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
606controller implementing GPIOs starting at #42) and have the following
607read-only attributes:
609    /sys/class/gpio/gpiochipN/
611        "base" ... same as N, the first GPIO managed by this chip
613        "label" ... provided for diagnostics (not always unique)
615        "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
617Board documentation should in most cases cover what GPIOs are used for
618what purposes. However, those numbers are not always stable; GPIOs on
619a daughtercard might be different depending on the base board being used,
620or other cards in the stack. In such cases, you may need to use the
621gpiochip nodes (possibly in conjunction with schematics) to determine
622the correct GPIO number to use for a given signal.
625Exporting from Kernel code
627Kernel code can explicitly manage exports of GPIOs which have already been
628requested using gpio_request():
630    /* export the GPIO to userspace */
631    int gpio_export(unsigned gpio, bool direction_may_change);
633    /* reverse gpio_export() */
634    void gpio_unexport();
636    /* create a sysfs link to an exported GPIO node */
637    int gpio_export_link(struct device *dev, const char *name,
638        unsigned gpio)
640    /* change the polarity of a GPIO node in sysfs */
641    int gpio_sysfs_set_active_low(unsigned gpio, int value);
643After a kernel driver requests a GPIO, it may only be made available in
644the sysfs interface by gpio_export(). The driver can control whether the
645signal direction may change. This helps drivers prevent userspace code
646from accidentally clobbering important system state.
648This explicit exporting can help with debugging (by making some kinds
649of experiments easier), or can provide an always-there interface that's
650suitable for documenting as part of a board support package.
652After the GPIO has been exported, gpio_export_link() allows creating
653symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
654use this to provide the interface under their own device in sysfs with
655a descriptive name.
657Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
658differences between boards from user space. This only affects the
659sysfs interface. Polarity change can be done both before and after
660gpio_export(), and previously enabled poll(2) support for either
661rising or falling edge will be reconfigured to follow this setting.

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