linux/Documentation/gpio.txt
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   1GPIO Interfaces
   2
   3This provides an overview of GPIO access conventions on Linux.
   4
   5These calls use the gpio_* naming prefix.  No other calls should use that
   6prefix, or the related __gpio_* prefix.
   7
   8
   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
  33    for the other value (notably, "open drain" signaling).
  34
  35  - Input values are likewise readable (1, 0).  Some chips support readback
  36    of pins configured as "output", which is very useful in such "wire-OR"
  37    cases (to support bidirectional signaling).  GPIO controllers may have
  38    input de-glitch/debounce logic, sometimes with software controls.
  39
  40  - Inputs can often be used as IRQ signals, often edge triggered but
  41    sometimes level triggered.  Such IRQs may be configurable as system
  42    wakeup events, to wake the system from a low power state.
  43
  44  - Usually a GPIO will be configurable as either input or output, as needed
  45    by different product boards; single direction ones exist too.
  46
  47  - Most GPIOs can be accessed while holding spinlocks, but those accessed
  48    through a serial bus normally can't.  Some systems support both types.
  49
  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
  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.)
  73
  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:
  80
  81        #include <linux/gpio.h>
  82
  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
  86Note that these operations include I/O barriers on platforms which need to
  87use them; drivers don't need to add them explicitly.
  88
  89
  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
  94"not available on this board", or indicating faults.  Code that doesn't
  95touch the underlying hardware treats these integers as opaque cookies.
  96
  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
 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:
 114
 115        int gpio_is_valid(int number);
 116
 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.
 120
 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.
 125
 126Using GPIOs
 127-----------
 128The first thing a system should do with a GPIO is allocate it, using
 129the gpio_request() call; see later.
 130
 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:
 133
 134        /* set as input or output, returning 0 or negative errno */
 135        int gpio_direction_input(unsigned gpio);
 136        int gpio_direction_output(unsigned gpio, int value);
 137
 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.
 143
 144For output GPIOs, the value provided becomes the initial output value.
 145This helps avoid signal glitching during system startup.
 146
 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.
 151
 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.)
 158
 159
 160Spinlock-Safe GPIO access
 161-------------------------
 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.
 165
 166Use the following calls to access such GPIOs,
 167for which gpio_cansleep() will always return false (see below):
 168
 169        /* GPIO INPUT:  return zero or nonzero */
 170        int gpio_get_value(unsigned gpio);
 171
 172        /* GPIO OUTPUT */
 173        void gpio_set_value(unsigned gpio, int value);
 174
 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.
 179
 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.
 185
 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.
 193
 194
 195GPIO access that may sleep
 196--------------------------
 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.
 201
 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):
 205
 206        int gpio_cansleep(unsigned gpio);
 207
 208To access such GPIOs, a different set of accessors is defined:
 209
 210        /* GPIO INPUT:  return zero or nonzero, might sleep */
 211        int gpio_get_value_cansleep(unsigned gpio);
 212
 213        /* GPIO OUTPUT, might sleep */
 214        void gpio_set_value_cansleep(unsigned gpio, int value);
 215
 216
 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.
 220
 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.
 224
 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.)
 229
 230        gpio_direction_input()
 231        gpio_direction_output()
 232        gpio_request()
 233
 234##      gpio_request_one()
 235##      gpio_request_array()
 236##      gpio_free_array()
 237
 238        gpio_free()
 239        gpio_set_debounce()
 240
 241
 242
 243Claiming and Releasing GPIOs
 244----------------------------
 245To help catch system configuration errors, two calls are defined.
 246
 247        /* request GPIO, returning 0 or negative errno.
 248         * non-null labels may be useful for diagnostics.
 249         */
 250        int gpio_request(unsigned gpio, const char *label);
 251
 252        /* release previously-claimed GPIO */
 253        void gpio_free(unsigned gpio);
 254
 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.
 260
 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.
 269
 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.
 273
 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).
 277
 278Also note that it's your responsibility to have stopped using a GPIO
 279before you free it.
 280
 281Considering in most cases GPIOs are actually configured right after they
 282are claimed, three additional calls are defined:
 283
 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);
 289
 290        /* request multiple GPIOs in a single call
 291         */
 292        int gpio_request_array(struct gpio *array, size_t num);
 293
 294        /* release multiple GPIOs in a single call
 295         */
 296        void gpio_free_array(struct gpio *array, size_t num);
 297
 298where 'flags' is currently defined to specify the following properties:
 299
 300        * GPIOF_DIR_IN          - to configure direction as input
 301        * GPIOF_DIR_OUT         - to configure direction as output
 302
 303        * GPIOF_INIT_LOW        - as output, set initial level to LOW
 304        * GPIOF_INIT_HIGH       - as output, set initial level to HIGH
 305
 306since GPIOF_INIT_* are only valid when configured as output, so group valid
 307combinations as:
 308
 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
 312
 313In the future, these flags can be extended to support more properties such
 314as open-drain status.
 315
 316Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
 317introduced to encapsulate all three fields as:
 318
 319        struct gpio {
 320                unsigned        gpio;
 321                unsigned long   flags;
 322                const char      *label;
 323        };
 324
 325A typical example of usage:
 326
 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        };
 334
 335        err = gpio_request_one(31, GPIOF_IN, "Reset Button");
 336        if (err)
 337                ...
 338
 339        err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
 340        if (err)
 341                ...
 342
 343        gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
 344
 345
 346GPIOs mapped to IRQs
 347--------------------
 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:
 351
 352        /* map GPIO numbers to IRQ numbers */
 353        int gpio_to_irq(unsigned gpio);
 354
 355        /* map IRQ numbers to GPIO numbers (avoid using this) */
 356        int irq_to_gpio(unsigned irq);
 357
 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().
 363
 364These two mapping calls are expected to cost on the order of a single
 365addition or subtraction.  They're not allowed to sleep.
 366
 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.
 372
 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.
 377
 378
 379Emulating Open Drain Signals
 380----------------------------
 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".
 386
 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.
 389
 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:
 394
 395 LOW:   gpio_direction_output(gpio, 0) ... this drives the signal
 396        and overrides the pullup.
 397
 398 HIGH:  gpio_direction_input(gpio) ... this turns off the output,
 399        so the pullup (or some other device) controls the signal.
 400
 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.
 407
 408
 409What do these conventions omit?
 410===============================
 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.)
 417
 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.
 426
 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.
 435
 436Dynamic definition of GPIOs is not currently standard; for example, as
 437a side effect of configuring an add-on board with some GPIO expanders.
 438
 439
 440GPIO implementor's framework (OPTIONAL)
 441=======================================
 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".
 445
 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.
 449
 450
 451Controller Drivers: gpio_chip
 452-----------------------------
 453In this framework each GPIO controller is packaged as a "struct gpio_chip"
 454with information common to each controller of that type:
 455
 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
 461
 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.
 464
 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.
 469
 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.
 473
 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.
 477
 478
 479Platform Support
 480----------------
 481To support this framework, a platform's Kconfig will "select" either
 482ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
 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().
 485
 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.
 490
 491ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
 492into the kernel on that architecture.
 493
 494ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user
 495can enable it and build it into the kernel optionally.
 496
 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.
 499
 500Trivial implementations of those functions can directly use framework
 501code, which always dispatches through the gpio_chip:
 502
 503  #define gpio_get_value        __gpio_get_value
 504  #define gpio_set_value        __gpio_set_value
 505  #define gpio_cansleep         __gpio_cansleep
 506
 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.
 514
 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.
 521
 522
 523Board Support
 524-------------
 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.
 530
 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().
 535
 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.
 544
 545
 546Sysfs Interface for Userspace (OPTIONAL)
 547========================================
 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.
 553
 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.
 561
 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.
 566
 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.
 571
 572
 573Paths in Sysfs
 574--------------
 575There are three kinds of entry in /sys/class/gpio:
 576
 577   -    Control interfaces used to get userspace control over GPIOs;
 578
 579   -    GPIOs themselves; and
 580
 581   -    GPIO controllers ("gpio_chip" instances).
 582
 583That's in addition to standard files including the "device" symlink.
 584
 585The control interfaces are write-only:
 586
 587    /sys/class/gpio/
 588
 589        "export" ... Userspace may ask the kernel to export control of
 590                a GPIO to userspace by writing its number to this file.
 591
 592                Example:  "echo 19 > export" will create a "gpio19" node
 593                for GPIO #19, if that's not requested by kernel code.
 594
 595        "unexport" ... Reverses the effect of exporting to userspace.
 596
 597                Example:  "echo 19 > unexport" will remove a "gpio19"
 598                node exported using the "export" file.
 599
 600GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
 601and have the following read/write attributes:
 602
 603    /sys/class/gpio/gpioN/
 604
 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.
 610
 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.
 615
 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.
 619
 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.
 629
 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.
 633
 634                This file exists only if the pin can be configured as an
 635                interrupt generating input pin.
 636
 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.
 643
 644GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
 645controller implementing GPIOs starting at #42) and have the following
 646read-only attributes:
 647
 648    /sys/class/gpio/gpiochipN/
 649
 650        "base" ... same as N, the first GPIO managed by this chip
 651
 652        "label" ... provided for diagnostics (not always unique)
 653
 654        "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
 655
 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.
 662
 663
 664Exporting from Kernel code
 665--------------------------
 666Kernel code can explicitly manage exports of GPIOs which have already been
 667requested using gpio_request():
 668
 669        /* export the GPIO to userspace */
 670        int gpio_export(unsigned gpio, bool direction_may_change);
 671
 672        /* reverse gpio_export() */
 673        void gpio_unexport();
 674
 675        /* create a sysfs link to an exported GPIO node */
 676        int gpio_export_link(struct device *dev, const char *name,
 677                unsigned gpio)
 678
 679        /* change the polarity of a GPIO node in sysfs */
 680        int gpio_sysfs_set_active_low(unsigned gpio, int value);
 681
 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.
 686
 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.
 690
 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.
 695
 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.
 701
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