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()href=, L118nts t.
 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 cahave errbe used ion/g.txtode.  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 multiplexn/g.txpin 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 nts ncies.
 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  href="Documen6"
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