GPIO Interfaces
  
  This provides an overview of GPIO access conventions on Linux.
  
  
  What is a GPIO?
  ===============
  A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
  digital signal.  They are provided from many kinds of chip, and are familiar
  to Linux developers working with embedded and custom hardware.  Each GPIO
  represents a bit connected to a particular pin, or "ball" on Ball Grid Array
  (BGA) packages.  Board schematics show which external hardware connects to
  which GPIOs.  Drivers can be written generically, so that board setup code
  passes such pin configuration data to drivers.
  
  System-on-Chip (SOC) processors heavily rely on GPIOs.  In some cases, every
  non-dedicated pin can be configured as a GPIO; and most chips have at least
  several dozen of them.  Programmable logic devices (like FPGAs) can easily
  provide GPIOs; multifunction chips like power managers, and audio codecs
  often have a few such pins to help with pin scarcity on SOCs; and there are
  also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
  Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
  firmware knowing how they're used).
  
  The exact capabilities of GPIOs vary between systems.  Common options:
  
    - Output values are writable (high=1, low=0).  Some chips also have
      options about how that value is driven, so that for example only one
      value might be driven ... supporting "wire-OR" and similar schemes

      for the other value (notably, "open drain" signaling).

  
    - Input values are likewise readable (1, 0).  Some chips support readback
      of pins configured as "output", which is very useful in such "wire-OR"
      cases (to support bidirectional signaling).  GPIO controllers may have
      input de-glitch logic, sometimes with software controls.
  
    - Inputs can often be used as IRQ signals, often edge triggered but
      sometimes level triggered.  Such IRQs may be configurable as system
      wakeup events, to wake the system from a low power state.
  
    - Usually a GPIO will be configurable as either input or output, as needed
      by different product boards; single direction ones exist too.
  
    - Most GPIOs can be accessed while holding spinlocks, but those accessed
      through a serial bus normally can't.  Some systems support both types.
  
  On a given board each GPIO is used for one specific purpose like monitoring
  MMC/SD card insertion/removal, detecting card writeprotect status, driving
  a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
  watchdog, sensing a switch, and so on.
  
  
  GPIO conventions
  ================
  Note that this is called a "convention" because you don't need to do it this
  way, and it's no crime if you don't.  There **are** cases where portability
  is not the main issue; GPIOs are often used for the kind of board-specific
  glue logic that may even change between board revisions, and can't ever be
  used on a board that's wired differently.  Only least-common-denominator
  functionality can be very portable.  Other features are platform-specific,
  and that can be critical for glue logic.
  
  Plus, this doesn't define an implementation framework, just an interface.
  One platform might implement it as simple inline functions accessing chip
  registers; another might implement it by delegating through abstractions
  used for several very different kinds of GPIO controller.
  
  That said, if the convention is supported on their platform, drivers should

  use it when possible.  Platforms should declare GENERIC_GPIO support in
  Kconfig (boolean true), which multi-platform drivers can depend on when
  using the include file:

  
  	#include <asm/gpio.h>
  
  If you stick to this convention then it'll be easier for other developers to
  see what your code is doing, and help maintain it.

  Note that these operations include I/O barriers on platforms which need to
  use them; drivers don't need to add them explicitly.

  
  Identifying GPIOs
  -----------------
  GPIOs are identified by unsigned integers in the range 0..MAX_INT.  That
  reserves "negative" numbers for other purposes like marking signals as

  "not available on this board", or indicating faults.  Code that doesn't
  touch the underlying hardware treats these integers as opaque cookies.

  
  Platforms define how they use those integers, and usually #define symbols
  for the GPIO lines so that board-specific setup code directly corresponds
  to the relevant schematics.  In contrast, drivers should only use GPIO
  numbers passed to them from that setup code, using platform_data to hold
  board-specific pin configuration data (along with other board specific
  data they need).  That avoids portability problems.
  
  So for example one platform uses numbers 32-159 for GPIOs; while another
  uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
  type of GPIO controller, and on one particular board 80-95 with an FPGA.
  The numbers need not be contiguous; either of those platforms could also
  use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
  
  Whether a platform supports multiple GPIO controllers is currently a
  platform-specific implementation issue.
  
  
  Using GPIOs
  -----------
  One of the first things to do with a GPIO, often in board setup code when
  setting up a platform_device using the GPIO, is mark its direction:
  
  	/* set as input or output, returning 0 or negative errno */
  	int gpio_direction_input(unsigned gpio);

  	int gpio_direction_output(unsigned gpio, int value);

  
  The return value is zero for success, else a negative errno.  It should
  be checked, since the get/set calls don't have error returns and since

  misconfiguration is possible.  You should normally issue these calls from
  a task context.  However, for spinlock-safe GPIOs it's OK to use them
  before tasking is enabled, as part of early board setup.

  For output GPIOs, the value provided becomes the initial output value.
  This helps avoid signal glitching during system startup.

  Setting the direction can fail if the GPIO number is invalid, or when
  that particular GPIO can't be used in that mode.  It's generally a bad
  idea to rely on boot firmware to have set the direction correctly, since
  it probably wasn't validated to do more than boot Linux.  (Similarly,
  that board setup code probably needs to multiplex that pin as a GPIO,
  and configure pullups/pulldowns appropriately.)
  
  
  Spinlock-Safe GPIO access
  -------------------------
  Most GPIO controllers can be accessed with memory read/write instructions.
  That doesn't need to sleep, and can safely be done from inside IRQ handlers.
  
  Use these calls to access such GPIOs:
  
  	/* GPIO INPUT:  return zero or nonzero */
  	int gpio_get_value(unsigned gpio);
  
  	/* GPIO OUTPUT */
  	void gpio_set_value(unsigned gpio, int value);
  
  The values are boolean, zero for low, nonzero for high.  When reading the
  value of an output pin, the value returned should be what's seen on the
  pin ... that won't always match the specified output value, because of
  issues including wire-OR and output latencies.
  
  The get/set calls have no error returns because "invalid GPIO" should have

  been reported earlier from gpio_direction_*().  However, note that not all

  platforms can read the value of output pins; those that can't should always

  return zero.  Also, using these calls for GPIOs that can't safely be accessed
  without sleeping (see below) is an error.

  Platform-specific implementations are encouraged to optimize the two

  calls to access the GPIO value in cases where the GPIO number (and for
  output, value) are constant.  It's normal for them to need only a couple
  of instructions in such cases (reading or writing a hardware register),
  and not to need spinlocks.  Such optimized calls can make bitbanging
  applications a lot more efficient (in both space and time) than spending
  dozens of instructions on subroutine calls.
  
  
  GPIO access that may sleep
  --------------------------
  Some GPIO controllers must be accessed using message based busses like I2C
  or SPI.  Commands to read or write those GPIO values require waiting to
  get to the head of a queue to transmit a command and get its response.
  This requires sleeping, which can't be done from inside IRQ handlers.
  
  Platforms that support this type of GPIO distinguish them from other GPIOs
  by returning nonzero from this call:
  
  	int gpio_cansleep(unsigned gpio);
  
  To access such GPIOs, a different set of accessors is defined:
  
  	/* GPIO INPUT:  return zero or nonzero, might sleep */
  	int gpio_get_value_cansleep(unsigned gpio);
  
  	/* GPIO OUTPUT, might sleep */
  	void gpio_set_value_cansleep(unsigned gpio, int value);
  
  Other than the fact that these calls might sleep, and will not be ignored
  for GPIOs that can't be accessed from IRQ handlers, these calls act the
  same as the spinlock-safe calls.
  
  
  Claiming and Releasing GPIOs (OPTIONAL)
  ---------------------------------------
  To help catch system configuration errors, two calls are defined.
  However, many platforms don't currently support this mechanism.
  
  	/* request GPIO, returning 0 or negative errno.
  	 * non-null labels may be useful for diagnostics.
  	 */
  	int gpio_request(unsigned gpio, const char *label);
  
  	/* release previously-claimed GPIO */
  	void gpio_free(unsigned gpio);
  
  Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
  GPIOs that have already been claimed with that call.  The return value of

  gpio_request() must be checked.  You should normally issue these calls from
  a task context.  However, for spinlock-safe GPIOs it's OK to request GPIOs
  before tasking is enabled, as part of early board setup.

  
  These calls serve two basic purposes.  One is marking the signals which
  are actually in use as GPIOs, for better diagnostics; systems may have
  several hundred potential GPIOs, but often only a dozen are used on any
  given board.  Another is to catch conflicts between drivers, reporting
  errors when drivers wrongly think they have exclusive use of that signal.
  
  These two calls are optional because not not all current Linux platforms
  offer such functionality in their GPIO support; a valid implementation
  could return success for all gpio_request() calls.  Unlike the other calls,
  the state they represent doesn't normally match anything from a hardware
  register; it's just a software bitmap which clearly is not necessary for
  correct operation of hardware or (bug free) drivers.
  
  Note that requesting a GPIO does NOT cause it to be configured in any
  way; it just marks that GPIO as in use.  Separate code must handle any
  pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
  
  
  GPIOs mapped to IRQs
  --------------------
  GPIO numbers are unsigned integers; so are IRQ numbers.  These make up
  two logically distinct namespaces (GPIO 0 need not use IRQ 0).  You can
  map between them using calls like:
  
  	/* map GPIO numbers to IRQ numbers */
  	int gpio_to_irq(unsigned gpio);
  
  	/* map IRQ numbers to GPIO numbers */
  	int irq_to_gpio(unsigned irq);
  
  Those return either the corresponding number in the other namespace, or
  else a negative errno code if the mapping can't be done.  (For example,
  some GPIOs can't used as IRQs.)  It is an unchecked error to use a GPIO

  number that wasn't set up as an input using gpio_direction_input(), or

  to use an IRQ number that didn't originally come from gpio_to_irq().
  
  These two mapping calls are expected to cost on the order of a single
  addition or subtraction.  They're not allowed to sleep.
  
  Non-error values returned from gpio_to_irq() can be passed to request_irq()
  or free_irq().  They will often be stored into IRQ resources for platform
  devices, by the board-specific initialization code.  Note that IRQ trigger
  options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
  system wakeup capabilities.
  
  Non-error values returned from irq_to_gpio() would most commonly be used

  with gpio_get_value(), for example to initialize or update driver state
  when the IRQ is edge-triggered.

  Emulating Open Drain Signals
  ----------------------------
  Sometimes shared signals need to use "open drain" signaling, where only the
  low signal level is actually driven.  (That term applies to CMOS transistors;
  "open collector" is used for TTL.)  A pullup resistor causes the high signal
  level.  This is sometimes called a "wire-AND"; or more practically, from the
  negative logic (low=true) perspective this is a "wire-OR".
  
  One common example of an open drain signal is a shared active-low IRQ line.
  Also, bidirectional data bus signals sometimes use open drain signals.
  
  Some GPIO controllers directly support open drain outputs; many don't.  When
  you need open drain signaling but your hardware doesn't directly support it,
  there's a common idiom you can use to emulate it with any GPIO pin that can
  be used as either an input or an output:
  
   LOW:	gpio_direction_output(gpio, 0) ... this drives the signal
  	and overrides the pullup.
  
   HIGH:	gpio_direction_input(gpio) ... this turns off the output,
  	so the pullup (or some other device) controls the signal.
  
  If you are "driving" the signal high but gpio_get_value(gpio) reports a low
  value (after the appropriate rise time passes), you know some other component
  is driving the shared signal low.  That's not necessarily an error.  As one
  common example, that's how I2C clocks are stretched:  a slave that needs a
  slower clock delays the rising edge of SCK, and the I2C master adjusts its
  signaling rate accordingly.

  
  What do these conventions omit?
  ===============================
  One of the biggest things these conventions omit is pin multiplexing, since
  this is highly chip-specific and nonportable.  One platform might not need
  explicit multiplexing; another might have just two options for use of any
  given pin; another might have eight options per pin; another might be able
  to route a given GPIO to any one of several pins.  (Yes, those examples all
  come from systems that run Linux today.)
  
  Related to multiplexing is configuration and enabling of the pullups or
  pulldowns integrated on some platforms.  Not all platforms support them,
  or support them in the same way; and any given board might use external
  pullups (or pulldowns) so that the on-chip ones should not be used.
  
  There are other system-specific mechanisms that are not specified here,
  like the aforementioned options for input de-glitching and wire-OR output.
  Hardware may support reading or writing GPIOs in gangs, but that's usually

  configuration dependent:  for GPIOs sharing the same bank.  (GPIOs are

  commonly grouped in banks of 16 or 32, with a given SOC having several such

  banks.)  Some systems can trigger IRQs from output GPIOs.  Code relying on
  such mechanisms will necessarily be nonportable.

  
  Dynamic definition of GPIOs is not currently supported; for example, as
  a side effect of configuring an add-on board with some GPIO expanders.
  
  These calls are purely for kernel space, but a userspace API could be built
  on top of it.