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:

	#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.


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.  (These calls could sleep.)

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 in gpio_set_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.  (These calls could sleep.)

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 hasn't been marked as an input using gpio_set_direction(), 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.
