linux-stable/Documentation/gpio/board.txt

GPIO Mappings
=============

This document explains how GPIOs can be assigned to given devices and functions.
Note that it only applies to the new descriptor-based interface. For a
description of the deprecated integer-based GPIO interface please refer to
gpio-legacy.txt (actually, there is no real mapping possible with the old
interface; you just fetch an integer from somewhere and request the
corresponding GPIO).

All platforms can enable the GPIO library, but if the platform strictly
requires GPIO functionality to be present, it needs to select GPIOLIB from its
Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
describe its hardware layout. Currently, mappings can be defined through device
tree, ACPI, and platform data.

Device Tree
-----------
GPIOs can easily be mapped to devices and functions in the device tree. The
exact way to do it depends on the GPIO controller providing the GPIOs, see the
device tree bindings for your controller.

GPIOs mappings are defined in the consumer device's node, in a property named
<function>-gpios, where <function> is the function the driver will request
through gpiod_get(). For example:

	foo_device {
		compatible = "acme,foo";
		...
		led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
			    <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
			    <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */

		power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
	};

Properties named <function>-gpio are also considered valid and old bindings use
it but are only supported for compatibility reasons and should not be used for
newer bindings since it has been deprecated.

This property will make GPIOs 15, 16 and 17 available to the driver under the
"led" function, and GPIO 1 as the "power" GPIO:

	struct gpio_desc *red, *green, *blue, *power;

	red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
	green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
	blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);

	power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);

The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.
gpiod_is_active_low(power) will be true).

The second parameter of the gpiod_get() functions, the con_id string, has to be
the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically
looked up by the gpiod functions internally) used in the device tree. With above
"led-gpios" example, use the prefix without the "-" as con_id parameter: "led".

Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio")
with the string passed in con_id to get the resulting string
(snprintf(... "%s-%s", con_id, gpio_suffixes[]).

ACPI
----
ACPI also supports function names for GPIOs in a similar fashion to DT.
The above DT example can be converted to an equivalent ACPI description
with the help of _DSD (Device Specific Data), introduced in ACPI 5.1:

	Device (FOO) {
		Name (_CRS, ResourceTemplate () {
			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
				"\\_SB.GPI0") {15} // red
			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
				"\\_SB.GPI0") {16} // green
			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
				"\\_SB.GPI0") {17} // blue
			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
				"\\_SB.GPI0") {1} // power
		})

		Name (_DSD, Package () {
			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
			Package () {
				Package () {
					"led-gpios",
					Package () {
						^FOO, 0, 0, 1,
						^FOO, 1, 0, 1,
						^FOO, 2, 0, 1,
					}
				},
				Package () {
					"power-gpios",
					Package () {^FOO, 3, 0, 0},
				},
			}
		})
	}

For more information about the ACPI GPIO bindings see
Documentation/acpi/gpio-properties.txt.

Platform Data
-------------
Finally, GPIOs can be bound to devices and functions using platform data. Board
files that desire to do so need to include the following header:

	#include <linux/gpio/machine.h>

GPIOs are mapped by the means of tables of lookups, containing instances of the
gpiod_lookup structure. Two macros are defined to help declaring such mappings:

	GPIO_LOOKUP(chip_label, chip_hwnum, con_id, flags)
	GPIO_LOOKUP_IDX(chip_label, chip_hwnum, con_id, idx, flags)

where

  - chip_label is the label of the gpiod_chip instance providing the GPIO
  - chip_hwnum is the hardware number of the GPIO within the chip
  - con_id is the name of the GPIO function from the device point of view. It
	can be NULL, in which case it will match any function.
  - idx is the index of the GPIO within the function.
  - flags is defined to specify the following properties:
	* GPIOF_ACTIVE_LOW	- to configure the GPIO as active-low
	* GPIOF_OPEN_DRAIN	- GPIO pin is open drain type.
	* GPIOF_OPEN_SOURCE	- GPIO pin is open source type.

In the future, these flags might be extended to support more properties.

Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.

A lookup table can then be defined as follows, with an empty entry defining its
end. The 'dev_id' field of the table is the identifier of the device that will
make use of these GPIOs. It can be NULL, in which case it will be matched for
calls to gpiod_get() with a NULL device.

struct gpiod_lookup_table gpios_table = {
	.dev_id = "foo.0",
	.table = {
		GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH),
		GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH),
		GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH),
		GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW),
		{ },
	},
};

And the table can be added by the board code as follows:

	gpiod_add_lookup_table(&gpios_table);

The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:

	struct gpio_desc *red, *green, *blue, *power;

	red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
	green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
	blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);

	power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);

Since the "led" GPIOs are mapped as active-high, this example will switch their
signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped
as active-low, its actual signal will be 0 after this code. Contrary to the
legacy integer GPIO interface, the active-low property is handled during
mapping and is thus transparent to GPIO consumers.

A set of functions such as gpiod_set_value() is available to work with
the new descriptor-oriented interface.