1GPIO Mappings
2=============
3
4This document explains how GPIOs can be assigned to given devices and functions.
5Note that it only applies to the new descriptor-based interface. For a
6description of the deprecated integer-based GPIO interface please refer to
7gpio-legacy.txt (actually, there is no real mapping possible with the old
8interface; you just fetch an integer from somewhere and request the
9corresponding GPIO.
10
11Platforms that make use of GPIOs must select ARCH_REQUIRE_GPIOLIB (if GPIO usage
12is mandatory) or ARCH_WANT_OPTIONAL_GPIOLIB (if GPIO support can be omitted) in
13their Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
14describe its hardware layout. Currently, mappings can be defined through device
15tree, ACPI, and platform data.
16
17Device Tree
18-----------
19GPIOs can easily be mapped to devices and functions in the device tree. The
20exact way to do it depends on the GPIO controller providing the GPIOs, see the
21device tree bindings for your controller.
22
23GPIOs mappings are defined in the consumer device's node, in a property named
24<function>-gpios, where <function> is the function the driver will request
25through gpiod_get(). For example:
26
27	foo_device {
28		compatible = "acme,foo";
29		...
30		led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
31			    <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
32			    <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
33
34		power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
35	};
36
37This property will make GPIOs 15, 16 and 17 available to the driver under the
38"led" function, and GPIO 1 as the "power" GPIO:
39
40	struct gpio_desc *red, *green, *blue, *power;
41
42	red = gpiod_get_index(dev, "led", 0);
43	green = gpiod_get_index(dev, "led", 1);
44	blue = gpiod_get_index(dev, "led", 2);
45
46	power = gpiod_get(dev, "power");
47
48The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.
49gpiod_is_active_low(power) will be true).
50
51ACPI
52----
53ACPI also supports function names for GPIOs in a similar fashion to DT.
54The above DT example can be converted to an equivalent ACPI description
55with the help of _DSD (Device Specific Data), introduced in ACPI 5.1:
56
57	Device (FOO) {
58		Name (_CRS, ResourceTemplate () {
59			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
60				"\\_SB.GPI0") {15} // red
61			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
62				"\\_SB.GPI0") {16} // green
63			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
64				"\\_SB.GPI0") {17} // blue
65			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
66				"\\_SB.GPI0") {1} // power
67		})
68
69		Name (_DSD, Package () {
70			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
71			Package () {
72				Package () {
73					"led-gpios",
74					Package () {
75						^FOO, 0, 0, 1,
76						^FOO, 1, 0, 1,
77						^FOO, 2, 0, 1,
78					}
79				},
80				Package () {
81					"power-gpios",
82					Package () {^FOO, 3, 0, 0},
83				},
84			}
85		})
86	}
87
88For more information about the ACPI GPIO bindings see
89Documentation/acpi/gpio-properties.txt.
90
91Platform Data
92-------------
93Finally, GPIOs can be bound to devices and functions using platform data. Board
94files that desire to do so need to include the following header:
95
96	#include <linux/gpio/machine.h>
97
98GPIOs are mapped by the means of tables of lookups, containing instances of the
99gpiod_lookup structure. Two macros are defined to help declaring such mappings:
100
101	GPIO_LOOKUP(chip_label, chip_hwnum, dev_id, con_id, flags)
102	GPIO_LOOKUP_IDX(chip_label, chip_hwnum, dev_id, con_id, idx, flags)
103
104where
105
106  - chip_label is the label of the gpiod_chip instance providing the GPIO
107  - chip_hwnum is the hardware number of the GPIO within the chip
108  - dev_id is the identifier of the device that will make use of this GPIO. It
109	can be NULL, in which case it will be matched for calls to gpiod_get()
110	with a NULL device.
111  - con_id is the name of the GPIO function from the device point of view. It
112	can be NULL, in which case it will match any function.
113  - idx is the index of the GPIO within the function.
114  - flags is defined to specify the following properties:
115	* GPIOF_ACTIVE_LOW	- to configure the GPIO as active-low
116	* GPIOF_OPEN_DRAIN	- GPIO pin is open drain type.
117	* GPIOF_OPEN_SOURCE	- GPIO pin is open source type.
118
119In the future, these flags might be extended to support more properties.
120
121Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
122
123A lookup table can then be defined as follows, with an empty entry defining its
124end:
125
126struct gpiod_lookup_table gpios_table = {
127	.dev_id = "foo.0",
128	.table = {
129		GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH),
130		GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH),
131		GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH),
132		GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW),
133		{ },
134	},
135};
136
137And the table can be added by the board code as follows:
138
139	gpiod_add_lookup_table(&gpios_table);
140
141The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:
142
143	struct gpio_desc *red, *green, *blue, *power;
144
145	red = gpiod_get_index(dev, "led", 0);
146	green = gpiod_get_index(dev, "led", 1);
147	blue = gpiod_get_index(dev, "led", 2);
148
149	power = gpiod_get(dev, "power");
150	gpiod_direction_output(power, 1);
151
152Since the "power" GPIO is mapped as active-low, its actual signal will be 0
153after this code. Contrary to the legacy integer GPIO interface, the active-low
154property is handled during mapping and is thus transparent to GPIO consumers.
155