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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	GPIO Mappings
2	=============
3	
4	This document explains how GPIOs can be assigned to given devices and functions.
5	Note that it only applies to the new descriptor-based interface. For a
6	description of the deprecated integer-based GPIO interface please refer to
7	gpio-legacy.txt (actually, there is no real mapping possible with the old
8	interface; you just fetch an integer from somewhere and request the
9	corresponding GPIO.
10	
11	Platforms that make use of GPIOs must select ARCH_REQUIRE_GPIOLIB (if GPIO usage
12	is mandatory) or ARCH_WANT_OPTIONAL_GPIOLIB (if GPIO support can be omitted) in
13	their Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
14	describe its hardware layout. Currently, mappings can be defined through device
15	tree, ACPI, and platform data.
16	
17	Device Tree
18	-----------
19	GPIOs can easily be mapped to devices and functions in the device tree. The
20	exact way to do it depends on the GPIO controller providing the GPIOs, see the
21	device tree bindings for your controller.
22	
23	GPIOs 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
25	through 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-gpio = <&gpio 1 GPIO_ACTIVE_LOW>;
35		};
36	
37	This 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	
48	The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.
49	gpiod_is_active_low(power) will be true).
50	
51	ACPI
52	----
53	ACPI does not support function names for GPIOs. Therefore, only the "idx"
54	argument of gpiod_get_index() is useful to discriminate between GPIOs assigned
55	to a device. The "con_id" argument can still be set for debugging purposes (it
56	will appear under error messages as well as debug and sysfs nodes).
57	
58	Platform Data
59	-------------
60	Finally, GPIOs can be bound to devices and functions using platform data. Board
61	files that desire to do so need to include the following header:
62	
63		#include <linux/gpio/driver.h>
64	
65	GPIOs are mapped by the means of tables of lookups, containing instances of the
66	gpiod_lookup structure. Two macros are defined to help declaring such mappings:
67	
68		GPIO_LOOKUP(chip_label, chip_hwnum, dev_id, con_id, flags)
69		GPIO_LOOKUP_IDX(chip_label, chip_hwnum, dev_id, con_id, idx, flags)
70	
71	where
72	
73	  - chip_label is the label of the gpiod_chip instance providing the GPIO
74	  - chip_hwnum is the hardware number of the GPIO within the chip
75	  - dev_id is the identifier of the device that will make use of this GPIO. It
76		can be NULL, in which case it will be matched for calls to gpiod_get()
77		with a NULL device.
78	  - con_id is the name of the GPIO function from the device point of view. It
79		can be NULL, in which case it will match any function.
80	  - idx is the index of the GPIO within the function.
81	  - flags is defined to specify the following properties:
82		* GPIOF_ACTIVE_LOW	- to configure the GPIO as active-low
83		* GPIOF_OPEN_DRAIN	- GPIO pin is open drain type.
84		* GPIOF_OPEN_SOURCE	- GPIO pin is open source type.
85	
86	In the future, these flags might be extended to support more properties.
87	
88	Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
89	
90	A lookup table can then be defined as follows, with an empty entry defining its
91	end:
92	
93	struct gpiod_lookup_table gpios_table = {
94		.dev_id = "foo.0",
95		.table = {
96			GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH),
97			GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH),
98			GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH),
99			GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW),
100			{ },
101		},
102	};
103	
104	And the table can be added by the board code as follows:
105	
106		gpiod_add_lookup_table(&gpios_table);
107	
108	The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:
109	
110		struct gpio_desc *red, *green, *blue, *power;
111	
112		red = gpiod_get_index(dev, "led", 0);
113		green = gpiod_get_index(dev, "led", 1);
114		blue = gpiod_get_index(dev, "led", 2);
115	
116		power = gpiod_get(dev, "power");
117		gpiod_direction_output(power, 1);
118	
119	Since the "power" GPIO is mapped as active-low, its actual signal will be 0
120	after this code. Contrary to the legacy integer GPIO interface, the active-low
121	property is handled during mapping and is thus transparent to GPIO consumers.
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