1 =============
2 GPIO Mappings
3 =============
4
5 This document explains how GPIOs can be assigned to given devices and functions.
6
7 All platforms can enable the GPIO library, but if the platform strictly
8 requires GPIO functionality to be present, it needs to select GPIOLIB from its
9 Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
10 describe its hardware layout. Currently, mappings can be defined through device
11 tree, ACPI, and platform data.
12
13 Device Tree
14 -----------
15 GPIOs can easily be mapped to devices and functions in the device tree. The
16 exact way to do it depends on the GPIO controller providing the GPIOs, see the
17 device tree bindings for your controller.
18
19 GPIOs mappings are defined in the consumer device's node, in a property named
20 <function>-gpios, where <function> is the function the driver will request
21 through gpiod_get(). For example::
22
23 foo_device {
24 compatible = "acme,foo";
25 ...
26 led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
27 <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
28 <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
29
30 power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
31 };
32
33 Properties named <function>-gpio are also considered valid and old bindings use
34 it but are only supported for compatibility reasons and should not be used for
35 newer bindings since it has been deprecated.
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, GPIOD_OUT_HIGH);
43 green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
44 blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
45
46 power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
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 The second parameter of the gpiod_get() functions, the con_id string, has to be
52 the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically
53 looked up by the gpiod functions internally) used in the device tree. With above
54 "led-gpios" example, use the prefix without the "-" as con_id parameter: "led".
55
56 Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio")
57 with the string passed in con_id to get the resulting string
58 (``snprintf(... "%s-%s", con_id, gpio_suffixes[]``).
59
60 ACPI
61 ----
62 ACPI also supports function names for GPIOs in a similar fashion to DT.
63 The above DT example can be converted to an equivalent ACPI description
64 with the help of _DSD (Device Specific Data), introduced in ACPI 5.1::
65
66 Device (FOO) {
67 Name (_CRS, ResourceTemplate () {
68 GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
69 "\\_SB.GPI0", 0, ResourceConsumer) { 15 } // red
70 GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
71 "\\_SB.GPI0", 0, ResourceConsumer) { 16 } // green
72 GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
73 "\\_SB.GPI0", 0, ResourceConsumer) { 17 } // blue
74 GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionOutputOnly,
75 "\\_SB.GPI0", 0, ResourceConsumer) { 1 } // power
76 })
77
78 Name (_DSD, Package () {
79 ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
80 Package () {
81 Package () {
82 "led-gpios",
83 Package () {
84 ^FOO, 0, 0, 1,
85 ^FOO, 1, 0, 1,
86 ^FOO, 2, 0, 1,
87 }
88 },
89 Package () { "power-gpios", Package () { ^FOO, 3, 0, 0 } },
90 }
91 })
92 }
93
94 For more information about the ACPI GPIO bindings see
95 Documentation/firmware-guide/acpi/gpio-properties.rst.
96
97 Platform Data
98 -------------
99 Finally, GPIOs can be bound to devices and functions using platform data. Board
100 files that desire to do so need to include the following header::
101
102 #include <linux/gpio/machine.h>
103
104 GPIOs are mapped by the means of tables of lookups, containing instances of the
105 gpiod_lookup structure. Two macros are defined to help declaring such mappings::
106
107 GPIO_LOOKUP(key, chip_hwnum, con_id, flags)
108 GPIO_LOOKUP_IDX(key, chip_hwnum, con_id, idx, flags)
109
110 where
111
112 - key is either the label of the gpiod_chip instance providing the GPIO, or
113 the GPIO line name
114 - chip_hwnum is the hardware number of the GPIO within the chip, or U16_MAX
115 to indicate that key is a GPIO line name
116 - con_id is the name of the GPIO function from the device point of view. It
117 can be NULL, in which case it will match any function.
118 - idx is the index of the GPIO within the function.
119 - flags is defined to specify the following properties:
120 * GPIO_ACTIVE_HIGH - GPIO line is active high
121 * GPIO_ACTIVE_LOW - GPIO line is active low
122 * GPIO_OPEN_DRAIN - GPIO line is set up as open drain
123 * GPIO_OPEN_SOURCE - GPIO line is set up as open source
124 * GPIO_PERSISTENT - GPIO line is persistent during
125 suspend/resume and maintains its value
126 * GPIO_TRANSITORY - GPIO line is transitory and may loose its
127 electrical state during suspend/resume
128
129 In the future, these flags might be extended to support more properties.
130
131 Note that:
132 1. GPIO line names are not guaranteed to be globally unique, so the first
133 match found will be used.
134 2. GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
135
136 A lookup table can then be defined as follows, with an empty entry defining its
137 end. The 'dev_id' field of the table is the identifier of the device that will
138 make use of these GPIOs. It can be NULL, in which case it will be matched for
139 calls to gpiod_get() with a NULL device.
140
141 .. code-block:: c
142
143 struct gpiod_lookup_table gpios_table = {
144 .dev_id = "foo.0",
145 .table = {
146 GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH),
147 GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH),
148 GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH),
149 GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW),
150 { },
151 },
152 };
153
154 And the table can be added by the board code as follows::
155
156 gpiod_add_lookup_table(&gpios_table);
157
158 The driver controlling "foo.0" will then be able to obtain its GPIOs as follows::
159
160 struct gpio_desc *red, *green, *blue, *power;
161
162 red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
163 green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
164 blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
165
166 power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
167
168 Since the "led" GPIOs are mapped as active-high, this example will switch their
169 signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped
170 as active-low, its actual signal will be 0 after this code. Contrary to the
171 legacy integer GPIO interface, the active-low property is handled during
172 mapping and is thus transparent to GPIO consumers.
173
174 A set of functions such as gpiod_set_value() is available to work with
175 the new descriptor-oriented interface.
176
177 Boards using platform data can also hog GPIO lines by defining GPIO hog tables.
178
179 .. code-block:: c
180
181 struct gpiod_hog gpio_hog_table[] = {
182 GPIO_HOG("gpio.0", 10, "foo", GPIO_ACTIVE_LOW, GPIOD_OUT_HIGH),
183 { }
184 };
185
186 And the table can be added to the board code as follows::
187
188 gpiod_add_hogs(gpio_hog_table);
189
190 The line will be hogged as soon as the gpiochip is created or - in case the
191 chip was created earlier - when the hog table is registered.
192
193 Arrays of pins
194 --------------
195 In addition to requesting pins belonging to a function one by one, a device may
196 also request an array of pins assigned to the function. The way those pins are
197 mapped to the device determines if the array qualifies for fast bitmap
198 processing. If yes, a bitmap is passed over get/set array functions directly
199 between a caller and a respective .get/set_multiple() callback of a GPIO chip.
200
201 In order to qualify for fast bitmap processing, the array must meet the
202 following requirements:
203
204 - pin hardware number of array member 0 must also be 0,
205 - pin hardware numbers of consecutive array members which belong to the same
206 chip as member 0 does must also match their array indexes.
207
208 Otherwise fast bitmap processing path is not used in order to avoid consecutive
209 pins which belong to the same chip but are not in hardware order being processed
210 separately.
211
212 If the array applies for fast bitmap processing path, pins which belong to
213 different chips than member 0 does, as well as those with indexes different from
214 their hardware pin numbers, are excluded from the fast path, both input and
215 output. Moreover, open drain and open source pins are excluded from fast bitmap
216 output processing.
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