1============= 2GPIO Mappings 3============= 4 5This document explains how GPIOs can be assigned to given devices and functions. 6 7All platforms can enable the GPIO library, but if the platform strictly 8requires GPIO functionality to be present, it needs to select GPIOLIB from its 9Kconfig. Then, how GPIOs are mapped depends on what the platform uses to 10describe its hardware layout. Currently, mappings can be defined through device 11tree, ACPI, and platform data. 12 13Device Tree 14----------- 15GPIOs can easily be mapped to devices and functions in the device tree. The 16exact way to do it depends on the GPIO controller providing the GPIOs, see the 17device tree bindings for your controller. 18 19GPIOs 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 21through 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 33Properties named <function>-gpio are also considered valid and old bindings use 34it but are only supported for compatibility reasons and should not be used for 35newer bindings since it has been deprecated. 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, 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 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 51The second parameter of the gpiod_get() functions, the con_id string, has to be 52the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically 53looked 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 56Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio") 57with the string passed in con_id to get the resulting string 58(``snprintf(... "%s-%s", con_id, gpio_suffixes[]``). 59 60ACPI 61---- 62ACPI also supports function names for GPIOs in a similar fashion to DT. 63The above DT example can be converted to an equivalent ACPI description 64with 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 94For more information about the ACPI GPIO bindings see 95Documentation/firmware-guide/acpi/gpio-properties.rst. 96 97Platform Data 98------------- 99Finally, GPIOs can be bound to devices and functions using platform data. Board 100files that desire to do so need to include the following header:: 101 102 #include <linux/gpio/machine.h> 103 104GPIOs are mapped by the means of tables of lookups, containing instances of the 105gpiod_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 110where 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 129In the future, these flags might be extended to support more properties. 130 131Note 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 136A lookup table can then be defined as follows, with an empty entry defining its 137end. The 'dev_id' field of the table is the identifier of the device that will 138make use of these GPIOs. It can be NULL, in which case it will be matched for 139calls 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 154And the table can be added by the board code as follows:: 155 156 gpiod_add_lookup_table(&gpios_table); 157 158The 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 168Since the "led" GPIOs are mapped as active-high, this example will switch their 169signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped 170as active-low, its actual signal will be 0 after this code. Contrary to the 171legacy integer GPIO interface, the active-low property is handled during 172mapping and is thus transparent to GPIO consumers. 173 174A set of functions such as gpiod_set_value() is available to work with 175the new descriptor-oriented interface. 176 177Boards 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 186And the table can be added to the board code as follows:: 187 188 gpiod_add_hogs(gpio_hog_table); 189 190The line will be hogged as soon as the gpiochip is created or - in case the 191chip was created earlier - when the hog table is registered. 192 193Arrays of pins 194-------------- 195In addition to requesting pins belonging to a function one by one, a device may 196also request an array of pins assigned to the function. The way those pins are 197mapped to the device determines if the array qualifies for fast bitmap 198processing. If yes, a bitmap is passed over get/set array functions directly 199between a caller and a respective .get/set_multiple() callback of a GPIO chip. 200 201In order to qualify for fast bitmap processing, the array must meet the 202following 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 208Otherwise fast bitmap processing path is not used in order to avoid consecutive 209pins which belong to the same chip but are not in hardware order being processed 210separately. 211 212If the array applies for fast bitmap processing path, pins which belong to 213different chips than member 0 does, as well as those with indexes different from 214their hardware pin numbers, are excluded from the fast path, both input and 215output. Moreover, open drain and open source pins are excluded from fast bitmap 216output processing. 217