1============= 2GPIO Mappings 3============= 4 5This document explains how GPIOs can be assigned to given devices and functions. 6 7Note that it only applies to the new descriptor-based interface. For a 8description of the deprecated integer-based GPIO interface please refer to 9gpio-legacy.txt (actually, there is no real mapping possible with the old 10interface; you just fetch an integer from somewhere and request the 11corresponding GPIO). 12 13All platforms can enable the GPIO library, but if the platform strictly 14requires GPIO functionality to be present, it needs to select GPIOLIB from its 15Kconfig. Then, how GPIOs are mapped depends on what the platform uses to 16describe its hardware layout. Currently, mappings can be defined through device 17tree, ACPI, and platform data. 18 19Device Tree 20----------- 21GPIOs can easily be mapped to devices and functions in the device tree. The 22exact way to do it depends on the GPIO controller providing the GPIOs, see the 23device tree bindings for your controller. 24 25GPIOs mappings are defined in the consumer device's node, in a property named 26<function>-gpios, where <function> is the function the driver will request 27through gpiod_get(). For example:: 28 29 foo_device { 30 compatible = "acme,foo"; 31 ... 32 led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */ 33 <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */ 34 <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */ 35 36 power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>; 37 }; 38 39Properties named <function>-gpio are also considered valid and old bindings use 40it but are only supported for compatibility reasons and should not be used for 41newer bindings since it has been deprecated. 42 43This property will make GPIOs 15, 16 and 17 available to the driver under the 44"led" function, and GPIO 1 as the "power" GPIO:: 45 46 struct gpio_desc *red, *green, *blue, *power; 47 48 red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH); 49 green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH); 50 blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH); 51 52 power = gpiod_get(dev, "power", GPIOD_OUT_HIGH); 53 54The led GPIOs will be active high, while the power GPIO will be active low (i.e. 55gpiod_is_active_low(power) will be true). 56 57The second parameter of the gpiod_get() functions, the con_id string, has to be 58the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically 59looked up by the gpiod functions internally) used in the device tree. With above 60"led-gpios" example, use the prefix without the "-" as con_id parameter: "led". 61 62Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio") 63with the string passed in con_id to get the resulting string 64(``snprintf(... "%s-%s", con_id, gpio_suffixes[]``). 65 66ACPI 67---- 68ACPI also supports function names for GPIOs in a similar fashion to DT. 69The above DT example can be converted to an equivalent ACPI description 70with the help of _DSD (Device Specific Data), introduced in ACPI 5.1:: 71 72 Device (FOO) { 73 Name (_CRS, ResourceTemplate () { 74 GpioIo (Exclusive, ..., IoRestrictionOutputOnly, 75 "\\_SB.GPI0") {15} // red 76 GpioIo (Exclusive, ..., IoRestrictionOutputOnly, 77 "\\_SB.GPI0") {16} // green 78 GpioIo (Exclusive, ..., IoRestrictionOutputOnly, 79 "\\_SB.GPI0") {17} // blue 80 GpioIo (Exclusive, ..., IoRestrictionOutputOnly, 81 "\\_SB.GPI0") {1} // power 82 }) 83 84 Name (_DSD, Package () { 85 ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"), 86 Package () { 87 Package () { 88 "led-gpios", 89 Package () { 90 ^FOO, 0, 0, 1, 91 ^FOO, 1, 0, 1, 92 ^FOO, 2, 0, 1, 93 } 94 }, 95 Package () { 96 "power-gpios", 97 Package () {^FOO, 3, 0, 0}, 98 }, 99 } 100 }) 101 } 102 103For more information about the ACPI GPIO bindings see 104Documentation/firmware-guide/acpi/gpio-properties.rst. 105 106Platform Data 107------------- 108Finally, GPIOs can be bound to devices and functions using platform data. Board 109files that desire to do so need to include the following header:: 110 111 #include <linux/gpio/machine.h> 112 113GPIOs are mapped by the means of tables of lookups, containing instances of the 114gpiod_lookup structure. Two macros are defined to help declaring such mappings:: 115 116 GPIO_LOOKUP(key, chip_hwnum, con_id, flags) 117 GPIO_LOOKUP_IDX(key, chip_hwnum, con_id, idx, flags) 118 119where 120 121 - key is either the label of the gpiod_chip instance providing the GPIO, or 122 the GPIO line name 123 - chip_hwnum is the hardware number of the GPIO within the chip, or U16_MAX 124 to indicate that key is a GPIO line name 125 - con_id is the name of the GPIO function from the device point of view. It 126 can be NULL, in which case it will match any function. 127 - idx is the index of the GPIO within the function. 128 - flags is defined to specify the following properties: 129 * GPIO_ACTIVE_HIGH - GPIO line is active high 130 * GPIO_ACTIVE_LOW - GPIO line is active low 131 * GPIO_OPEN_DRAIN - GPIO line is set up as open drain 132 * GPIO_OPEN_SOURCE - GPIO line is set up as open source 133 * GPIO_PERSISTENT - GPIO line is persistent during 134 suspend/resume and maintains its value 135 * GPIO_TRANSITORY - GPIO line is transitory and may loose its 136 electrical state during suspend/resume 137 138In the future, these flags might be extended to support more properties. 139 140Note that: 141 1. GPIO line names are not guaranteed to be globally unique, so the first 142 match found will be used. 143 2. GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0. 144 145A lookup table can then be defined as follows, with an empty entry defining its 146end. The 'dev_id' field of the table is the identifier of the device that will 147make use of these GPIOs. It can be NULL, in which case it will be matched for 148calls to gpiod_get() with a NULL device. 149 150.. code-block:: c 151 152 struct gpiod_lookup_table gpios_table = { 153 .dev_id = "foo.0", 154 .table = { 155 GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH), 156 GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH), 157 GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH), 158 GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW), 159 { }, 160 }, 161 }; 162 163And the table can be added by the board code as follows:: 164 165 gpiod_add_lookup_table(&gpios_table); 166 167The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:: 168 169 struct gpio_desc *red, *green, *blue, *power; 170 171 red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH); 172 green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH); 173 blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH); 174 175 power = gpiod_get(dev, "power", GPIOD_OUT_HIGH); 176 177Since the "led" GPIOs are mapped as active-high, this example will switch their 178signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped 179as active-low, its actual signal will be 0 after this code. Contrary to the 180legacy integer GPIO interface, the active-low property is handled during 181mapping and is thus transparent to GPIO consumers. 182 183A set of functions such as gpiod_set_value() is available to work with 184the new descriptor-oriented interface. 185 186Boards using platform data can also hog GPIO lines by defining GPIO hog tables. 187 188.. code-block:: c 189 190 struct gpiod_hog gpio_hog_table[] = { 191 GPIO_HOG("gpio.0", 10, "foo", GPIO_ACTIVE_LOW, GPIOD_OUT_HIGH), 192 { } 193 }; 194 195And the table can be added to the board code as follows:: 196 197 gpiod_add_hogs(gpio_hog_table); 198 199The line will be hogged as soon as the gpiochip is created or - in case the 200chip was created earlier - when the hog table is registered. 201 202Arrays of pins 203-------------- 204In addition to requesting pins belonging to a function one by one, a device may 205also request an array of pins assigned to the function. The way those pins are 206mapped to the device determines if the array qualifies for fast bitmap 207processing. If yes, a bitmap is passed over get/set array functions directly 208between a caller and a respective .get/set_multiple() callback of a GPIO chip. 209 210In order to qualify for fast bitmap processing, the array must meet the 211following requirements: 212 213- pin hardware number of array member 0 must also be 0, 214- pin hardware numbers of consecutive array members which belong to the same 215 chip as member 0 does must also match their array indexes. 216 217Otherwise fast bitmap processing path is not used in order to avoid consecutive 218pins which belong to the same chip but are not in hardware order being processed 219separately. 220 221If the array applies for fast bitmap processing path, pins which belong to 222different chips than member 0 does, as well as those with indexes different from 223their hardware pin numbers, are excluded from the fast path, both input and 224output. Moreover, open drain and open source pins are excluded from fast bitmap 225output processing. 226