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/acpi/gpio-properties.txt. 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(chip_label, chip_hwnum, con_id, flags) 117 GPIO_LOOKUP_IDX(chip_label, chip_hwnum, con_id, idx, flags) 118 119where 120 121 - chip_label is the label of the gpiod_chip instance providing the GPIO 122 - chip_hwnum is the hardware number of the GPIO within the chip 123 - con_id is the name of the GPIO function from the device point of view. It 124 can be NULL, in which case it will match any function. 125 - idx is the index of the GPIO within the function. 126 - flags is defined to specify the following properties: 127 * GPIO_ACTIVE_HIGH - GPIO line is active high 128 * GPIO_ACTIVE_LOW - GPIO line is active low 129 * GPIO_OPEN_DRAIN - GPIO line is set up as open drain 130 * GPIO_OPEN_SOURCE - GPIO line is set up as open source 131 * GPIO_PERSISTENT - GPIO line is persistent during 132 suspend/resume and maintains its value 133 * GPIO_TRANSITORY - GPIO line is transitory and may loose its 134 electrical state during suspend/resume 135 136In the future, these flags might be extended to support more properties. 137 138Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0. 139 140A lookup table can then be defined as follows, with an empty entry defining its 141end. The 'dev_id' field of the table is the identifier of the device that will 142make use of these GPIOs. It can be NULL, in which case it will be matched for 143calls to gpiod_get() with a NULL device. 144 145.. code-block:: c 146 147 struct gpiod_lookup_table gpios_table = { 148 .dev_id = "foo.0", 149 .table = { 150 GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH), 151 GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH), 152 GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH), 153 GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW), 154 { }, 155 }, 156 }; 157 158And the table can be added by the board code as follows:: 159 160 gpiod_add_lookup_table(&gpios_table); 161 162The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:: 163 164 struct gpio_desc *red, *green, *blue, *power; 165 166 red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH); 167 green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH); 168 blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH); 169 170 power = gpiod_get(dev, "power", GPIOD_OUT_HIGH); 171 172Since the "led" GPIOs are mapped as active-high, this example will switch their 173signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped 174as active-low, its actual signal will be 0 after this code. Contrary to the 175legacy integer GPIO interface, the active-low property is handled during 176mapping and is thus transparent to GPIO consumers. 177 178A set of functions such as gpiod_set_value() is available to work with 179the new descriptor-oriented interface. 180 181Boards using platform data can also hog GPIO lines by defining GPIO hog tables. 182 183.. code-block:: c 184 185 struct gpiod_hog gpio_hog_table[] = { 186 GPIO_HOG("gpio.0", 10, "foo", GPIO_ACTIVE_LOW, GPIOD_OUT_HIGH), 187 { } 188 }; 189 190And the table can be added to the board code as follows:: 191 192 gpiod_add_hogs(gpio_hog_table); 193 194The line will be hogged as soon as the gpiochip is created or - in case the 195chip was created earlier - when the hog table is registered. 196 197Arrays of pins 198-------------- 199In addition to requesting pins belonging to a function one by one, a device may 200also request an array of pins assigned to the function. The way those pins are 201mapped to the device determines if the array qualifies for fast bitmap 202processing. If yes, a bitmap is passed over get/set array functions directly 203between a caller and a respective .get/set_multiple() callback of a GPIO chip. 204 205In order to qualify for fast bitmap processing, the array must meet the 206following requirements: 207 208- pin hardware number of array member 0 must also be 0, 209- pin hardware numbers of consecutive array members which belong to the same 210 chip as member 0 does must also match their array indexes. 211 212Otherwise fast bitmap processing path is not used in order to avoid consecutive 213pins which belong to the same chip but are not in hardware order being processed 214separately. 215 216If the array applies for fast bitmap processing path, pins which belong to 217different chips than member 0 does, as well as those with indexes different from 218their hardware pin numbers, are excluded from the fast path, both input and 219output. Moreover, open drain and open source pins are excluded from fast bitmap 220output processing. 221