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Diffstat (limited to 'kernel/Documentation/gpio/board.txt')
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diff --git a/kernel/Documentation/gpio/board.txt b/kernel/Documentation/gpio/board.txt new file mode 100644 index 000000000..b80606de5 --- /dev/null +++ b/kernel/Documentation/gpio/board.txt @@ -0,0 +1,154 @@ +GPIO Mappings +============= + +This document explains how GPIOs can be assigned to given devices and functions. +Note that it only applies to the new descriptor-based interface. For a +description of the deprecated integer-based GPIO interface please refer to +gpio-legacy.txt (actually, there is no real mapping possible with the old +interface; you just fetch an integer from somewhere and request the +corresponding GPIO. + +Platforms that make use of GPIOs must select ARCH_REQUIRE_GPIOLIB (if GPIO usage +is mandatory) or ARCH_WANT_OPTIONAL_GPIOLIB (if GPIO support can be omitted) in +their Kconfig. Then, how GPIOs are mapped depends on what the platform uses to +describe its hardware layout. Currently, mappings can be defined through device +tree, ACPI, and platform data. + +Device Tree +----------- +GPIOs can easily be mapped to devices and functions in the device tree. The +exact way to do it depends on the GPIO controller providing the GPIOs, see the +device tree bindings for your controller. + +GPIOs mappings are defined in the consumer device's node, in a property named +<function>-gpios, where <function> is the function the driver will request +through gpiod_get(). For example: + + foo_device { + compatible = "acme,foo"; + ... + led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */ + <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */ + <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */ + + power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>; + }; + +This property will make GPIOs 15, 16 and 17 available to the driver under the +"led" function, and GPIO 1 as the "power" GPIO: + + struct gpio_desc *red, *green, *blue, *power; + + red = gpiod_get_index(dev, "led", 0); + green = gpiod_get_index(dev, "led", 1); + blue = gpiod_get_index(dev, "led", 2); + + power = gpiod_get(dev, "power"); + +The led GPIOs will be active-high, while the power GPIO will be active-low (i.e. +gpiod_is_active_low(power) will be true). + +ACPI +---- +ACPI also supports function names for GPIOs in a similar fashion to DT. +The above DT example can be converted to an equivalent ACPI description +with the help of _DSD (Device Specific Data), introduced in ACPI 5.1: + + Device (FOO) { + Name (_CRS, ResourceTemplate () { + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {15} // red + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {16} // green + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {17} // blue + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {1} // power + }) + + Name (_DSD, Package () { + ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"), + Package () { + Package () { + "led-gpios", + Package () { + ^FOO, 0, 0, 1, + ^FOO, 1, 0, 1, + ^FOO, 2, 0, 1, + } + }, + Package () { + "power-gpios", + Package () {^FOO, 3, 0, 0}, + }, + } + }) + } + +For more information about the ACPI GPIO bindings see +Documentation/acpi/gpio-properties.txt. + +Platform Data +------------- +Finally, GPIOs can be bound to devices and functions using platform data. Board +files that desire to do so need to include the following header: + + #include <linux/gpio/machine.h> + +GPIOs are mapped by the means of tables of lookups, containing instances of the +gpiod_lookup structure. Two macros are defined to help declaring such mappings: + + GPIO_LOOKUP(chip_label, chip_hwnum, dev_id, con_id, flags) + GPIO_LOOKUP_IDX(chip_label, chip_hwnum, dev_id, con_id, idx, flags) + +where + + - chip_label is the label of the gpiod_chip instance providing the GPIO + - chip_hwnum is the hardware number of the GPIO within the chip + - dev_id is the identifier of the device that will make use of this GPIO. It + can be NULL, in which case it will be matched for calls to gpiod_get() + with a NULL device. + - con_id is the name of the GPIO function from the device point of view. It + can be NULL, in which case it will match any function. + - idx is the index of the GPIO within the function. + - flags is defined to specify the following properties: + * GPIOF_ACTIVE_LOW - to configure the GPIO as active-low + * GPIOF_OPEN_DRAIN - GPIO pin is open drain type. + * GPIOF_OPEN_SOURCE - GPIO pin is open source type. + +In the future, these flags might be extended to support more properties. + +Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0. + +A lookup table can then be defined as follows, with an empty entry defining its +end: + +struct gpiod_lookup_table gpios_table = { + .dev_id = "foo.0", + .table = { + GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH), + GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH), + GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH), + GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW), + { }, + }, +}; + +And the table can be added by the board code as follows: + + gpiod_add_lookup_table(&gpios_table); + +The driver controlling "foo.0" will then be able to obtain its GPIOs as follows: + + struct gpio_desc *red, *green, *blue, *power; + + red = gpiod_get_index(dev, "led", 0); + green = gpiod_get_index(dev, "led", 1); + blue = gpiod_get_index(dev, "led", 2); + + power = gpiod_get(dev, "power"); + gpiod_direction_output(power, 1); + +Since the "power" GPIO is mapped as active-low, its actual signal will be 0 +after this code. Contrary to the legacy integer GPIO interface, the active-low +property is handled during mapping and is thus transparent to GPIO consumers. |