171 lines
6.2 KiB
Plaintext
171 lines
6.2 KiB
Plaintext
GPIO Mappings
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=============
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This document explains how GPIOs can be assigned to given devices and functions.
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Note that it only applies to the new descriptor-based interface. For a
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description of the deprecated integer-based GPIO interface please refer to
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gpio-legacy.txt (actually, there is no real mapping possible with the old
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interface; you just fetch an integer from somewhere and request the
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corresponding GPIO).
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All platforms can enable the GPIO library, but if the platform strictly
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requires GPIO functionality to be present, it needs to select GPIOLIB from its
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Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
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describe its hardware layout. Currently, mappings can be defined through device
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tree, ACPI, and platform data.
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Device Tree
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-----------
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GPIOs can easily be mapped to devices and functions in the device tree. The
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exact way to do it depends on the GPIO controller providing the GPIOs, see the
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device tree bindings for your controller.
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GPIOs mappings are defined in the consumer device's node, in a property named
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<function>-gpios, where <function> is the function the driver will request
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through gpiod_get(). For example:
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foo_device {
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compatible = "acme,foo";
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...
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led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
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<&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
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<&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
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power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
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};
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Properties named <function>-gpio are also considered valid and old bindings use
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it but are only supported for compatibility reasons and should not be used for
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newer bindings since it has been deprecated.
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This property will make GPIOs 15, 16 and 17 available to the driver under the
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"led" function, and GPIO 1 as the "power" GPIO:
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struct gpio_desc *red, *green, *blue, *power;
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red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
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green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
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blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
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power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
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The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.
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gpiod_is_active_low(power) will be true).
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The second parameter of the gpiod_get() functions, the con_id string, has to be
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the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically
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looked up by the gpiod functions internally) used in the device tree. With above
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"led-gpios" example, use the prefix without the "-" as con_id parameter: "led".
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Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio")
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with the string passed in con_id to get the resulting string
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(snprintf(... "%s-%s", con_id, gpio_suffixes[]).
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ACPI
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----
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ACPI also supports function names for GPIOs in a similar fashion to DT.
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The above DT example can be converted to an equivalent ACPI description
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with the help of _DSD (Device Specific Data), introduced in ACPI 5.1:
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Device (FOO) {
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Name (_CRS, ResourceTemplate () {
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GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
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"\\_SB.GPI0") {15} // red
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GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
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"\\_SB.GPI0") {16} // green
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GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
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"\\_SB.GPI0") {17} // blue
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GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
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"\\_SB.GPI0") {1} // power
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})
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Name (_DSD, Package () {
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ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
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Package () {
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Package () {
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"led-gpios",
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Package () {
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^FOO, 0, 0, 1,
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^FOO, 1, 0, 1,
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^FOO, 2, 0, 1,
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}
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},
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Package () {
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"power-gpios",
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Package () {^FOO, 3, 0, 0},
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},
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}
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})
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}
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For more information about the ACPI GPIO bindings see
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Documentation/acpi/gpio-properties.txt.
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Platform Data
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-------------
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Finally, GPIOs can be bound to devices and functions using platform data. Board
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files that desire to do so need to include the following header:
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#include <linux/gpio/machine.h>
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GPIOs are mapped by the means of tables of lookups, containing instances of the
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gpiod_lookup structure. Two macros are defined to help declaring such mappings:
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GPIO_LOOKUP(chip_label, chip_hwnum, con_id, flags)
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GPIO_LOOKUP_IDX(chip_label, chip_hwnum, con_id, idx, flags)
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where
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- chip_label is the label of the gpiod_chip instance providing the GPIO
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- chip_hwnum is the hardware number of the GPIO within the chip
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- con_id is the name of the GPIO function from the device point of view. It
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can be NULL, in which case it will match any function.
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- idx is the index of the GPIO within the function.
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- flags is defined to specify the following properties:
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* GPIOF_ACTIVE_LOW - to configure the GPIO as active-low
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* GPIOF_OPEN_DRAIN - GPIO pin is open drain type.
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* GPIOF_OPEN_SOURCE - GPIO pin is open source type.
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In the future, these flags might be extended to support more properties.
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Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
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A lookup table can then be defined as follows, with an empty entry defining its
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end. The 'dev_id' field of the table is the identifier of the device that will
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make use of these GPIOs. It can be NULL, in which case it will be matched for
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calls to gpiod_get() with a NULL device.
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struct gpiod_lookup_table gpios_table = {
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.dev_id = "foo.0",
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.table = {
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GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH),
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GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH),
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GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH),
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GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW),
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{ },
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},
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};
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And the table can be added by the board code as follows:
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gpiod_add_lookup_table(&gpios_table);
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The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:
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struct gpio_desc *red, *green, *blue, *power;
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red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH);
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green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
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blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
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power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
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Since the "led" GPIOs are mapped as active-high, this example will switch their
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signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped
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as active-low, its actual signal will be 0 after this code. Contrary to the
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legacy integer GPIO interface, the active-low property is handled during
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mapping and is thus transparent to GPIO consumers.
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A set of functions such as gpiod_set_value() is available to work with
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the new descriptor-oriented interface.
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