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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:49:45 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:49:45 +0000
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tree848558de17fb3008cdf4d861b01ac7781903ce39 /Documentation/driver-api/gpio/board.rst
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Adding upstream version 6.1.76.upstream/6.1.76upstream
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+=============
+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
+legacy.rst (actually, there is no real mapping possible with the old
+interface; you just fetch an integer from somewhere and request the
+corresponding GPIO).
+
+All platforms can enable the GPIO library, but if the platform strictly
+requires GPIO functionality to be present, it needs to select GPIOLIB from its
+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>;
+ };
+
+Properties named <function>-gpio are also considered valid and old bindings use
+it but are only supported for compatibility reasons and should not be used for
+newer bindings since it has been deprecated.
+
+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, GPIOD_OUT_HIGH);
+ green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
+ blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
+
+ power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
+
+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).
+
+The second parameter of the gpiod_get() functions, the con_id string, has to be
+the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically
+looked up by the gpiod functions internally) used in the device tree. With above
+"led-gpios" example, use the prefix without the "-" as con_id parameter: "led".
+
+Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio")
+with the string passed in con_id to get the resulting string
+(``snprintf(... "%s-%s", con_id, gpio_suffixes[]``).
+
+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, PullUp, 0, 0, IoRestrictionOutputOnly,
+ "\\_SB.GPI0", 0, ResourceConsumer) { 15 } // red
+ GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
+ "\\_SB.GPI0", 0, ResourceConsumer) { 16 } // green
+ GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
+ "\\_SB.GPI0", 0, ResourceConsumer) { 17 } // blue
+ GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionOutputOnly,
+ "\\_SB.GPI0", 0, ResourceConsumer) { 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/firmware-guide/acpi/gpio-properties.rst.
+
+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(key, chip_hwnum, con_id, flags)
+ GPIO_LOOKUP_IDX(key, chip_hwnum, con_id, idx, flags)
+
+where
+
+ - key is either the label of the gpiod_chip instance providing the GPIO, or
+ the GPIO line name
+ - chip_hwnum is the hardware number of the GPIO within the chip, or U16_MAX
+ to indicate that key is a GPIO line name
+ - 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:
+ * GPIO_ACTIVE_HIGH - GPIO line is active high
+ * GPIO_ACTIVE_LOW - GPIO line is active low
+ * GPIO_OPEN_DRAIN - GPIO line is set up as open drain
+ * GPIO_OPEN_SOURCE - GPIO line is set up as open source
+ * GPIO_PERSISTENT - GPIO line is persistent during
+ suspend/resume and maintains its value
+ * GPIO_TRANSITORY - GPIO line is transitory and may loose its
+ electrical state during suspend/resume
+
+In the future, these flags might be extended to support more properties.
+
+Note that:
+ 1. GPIO line names are not guaranteed to be globally unique, so the first
+ match found will be used.
+ 2. 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. The 'dev_id' field of the table is the identifier of the device that will
+make use of these GPIOs. It can be NULL, in which case it will be matched for
+calls to gpiod_get() with a NULL device.
+
+.. code-block:: c
+
+ 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, GPIOD_OUT_HIGH);
+ green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH);
+ blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH);
+
+ power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);
+
+Since the "led" GPIOs are mapped as active-high, this example will switch their
+signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which 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.
+
+A set of functions such as gpiod_set_value() is available to work with
+the new descriptor-oriented interface.
+
+Boards using platform data can also hog GPIO lines by defining GPIO hog tables.
+
+.. code-block:: c
+
+ struct gpiod_hog gpio_hog_table[] = {
+ GPIO_HOG("gpio.0", 10, "foo", GPIO_ACTIVE_LOW, GPIOD_OUT_HIGH),
+ { }
+ };
+
+And the table can be added to the board code as follows::
+
+ gpiod_add_hogs(gpio_hog_table);
+
+The line will be hogged as soon as the gpiochip is created or - in case the
+chip was created earlier - when the hog table is registered.
+
+Arrays of pins
+--------------
+In addition to requesting pins belonging to a function one by one, a device may
+also request an array of pins assigned to the function. The way those pins are
+mapped to the device determines if the array qualifies for fast bitmap
+processing. If yes, a bitmap is passed over get/set array functions directly
+between a caller and a respective .get/set_multiple() callback of a GPIO chip.
+
+In order to qualify for fast bitmap processing, the array must meet the
+following requirements:
+
+- pin hardware number of array member 0 must also be 0,
+- pin hardware numbers of consecutive array members which belong to the same
+ chip as member 0 does must also match their array indexes.
+
+Otherwise fast bitmap processing path is not used in order to avoid consecutive
+pins which belong to the same chip but are not in hardware order being processed
+separately.
+
+If the array applies for fast bitmap processing path, pins which belong to
+different chips than member 0 does, as well as those with indexes different from
+their hardware pin numbers, are excluded from the fast path, both input and
+output. Moreover, open drain and open source pins are excluded from fast bitmap
+output processing.