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diff --git a/Documentation/driver-api/gpio/board.rst b/Documentation/driver-api/gpio/board.rst new file mode 100644 index 000000000..4ac1e12cf --- /dev/null +++ b/Documentation/driver-api/gpio/board.rst @@ -0,0 +1,278 @@ +============= +GPIO Mappings +============= + +This document explains how GPIOs can be assigned to given devices and functions. + +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. + +Software Nodes +-------------- + +Software nodes allow board-specific code to construct an in-memory, +device-tree-like structure using struct software_node and struct +property_entry. This structure can then be associated with a platform device, +allowing drivers to use the standard device properties API to query +configuration, just as they would on an ACPI or device tree system. + +Software-node-backed GPIOs are described using the ``PROPERTY_ENTRY_GPIO()`` +macro, which ties a software node representing the GPIO controller with +consumer device. It allows consumers to use regular gpiolib APIs, such as +gpiod_get(), gpiod_get_optional(). + +The software node representing a GPIO controller must be attached to the +GPIO controller device - either as the primary or the secondary firmware node. + +For example, here is how to describe a single GPIO-connected LED. This is an +alternative to using platform_data on legacy systems. + +.. code-block:: c + + #include <linux/property.h> + #include <linux/gpio/machine.h> + #include <linux/gpio/property.h> + + /* + * 1. Define a node for the GPIO controller. + */ + static const struct software_node gpio_controller_node = { + .name = "gpio-foo", + }; + + /* 2. Define the properties for the LED device. */ + static const struct property_entry led_device_props[] = { + PROPERTY_ENTRY_STRING("label", "myboard:green:status"), + PROPERTY_ENTRY_STRING("linux,default-trigger", "heartbeat"), + PROPERTY_ENTRY_GPIO("gpios", &gpio_controller_node, 42, GPIO_ACTIVE_HIGH), + { } + }; + + /* 3. Define the software node for the LED device. */ + static const struct software_node led_device_swnode = { + .name = "status-led", + .properties = led_device_props, + }; + + /* + * 4. Register the software nodes and the platform device. + */ + const struct software_node *swnodes[] = { + &gpio_controller_node, + &led_device_swnode, + NULL + }; + software_node_register_node_group(swnodes); + + /* + * 5. Attach the GPIO controller's software node to the device and + * register it. + */ + static void gpio_foo_register(void) + { + struct platform_device_info pdev_info = { + .name = "gpio-foo", + .id = PLATFORM_DEVID_NONE, + .swnode = &gpio_controller_node + }; + + platform_device_register_full(&pdev_info); + } + + // Then register a platform_device for "leds-gpio" and associate + // it with &led_device_swnode via .fwnode. + +For a complete guide on converting board files to use software nodes, see +Documentation/driver-api/gpio/legacy-boards.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. + +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. |