116 lines
4.4 KiB
Plaintext
116 lines
4.4 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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Platforms that make use of GPIOs must select ARCH_REQUIRE_GPIOLIB (if GPIO usage
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is mandatory) or ARCH_WANT_OPTIONAL_GPIOLIB (if GPIO support can be omitted) in
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their 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-gpio = <&gpio 1 GPIO_ACTIVE_LOW>;
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};
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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);
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green = gpiod_get_index(dev, "led", 1);
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blue = gpiod_get_index(dev, "led", 2);
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power = gpiod_get(dev, "power");
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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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ACPI
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----
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ACPI does not support function names for GPIOs. Therefore, only the "idx"
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argument of gpiod_get_index() is useful to discriminate between GPIOs assigned
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to a device. The "con_id" argument can still be set for debugging purposes (it
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will appear under error messages as well as debug and sysfs nodes).
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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/driver.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, dev_id, con_id, flags)
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GPIO_LOOKUP_IDX(chip_label, chip_hwnum, dev_id, 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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- dev_id is the identifier of the device that will make use of this GPIO. If
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NULL, the GPIO will be available to all devices.
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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.
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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:
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struct gpiod_lookup gpios_table[] = {
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GPIO_LOOKUP_IDX("gpio.0", 15, "foo.0", "led", 0, GPIO_ACTIVE_HIGH),
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GPIO_LOOKUP_IDX("gpio.0", 16, "foo.0", "led", 1, GPIO_ACTIVE_HIGH),
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GPIO_LOOKUP_IDX("gpio.0", 17, "foo.0", "led", 2, GPIO_ACTIVE_HIGH),
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GPIO_LOOKUP("gpio.0", 1, "foo.0", "power", GPIO_ACTIVE_LOW),
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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_table(gpios_table, ARRAY_SIZE(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);
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green = gpiod_get_index(dev, "led", 1);
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blue = gpiod_get_index(dev, "led", 2);
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power = gpiod_get(dev, "power");
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gpiod_direction_output(power, 1);
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Since the "power" GPIO is mapped as active-low, its actual signal will be 0
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after this code. Contrary to the legacy integer GPIO interface, the active-low
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property is handled during mapping and is thus transparent to GPIO consumers.
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