Documentation: gpiolib: document new interface

gpiolib now exports a new descriptor-based interface which deprecates
the older integer-based one. This patch documents this new interface and
also takes the opportunity to brush-up the GPIO documentation a little
bit.

The new descriptor-based interface follows the same consumer/driver
model as many other kernel subsystems (e.g. clock, regulator), so its
documentation has similarly been splitted into different files.

The content of the former documentation has been reused whenever it
made sense; however, some of its content did not apply to the new
interface anymore and have this been removed. Likewise, new sections
like the mapping of GPIOs to devices have been written from scratch.

The deprecated legacy-based documentation is still available, untouched,
under Documentation/gpio/gpio-legacy.txt.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

1 files changed, 115 insertions, 0 deletions

diff --git a/Documentation/gpio/board.txt b/Documentation/gpio/board.txt
new file mode 100644
index 000000000000..0d03506f2cc5
--- /dev/null
+++ b/Documentation/gpio/board.txt
@@ -0,0 +1,115 @@
+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-gpio = <&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 does not support function names for GPIOs. Therefore, only the "idx"
+argument of gpiod_get_index() is useful to discriminate between GPIOs assigned
+to a device. The "con_id" argument can still be set for debugging purposes (it
+will appear under error messages as well as debug and sysfs nodes).
+
+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/driver.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. If
+	NULL, the GPIO will be available to all devices.
+  - con_id is the name of the GPIO function from the device point of view. It
+	can be NULL.
+  - 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:
+
+	struct gpiod_lookup gpios_table[] = {
+	GPIO_LOOKUP_IDX("gpio.0", 15, "foo.0", "led", 0, GPIO_ACTIVE_HIGH),
+	GPIO_LOOKUP_IDX("gpio.0", 16, "foo.0", "led", 1, GPIO_ACTIVE_HIGH),
+	GPIO_LOOKUP_IDX("gpio.0", 17, "foo.0", "led", 2, GPIO_ACTIVE_HIGH),
+	GPIO_LOOKUP("gpio.0", 1, "foo.0", "power", GPIO_ACTIVE_LOW),
+	};
+
+And the table can be added by the board code as follows:
+
+	gpiod_add_table(gpios_table, ARRAY_SIZE(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.