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GPIO Descriptor Driver Interface
================================
This document serves as a guide for GPIO chip drivers writers. Note that it
describes the new descriptor-based interface. For a description of the
deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
Each GPIO controller driver needs to include the following header, which defines
the structures used to define a GPIO driver:
#include <linux/gpio/driver.h>
Internal Representation of GPIOs
================================
Inside a GPIO driver, individual GPIOs are identified by their hardware number,
which is a unique number between 0 and n, n being the number of GPIOs managed by
the chip. This number is purely internal: the hardware number of a particular
GPIO descriptor is never made visible outside of the driver.
On top of this internal number, each GPIO also need to have a global number in
the integer GPIO namespace so that it can be used with the legacy GPIO
interface. Each chip must thus have a "base" number (which can be automatically
assigned), and for each GPIO the global number will be (base + hardware number).
Although the integer representation is considered deprecated, it still has many
users and thus needs to be maintained.
So for example one platform could use numbers 32-159 for GPIOs, with a
controller defining 128 GPIOs at a "base" of 32 ; while another platform uses
numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO
controller, and on one particular board 80-95 with an FPGA. The numbers need not
be contiguous; either of those platforms could also use numbers 2000-2063 to
identify GPIOs in a bank of I2C GPIO expanders.
Controller Drivers: gpio_chip
=============================
In the gpiolib framework each GPIO controller is packaged as a "struct
gpio_chip" (see linux/gpio/driver.h for its complete definition) with members
common to each controller of that type:
- methods to establish GPIO direction
- methods used to access GPIO values
- method to return the IRQ number associated to a given GPIO
- flag saying whether calls to its methods may sleep
- optional debugfs dump method (showing extra state like pullup config)
- optional base number (will be automatically assigned if omitted)
- label for diagnostics and GPIOs mapping using platform data
The code implementing a gpio_chip should support multiple instances of the
controller, possibly using the driver model. That code will configure each
gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be rare;
use gpiochip_remove() when it is unavoidable.
Most often a gpio_chip is part of an instance-specific structure with state not
exposed by the GPIO interfaces, such as addressing, power management, and more.
Chips such as codecs will have complex non-GPIO state.
Any debugfs dump method should normally ignore signals which haven't been
requested as GPIOs. They can use gpiochip_is_requested(), which returns either
NULL or the label associated with that GPIO when it was requested.
Locking IRQ usage
-----------------
Input GPIOs can be used as IRQ signals. When this happens, a driver is requested
to mark the GPIO as being used as an IRQ:
int gpiod_lock_as_irq(struct gpio_desc *desc)
This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
is released:
void gpiod_unlock_as_irq(struct gpio_desc *desc)
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