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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.


GPIO drivers providing IRQs
---------------------------
It is custom that GPIO drivers (GPIO chips) are also providing interrupts,
most often cascaded off a parent interrupt controller, and in some special
cases the GPIO logic is melded with a SoC's primary interrupt controller.

The IRQ portions of the GPIO block are implemented using an irqchip, using
the header <linux/irq.h>. So basically such a driver is utilizing two sub-
systems simultaneously: gpio and irq.

GPIO irqchips usually fall in one of two categories:

* CHAINED GPIO irqchips: these are usually the type that is embedded on
  an SoC. This means that there is a fast IRQ handler for the GPIOs that
  gets called in a chain from the parent IRQ handler, most typically the
  system interrupt controller. This means the GPIO irqchip is registered
  using irq_set_chained_handler() or the corresponding
  gpiochip_set_chained_irqchip() helper function, and the GPIO irqchip
  handler will be called immediately from the parent irqchip, while
  holding the IRQs disabled. The GPIO irqchip will then end up calling
  something like this sequence in its interrupt handler:

  static irqreturn_t tc3589x_gpio_irq(int irq, void *data)
      chained_irq_enter(...);
      generic_handle_irq(...);
      chained_irq_exit(...);

  Chained GPIO irqchips typically can NOT set the .can_sleep flag on
  struct gpio_chip, as everything happens directly in the callbacks.

* NESTED THREADED GPIO irqchips: these are off-chip GPIO expanders and any
  other GPIO irqchip residing on the other side of a sleeping bus. Of course
  such drivers that need slow bus traffic to read out IRQ status and similar,
  traffic which may in turn incur other IRQs to happen, cannot be handled
  in a quick IRQ handler with IRQs disabled. Instead they need to spawn a
  thread and then mask the parent IRQ line until the interrupt is handled
  by the driver. The hallmark of this driver is to call something like
  this in its interrupt handler:

  static irqreturn_t tc3589x_gpio_irq(int irq, void *data)
      ...
      handle_nested_irq(irq);

  The hallmark of threaded GPIO irqchips is that they set the .can_sleep
  flag on struct gpio_chip to true, indicating that this chip may sleep
  when accessing the GPIOs.

To help out in handling the set-up and management of GPIO irqchips and the
associated irqdomain and resource allocation callbacks, the gpiolib has
some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig
symbol:

* gpiochip_irqchip_add(): adds an irqchip to a gpiochip. It will pass
  the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks
  need to embed the gpio_chip in its state container and obtain a pointer
  to the container using container_of().
  (See Documentation/driver-model/design-patterns.txt)

* gpiochip_set_chained_irqchip(): sets up a chained irq handler for a
  gpio_chip from a parent IRQ and passes the struct gpio_chip* as handler
  data. (Notice handler data, since the irqchip data is likely used by the
  parent irqchip!) This is for the chained type of chip.

To use the helpers please keep the following in mind:

- Make sure to assign all relevant members of the struct gpio_chip so that
  the irqchip can initialize. E.g. .dev and .can_sleep shall be set up
  properly.

It is legal for any IRQ consumer to request an IRQ from any irqchip no matter
if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and
irq_chip are orthogonal, and offering their services independent of each
other.

gpiod_to_irq() is just a convenience function to figure out the IRQ for a
certain GPIO line and should not be relied upon to have been called before
the IRQ is used.

So always prepare the hardware and make it ready for action in respective
callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having
been called first.

This orthogonality leads to ambiguities that we need to solve: if there is
competition inside the subsystem which side is using the resource (a certain
GPIO line and register for example) it needs to deny certain operations and
keep track of usage inside of the gpiolib subsystem. This is why the API
below exists.


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)

When implementing an irqchip inside a GPIO driver, these two functions should
typically be called in the .startup() and .shutdown() callbacks from the
irqchip.

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