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.

RT_FULL: GPIO driver should not use spinlock_t or any sleepable APIs
(like PM runtime) in its gpio_chip implementation (.get/.set and direction
control callbacks) if it is expected to call GPIO APIs from atomic context
on -RT (inside hard IRQ handlers and similar contexts). Normally this should
not be required.


GPIOs with open drain/source support
------------------------------------

Open drain (CMOS) or open collector (TTL) means the line is not actively driven
high: instead you provide the drain/collector as output, so when the transistor
is not open, it will present a high-impedance (tristate) to the external rail.


   CMOS CONFIGURATION      TTL CONFIGURATION

            ||--- out              +--- out
     in ----||                   |/
            ||--+         in ----|
                |                |\
               GND	           GND

This configuration is normally used as a way to achieve one of two things:

- Level-shifting: to reach a logical level higher than that of the silicon
  where the output resides.

- inverse wire-OR on an I/O line, for example a GPIO line, making it possible
  for any driving stage on the line to drive it low even if any other output
  to the same line is simultaneously driving it high. A special case of this
  is driving the SCL and SCA lines of an I2C bus, which is by definition a
  wire-OR bus.

Both usecases require that the line be equipped with a pull-up resistor. This
resistor will make the line tend to high level unless one of the transistors on
the rail actively pulls it down.

The level on the line will go as high as the VDD on the pull-up resistor, which
may be higher than the level supported by the transistor, achieveing a
level-shift to the higher VDD.

Integrated electronics often have an output driver stage in the form of a CMOS
"totem-pole" with one N-MOS and one P-MOS transistor where one of them drives
the line high and one of them drives the line low. This is called a push-pull
output. The "totem-pole" looks like so:

                 VDD
                  |
        OD    ||--+
     +--/ ---o||     P-MOS-FET
     |        ||--+
IN --+            +----- out
     |        ||--+
     +--/ ----||     N-MOS-FET
        OS    ||--+
                  |
                 GND

The desired output signal (e.g. coming directly from some GPIO output register)
arrives at IN. The switches named "OD" and "OS" are normally closed, creating
a push-pull circuit.

Consider the little "switches" named "OD" and "OS" that enable/disable the
P-MOS or N-MOS transistor right after the split of the input. As you can see,
either transistor will go totally numb if this switch is open. The totem-pole
is then halved and give high impedance instead of actively driving the line
high or low respectively. That is usually how software-controlled open
drain/source works.

Some GPIO hardware come in open drain / open source configuration. Some are
hard-wired lines that will only support open drain or open source no matter
what: there is only one transistor there. Some are software-configurable:
by flipping a bit in a register the output can be configured as open drain
or open source, in practice by flicking open the switches labeled "OD" and "OS"
in the drawing above.

By disabling the P-MOS transistor, the output can be driven between GND and
high impedance (open drain), and by disabling the N-MOS transistor, the output
can be driven between VDD and high impedance (open source). In the first case,
a pull-up resistor is needed on the outgoing rail to complete the circuit, and
in the second case, a pull-down resistor is needed on the rail.

Hardware that supports open drain or open source or both, can implement a
special callback in the gpio_chip: .set_single_ended() that takes an enum flag
telling whether to configure the line as open drain, open source or push-pull.
This will happen in response to the GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag
set in the machine file, or coming from other hardware descriptions.

If this state can not be configured in hardware, i.e. if the GPIO hardware does
not support open drain/open source in hardware, the GPIO library will instead
use a trick: when a line is set as output, if the line is flagged as open
drain, and the IN output value is low, it will be driven low as usual. But
if the IN output value is set to high, it will instead *NOT* be driven high,
instead it will be switched to input, as input mode is high impedance, thus
achieveing an "open drain emulation" of sorts: electrically the behaviour will
be identical, with the exception of possible hardware glitches when switching
the mode of the line.

For open source configuration the same principle is used, just that instead
of actively driving the line low, it is set to input.


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.

RT_FULL: GPIO driver should not use spinlock_t or any sleepable APIs
(like PM runtime) as part of its irq_chip implementation on -RT.
- spinlock_t should be replaced with raw_spinlock_t [1].
- If sleepable APIs have to be used, these can be done from the .irq_bus_lock()
  and .irq_bus_unlock() callbacks, as these are the only slowpath callbacks
  on an irqchip. Create the callbacks if needed [2].

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.

  RT_FULL: Note, chained IRQ handlers will not be forced threaded on -RT.
  As result, spinlock_t or any sleepable APIs (like PM runtime) can't be used
  in chained IRQ handler.
  if required (and if it can't be converted to the nested threaded GPIO irqchip)
  - chained IRQ handler can be converted to generic irq handler and this way
  it will be threaded IRQ handler on -RT and hard IRQ handler on non-RT
  (for example, see [3]).
  Know W/A: The generic_handle_irq() is expected to be called with IRQ disabled,
  so IRQ core will complain if it will be called from IRQ handler which is
  forced thread. The "fake?" raw lock can be used to W/A this problem:

	raw_spinlock_t wa_lock;
	static irqreturn_t omap_gpio_irq_handler(int irq, void *gpiobank)
		unsigned long wa_lock_flags;
		raw_spin_lock_irqsave(&bank->wa_lock, wa_lock_flags);
		generic_handle_irq(irq_find_mapping(bank->chip.irqdomain, bit));
		raw_spin_unlock_irqrestore(&bank->wa_lock, wa_lock_flags);

* GENERIC CHAINED GPIO irqchips: these are the same as "CHAINED GPIO irqchips",
  but chained IRQ handlers are not used. Instead GPIO IRQs dispatching is
  performed by generic IRQ handler which is configured using request_irq().
  The GPIO irqchip will then end up calling something like this sequence in
  its interrupt handler:

  static irqreturn_t gpio_rcar_irq_handler(int irq, void *dev_id)
	for each detected GPIO IRQ
		generic_handle_irq(...);

  RT_FULL: Such kind of handlers will be forced threaded on -RT, as result IRQ
  core will complain that generic_handle_irq() is called with IRQ enabled and
  the same W/A as for "CHAINED GPIO irqchips" can be applied.

* 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. This is also used
  to set up a nested irqchip if NULL is passed as handler.

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.

- Nominally set all handlers to handle_bad_irq() in the setup call and pass
  handle_bad_irq() as flow handler parameter in gpiochip_irqchip_add() if it is
  expected for GPIO driver that irqchip .set_type() callback have to be called
  before using/enabling GPIO IRQ. Then set the handler to handle_level_irq()
  and/or handle_edge_irq() in the irqchip .set_type() callback depending on
  what your controller supports.

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 gpiochip_lock_as_irq(struct gpio_chip *chip, unsigned int offset)

This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
is released:

	void gpiochip_unlock_as_irq(struct gpio_chip *chip, unsigned int offset)

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

Real-Time compliance for GPIO IRQ chips
---------------------------------------

Any provider of irqchips needs to be carefully tailored to support Real Time
preemption. It is desirable that all irqchips in the GPIO subsystem keep this
in mind and does the proper testing to assure they are real time-enabled.
So, pay attention on above " RT_FULL:" notes, please.
The following is a checklist to follow when preparing a driver for real
time-compliance:

- ensure spinlock_t is not used as part irq_chip implementation;
- ensure that sleepable APIs are not used as part irq_chip implementation.
  If sleepable APIs have to be used, these can be done from the .irq_bus_lock()
  and .irq_bus_unlock() callbacks;
- Chained GPIO irqchips: ensure spinlock_t or any sleepable APIs are not used
  from chained IRQ handler;
- Generic chained GPIO irqchips: take care about generic_handle_irq() calls and
  apply corresponding W/A;
- Chained GPIO irqchips: get rid of chained IRQ handler and use generic irq
  handler if possible :)
- regmap_mmio: Sry, but you are in trouble :( if MMIO regmap is used as for
  GPIO IRQ chip implementation;
- Test your driver with the appropriate in-kernel real time test cases for both
  level and edge IRQs.


Requesting self-owned GPIO pins
-------------------------------

Sometimes it is useful to allow a GPIO chip driver to request its own GPIO
descriptors through the gpiolib API. Using gpio_request() for this purpose
does not help since it pins the module to the kernel forever (it calls
try_module_get()). A GPIO driver can use the following functions instead
to request and free descriptors without being pinned to the kernel forever.

	struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc,
						    const char *label)

	void gpiochip_free_own_desc(struct gpio_desc *desc)

Descriptors requested with gpiochip_request_own_desc() must be released with
gpiochip_free_own_desc().

These functions must be used with care since they do not affect module use
count. Do not use the functions to request gpio descriptors not owned by the
calling driver.

[1] http://www.spinics.net/lists/linux-omap/msg120425.html
[2] https://lkml.org/lkml/2015/9/25/494
[3] https://lkml.org/lkml/2015/9/25/495
