This is a place for planning the ongoing long-term work in the GPIO
subsystem.


GPIO descriptors

Starting with commit 79a9becda894 the GPIO subsystem embarked on a journey
to move away from the global GPIO numberspace and toward a descriptor-based
approach. This means that GPIO consumers, drivers and machine descriptions
ideally have no use or idea of the global GPIO numberspace that has/was
used in the inception of the GPIO subsystem.

The numberspace issue is the same as to why irq is moving away from irq
numbers to IRQ descriptors.

The underlying motivation for this is that the GPIO numberspace has become
unmanageable: machine board files tend to become full of macros trying to
establish the numberspace at compile-time, making it hard to add any numbers
in the middle (such as if you missed a pin on a chip) without the numberspace
breaking.

Machine descriptions such as device tree or ACPI does not have a concept of the
Linux GPIO number as those descriptions are external to the Linux kernel
and treat GPIO lines as abstract entities.

The runtime-assigned GPIO numberspace (what you get if you assign the GPIO
base as -1 in struct gpio_chip) has also became unpredictable due to factors
such as probe ordering and the introduction of -EPROBE_DEFER making probe
ordering of independent GPIO chips essentially unpredictable, as their base
number will be assigned on a first come first serve basis.

The best way to get out of the problem is to make the global GPIO numbers
unimportant by simply not using them. GPIO descriptors deal with this.

Work items:

- Convert all GPIO device drivers to only #include <linux/gpio/driver.h>

- Convert all consumer drivers to only #include <linux/gpio/consumer.h>

- Convert all machine descriptors in "boardfiles" to only
  #include <linux/gpio/machine.h>, the other option being to convert it
  to a machine description such as device tree, ACPI or fwnode that
  implicitly does not use global GPIO numbers.

- When this work is complete (will require some of the items in the
  following ongoing work as well) we can delete the old global
  numberspace accessors from <linux/gpio.h> and eventually delete
  <linux/gpio.h> altogether.


Get rid of <linux/of_gpio.h>

This header and helpers appeared at one point when there was no proper
driver infrastructure for doing simpler MMIO GPIO devices and there was
no core support for parsing device tree GPIOs from the core library with
the [devm_]gpiod_get() calls we have today that will implicitly go into
the device tree back-end. It is legacy and should not be used in new code.

Work items:

- Get rid of struct of_mm_gpio_chip altogether: use the generic  MMIO
  GPIO for all current users (see below). Delete struct of_mm_gpio_chip,
  to_of_mm_gpio_chip(), of_mm_gpiochip_add_data(), of_mm_gpiochip_add()
  of_mm_gpiochip_remove() from the kernel.

- Change all consumer drivers that #include <linux/of_gpio.h> to
  #include <linux/gpio/consumer.h> and stop doing custom parsing of the
  GPIO lines from the device tree. This can be tricky and often ivolves
  changing boardfiles, etc.

- Pull semantics for legacy device tree (OF) GPIO lookups into
  gpiolib-of.c: in some cases subsystems are doing custom flags and
  lookups for polarity inversion, open drain and what not. As we now
  handle this with generic OF bindings, pull all legacy handling into
  gpiolib so the library API becomes narrow and deep and handle all
  legacy bindings internally. (See e.g. commits 6953c57ab172,
  6a537d48461d etc)

- Delete <linux/of_gpio.h> when all the above is complete and everything
  uses <linux/gpio/consumer.h> or <linux/gpio/driver.h> instead.


Get rid of <linux/gpio.h>

This legacy header is a one stop shop for anything GPIO is closely tied
to the global GPIO numberspace. The endgame of the above refactorings will
be the removal of <linux/gpio.h> and from that point only the specialized
headers under <linux/gpio/*.h> will be used. This requires all the above to
be completed and is expected to take a long time.


Collect drivers

Collect GPIO drivers from arch/* and other places that should be placed
in drivers/gpio/gpio-*. Augment platforms to create platform devices or
similar and probe a proper driver in the gpiolib subsystem.

In some cases it makes sense to create a GPIO chip from the local driver
for a few GPIOs. Those should stay where they are.

At the same time it makes sense to get rid of code duplication in existing or
new coming drivers. For example, gpio-ml-ioh should be incorporated into
gpio-pch.


Generic MMIO GPIO

The GPIO drivers can utilize the generic MMIO helper library in many
cases, and the helper library should be as helpful as possible for MMIO
drivers. (drivers/gpio/gpio-mmio.c)

Work items:

- Look over and identify any remaining easily converted drivers and
  dry-code conversions to MMIO GPIO for maintainers to test

- Expand the MMIO GPIO or write a new library for regmap-based I/O
  helpers for GPIO drivers on regmap that simply use offsets
  0..n in some register to drive GPIO lines

- Expand the MMIO GPIO or write a new library for port-mapped I/O
  helpers (x86 inb()/outb()) and convert port-mapped I/O drivers to use
  this with dry-coding and sending to maintainers to test


Generic regmap GPIO

In the very similar way to Generic MMIO GPIO convert the users which can
take advantage of using regmap over direct IO accessors. Note, even in
MMIO case the regmap MMIO with gpio-regmap.c is preferable over gpio-mmio.c.


GPIOLIB irqchip

The GPIOLIB irqchip is a helper irqchip for "simple cases" that should
try to cover any generic kind of irqchip cascaded from a GPIO.

- Look over and identify any remaining easily converted drivers and
  dry-code conversions to gpiolib irqchip for maintainers to test


Increase integration with pin control

There are already ways to use pin control as back-end for GPIO and
it may make sense to bring these subsystems closer. One reason for
creating pin control as its own subsystem was that we could avoid any
use of the global GPIO numbers. Once the above is complete, it may
make sense to simply join the subsystems into one and make pin
multiplexing, pin configuration, GPIO, etc selectable options in one
and the same pin control and GPIO subsystem.


Debugfs in place of sysfs

The old sysfs code that enables simple uses of GPIOs from the
command line is still popular despite the existance of the proper
character device. The reason is that it is simple to use on
root filesystems where you only have a minimal set of tools such
as "cat", "echo" etc.

The old sysfs still need to be strongly deprecated and removed
as it relies on the global GPIO numberspace that assume a strict
order of global GPIO numbers that do not change between boots
and is independent of probe order.

To solve this and provide an ABI that people can use for hacks
and development, implement a debugfs interface to manipulate
GPIO lines that can do everything that sysfs can do today: one
directory per gpiochip and one file entry per line:

/sys/kernel/debug/gpiochip/gpiochip0
/sys/kernel/debug/gpiochip/gpiochip0/gpio0
/sys/kernel/debug/gpiochip/gpiochip0/gpio1
/sys/kernel/debug/gpiochip/gpiochip0/gpio2
/sys/kernel/debug/gpiochip/gpiochip0/gpio3
...
/sys/kernel/debug/gpiochip/gpiochip1
/sys/kernel/debug/gpiochip/gpiochip1/gpio0
/sys/kernel/debug/gpiochip/gpiochip1/gpio1
...

The exact files and design of the debugfs interface can be
discussed but the idea is to provide a low-level access point
for debugging and hacking and to expose all lines without the
need of any exporting. Also provide ample ammunition to shoot
oneself in the foot, because this is debugfs after all.


Moving over to immutable irq_chip structures

Most of the gpio chips implementing interrupt support rely on gpiolib
intercepting some of the irq_chip callbacks, preventing the structures
from being made read-only and forcing duplication of structures that
should otherwise be unique.

The solution is to call into the gpiolib code when needed (resource
management, enable/disable or unmask/mask callbacks), and to let the
core code know about that by exposing a flag (IRQCHIP_IMMUTABLE) in
the irq_chip structure. The irq_chip structure can then be made unique
and const.

A small number of drivers have been converted (pl061, tegra186, msm,
amd, apple), and can be used as examples of how to proceed with this
conversion. Note that drivers using the generic irqchip framework
cannot be converted yet, but watch this space!