gpio: Add global TODO file for GPIO

With a new subsystem co-maintainer on board it is good to draw
up the ongoing changes and future plans for the subsystem, i.e.
what is in my head and being worked on long term.

What better way is there than simply adding a TODO right in the
code and send it out to the mailing list.

Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/TODO

+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 decriptor-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.
+
+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.
+
+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.
+
+
+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.
+
+
+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 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
+
+
+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
+
+- Support generic hierarchical GPIO interrupts: these are for the
+  non-cascading case where there is one IRQ per GPIO line, there is
+  currently no common infrastructure for this.
+
+
+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.