gpio: Add use guidance documentation

The text in this new document is a response to recurring questions
about the GPIO in-kernel API vs the userspace ABI. When do you use
one or the other? It can be a bit intuitive, but I tried to sum it
all up.
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200108001712.47500-1-linus.walleij@linaro.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/drivers-on-gpio.rst

@@ -95,7 +95,7 @@ to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
 MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
 usually connected directly to the flash.
 
-Use those instead of talking directly to the GPIOs using sysfs; they integrate
-with kernel frameworks better than your userspace code could. Needless to say,
-just using the appropriate kernel drivers will simplify and speed up your
-embedded hacking in particular by providing ready-made components.
+Use those instead of talking directly to the GPIOs from userspace; they
+integrate with kernel frameworks better than your userspace code could.
+Needless to say, just using the appropriate kernel drivers will simplify and
+speed up your embedded hacking in particular by providing ready-made components.

Documentation/driver-api/gpio/index.rst

@@ -8,6 +8,7 @@ Contents:
    :maxdepth: 2
 
    intro
+   using-gpio
    driver
    consumer
    board

Documentation/driver-api/gpio/using-gpio.rst

+=========================
+Using GPIO Lines in Linux
+=========================
+
+The Linux kernel exists to abstract and present hardware to users. GPIO lines
+as such are normally not user facing abstractions. The most obvious, natural
+and preferred way to use GPIO lines is to let kernel hardware drivers deal
+with them.
+
+For examples of already existing generic drivers that will also be good
+examples for any other kernel drivers you want to author, refer to
+:doc:`drivers-on-gpio`
+
+For any kind of mass produced system you want to support, such as servers,
+laptops, phones, tablets, routers, and any consumer or office or business goods
+using appropriate kernel drivers is paramount. Submit your code for inclusion
+in the upstream Linux kernel when you feel it is mature enough and you will get
+help to refine it, see :doc:`../../process/submitting-patches`.
+
+In Linux GPIO lines also have a userspace ABI.
+
+The userspace ABI is intended for one-off deployments. Examples are prototypes,
+factory lines, maker community projects, workshop specimen, production tools,
+industrial automation, PLC-type use cases, door controllers, in short a piece
+of specialized equipment that is not produced by the numbers, requiring
+operators to have a deep knowledge of the equipment and knows about the
+software-hardware interface to be set up. They should not have a natural fit
+to any existing kernel subsystem and not be a good fit for an operating system,
+because of not being reusable or abstract enough, or involving a lot of non
+computer hardware related policy.
+
+Applications that have a good reason to use the industrial I/O (IIO) subsystem
+from userspace will likely be a good fit for using GPIO lines from userspace as
+well.
+
+Do not under any circumstances abuse the GPIO userspace ABI to cut corners in
+any product development projects. If you use it for prototyping, then do not
+productify the prototype: rewrite it using proper kernel drivers. Do not under
+any circumstances deploy any uniform products using GPIO from userspace.
+
+The userspace ABI is a character device for each GPIO hardware unit (GPIO chip).
+These devices will appear on the system as ``/dev/gpiochip0`` thru
+``/dev/gpiochipN``. Examples of how to directly use the userspace ABI can be
+found in the kernel tree ``tools/gpio`` subdirectory.
+
+For structured and managed applications, we recommend that you make use of the
+libgpiod_ library. This provides helper abstractions, command line utlities
+and arbitration for multiple simultaneous consumers on the same GPIO chip.
+
+.. _libgpiod: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/