The GPIO LED trigger exposes a userspace ABI where a user
can echo a GPIO number from the global GPIO numberspace into
a file that will trigger a certain LED when active.

This is problematic because the global GPIO numberspace is
inherently instable. The trigger came about at a time when
systems had one GPIO controller that defined hard-wired
GPIOs numbered 0..N and this number space was stable.

We have since moved to dynamic allocation of GPIO numbers
and there is no real guarantee that a GPIO number will stay
consistent even across a reboot: consider a USB attached
GPIO controller for example. Or two. Or the effect of
probe order after adding -EPROBE_DEFER to the kernel.

The trigger was added to support keypad LEDs on the Nokia
n810 from the GPIO event when a user slides up/down the
keypad. This is arch/arm/boot/dts/omap2420-n810.dts.
A userspace script is needed to activate the trigger.
This will be broken unless the script was updated recently
since the OMAP GPIO controller now uses dynamic GPIO
number allocations.

I want to know that this trigger has active users that
cannot live without it if we are to continue to support it.

Option if this is really needed: I can develop a new trigger
that can associate GPIOs with LEDs as triggers using device
tree, which should also remove the use of userspace custom
scripts to achieve this and be much more trustworthy, if
someone with the Nokia n810 or a device with a similar need
is willing to test it.

Suggested-by Pavel Machek <pavel@ucw.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230314210059.419159-1-linus.walleij@linaro.org