When set to 0 this will stop tools like the backlight and rfkill tools to
restore state from previous boot. This is useful in case the stored state
is bogus to the extent that it is preventing you from resetting it (e.g.,
the backlight settings cause the screen to be off on boot on a system where
the backlight can not be adjusted directly from the keyboard).
Much like for rfkill devices we should provide some stability regarding
enumeration order, hence include the stable bits of the device path in
the file name we store settings under.
As many laptops don't save/restore screen brightness across reboots,
let's do this in systemd with a minimal tool, that restores the
brightness as early as possible, and saves it as late as possible. This
will cover consoles and graphical logins, but graphical desktops should
do their own per-user stuff probably.
This only touches firmware brightness controls for now.
Tom Gundersen
Lennart Poettering
Zbigniew Jędrzejewski-Szmek
Bastien Nocera