On virtually any newer Asus mainboard, the eeepc-wmi driver is loaded.
It exposes a backlight device despite the lack of any physical backlight
devices. This fake backlight device has max_brightness set to 0. Since
the introduction of the clamp_brightness function, systemd-backlight
tries to write '1' to brightness and fails.
This patch changes systemd-backlight to exit gracefully when
max_brightness is 0 before performing any action. This affects
both the load and save actions.
Some systems turn the backlight all the way off at the lowest levels.
Clamp saved brightness to at least 1 or 5% of max_brightness. This
avoids preserving an unreadably dim screen, which would otherwise force
the user to disable state restoration.
udev_device_get_sysattr_value returns NULL on failure, but doesn't
provide an error code; thus, when printing an error from it, don't print
an unrelated error code from a previous call.
If -flto is used then gcc will generate a lot more warnings than before,
among them a number of use-without-initialization warnings. Most of them
without are false positives, but let's make them go away, because it
doesn't really matter.
Instead of individually checking for containers in each user do this
once in a new call proc_cmdline() that read the file only if we are not
in a container.
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.
