[Bug 106228] amdgpu reads back brightness 0 (which is not true) if checked before a brightness has been explicitly set

https://bugs.freedesktop.org/show_bug.cgi?id=106228

--- Comment #6 from jian-hong@endlessm.com --- Apply "[PATCH] drm/amdgpu: Read back max backlight value at boot" to Linux 4.18.0-rc6 kernel.

I disable all of the brightness setting applications, including masking systemd-backlight service and than reboot. "cat /sys/class/backlight/amdgpu_bl0/brightness" before set brightness manually again and it reads back 255 (the maximum brightness).

If I unmask systemd-backlight service and than reboot for test, "cat /sys/class/backlight/amdgpu_bl0/brightness" before set brightness manually again and it reads back the brightness which is saved before last reboot.