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

[bugzilla-daemon at freedesktop.org]

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

--- Comment #6 from jian-hong at 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.

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