Legion 5 backlight control not working

Legion 5 backlight control not working

[Pedro Ribeiro]

I have a Lenovo Legion 5 15ARH05H and I'm trying to get the backlight
control working.

This is a laptop with an AMD APU and NVIDIA GPU. It works in a Hybrid
graphics mode with both GPU on, or a Discrete graphics mode where only
the NVIDIA GPU is on and the AMD one is disabled. I always run the
laptop in Discrete mode.

I have tried various NVIDIA driver versions, and was able to get the
backlight working on kernel 5.9.1, but I have no idea what I did to my
system, a few weeks later it stopped working and now I can't get it
back. Tried various kernel versions too, kernel configs, etc, nothing,
the backlight just doesn't work.

It doesn't work either via the /sys/class/backlight device nor via the
hardware keys. On Windows everything works fine.

Every time I boot, I get this in dmesg:
[    1.057696] ACPI BIOS Error (bug): Could not resolve symbol
[\_SB.PCI0.GP17.VGA.LCD._BCM.AFN7], AE_NOT_FOUND (20200717/psargs-330)
[    1.057753] ACPI Error: Aborting method \_SB.PCI0.GP17.VGA.LCD._BCM
due to previous error (AE_NOT_FOUND) (20200717/psparse-529)
[    1.057806] ACPI Error: Evaluating _BCM failed (20200717/video-357)

I believe this is related to the brightness control problem, can
anyone shed a light on this?

Regards,
Pedro