Comment # 15 on bug 1180591 from Takashi Iwai

Thanks.

I checked briefly between v5.7 and v5.8, but there is no change in either
driver/acpi/video_detect.c or drivers/acpi/acpi_video.c.  So the ACPI code (in
those areas) didn't change, and something else (also in ACPI) must be the cause
of the regression.  It's hard to tell for now.

And, I looked at the backlight support on Apple machines and they are...
well... messy.  There is apple-gmux driver that supports things on some devices
but it's only for ACPI APP000B, which isn't enabled on yours.

At this moment, the best thing we can do is to git bisect.  But of course it
takes time (and patience).