debugging backlight issues

[David Tombs]

Hello all,

As a member of bugsquad and a regular triager, I've been working on some
power management bugs the past few days. Naturally a few of these have
been backlight brightness issues but I haven't been able to find
helpful, up-to-date info on debugging backlight stuff. So I'm planning
on making a /DebuggingBacklight wiki page with some basic information,
and I have a few questions for you guys and girls if any of you have a sec.

1) Any backlight information found by the kernel will be in
/sys/class/backlight correct? Is all the info from /proc/acpi available
somewhere in /sys now?

2) I have a laptop where the brightness hotkeys work, but there's
nothing in /sys/backlight and /proc/acpi/video/*/backlight reads "<not
supported>". Do some laptops have the keys directly connected to the
hardware or something?

3) Is there any way to "blacklist" ACPI control of the backlight if it
doesn't work? This would be for laptops where acpi=off fixes the
brightness keys but the user naturally doesn't want to use this all the
time.

If there are any other tips you can point me to, particularly regarding
the ACPI information under /proc/acpi and /sys, that would be great.

Thanks in advance,
David