debugging backlight issues

[Stefan Bader]

On 06/15/2010 12:36 AM, David Tombs wrote:
> 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?

Yes and no. /sys is a bit more abstracted. In proc you will see discrete
percentage values and the current value in one file. The sysfs interface uses
index levels and has a separate variable for the maximum value.

> 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?

Yes, this is the case sometimes.

> 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.
> 
acpi_backlight=vendor could help. The opposite acpi_backlight=video had a bug
and will only work in a future Lucid (which includes upstream stable 2.6.32.16)
version or Maverick.

> 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.
> 

Both vendor specific and the acpi video driver should place the same interface
files into /sys/class/backlight/*/...

max_brightness would contain the highest number and echoing numbers into
brightness should change the brightness. If supported correctly. In hairy cases
asking the users for a dump (sudo acpidump -o acpidump-<laptop-model>.txt). This
can then extracted into various acpi tables (acpixtract) and disassembled with
iasl. Though reading the result is tedious.

> Thanks in advance,
> David
> 

-Stefan