debugging backlight issues

[David Tombs]

Hi Kamal,

Thanks for the extra info!

On 06/16/2010 03:43 PM, Kamal Mostafa wrote:

>> 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?
> 
> I imagine that's possible...  On that laptop, do you see the GUI
> "brightness notification popup"?  If you do then the software must be
> involved somehow; if you don't then maybe the keys are indeed directly
> wired to the hardware.

Nope, no notification popup. Hardwired, I guess.

>> 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.
> 
> I think the param "acpi.brightness_switch_enabled=0" does what you're
> looking for.

Do you know of any documentation for this? I couldn't find the parameter
in kernel-parameters.txt [1] and the patch I found Googling [2] seems to
imply that it just disables automatic brightness changes or something,
but won't let the hw take over completely like acpi=off does.

Also, is this actually supported in Ubuntu? I couldn't find a parameter
for it anywhere in sysfs.

Thanks,
David

[1] http://www.mjmwired.net/kernel/Documentation/kernel-parameters.txt
[2] http://www.mail-archive.com/linux-acpi@vger.kernel.org/msg11894.html