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Old 06-16-2010, 08:14 PM
David Tombs
Default Fwd: debugging backlight issues

Sorry, didn't include the mailing list on this reply.


-------- Original Message --------
Subject: Re: debugging backlight issues
Date: Wed, 16 Jun 2010 15:13:42 -0400
From: David Tombs <cyan.spam@gmail.com>
To: Stefan Bader <stefan.bader@canonical.com>

On 06/15/2010 09:03 AM, Stefan Bader wrote:
> 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
> 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

Hi Stefan,

Excellent, thanks for the information. I see you already did my work for
me and created <https://wiki.ubuntu.com/Kernel/Debugging/Backlight>. So,
double thanks!


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