On 2011-11-10 at 19:36 -0500, Taylor Hedberg wrote:
> Sebastian Schwarz, Thu 2011-11-10 @ 16:36:54+0100:
> > The bug can easily triggered in X by running:
> >
> > xset dpms force off
>
> Yes, this is the same behavior that I'm seeing. Writing the
> max_brightness value into the sysfs brightness file restores
> normal backlight brightness. I can also restore it with
> `xbacklight -set 100`.
After running `xbacklight -set 100` my backlight is at the
previous brightness level, which is 57% and not at the 100%
xbacklight should set it to.
> > I found this I my dmesg output:
> >
> > [Firmware Bug]: Duplicate ACPI video bus devices for the same VGA
> > controller, please try module parameter
> > "video.allow_duplicates=1"if the current driver doesn't work.
> >
> > But I also haven't tried this, as the incorrect brightness has not
> > annoyed me enough, yet.
>
> I have this message in my dmesg output as well, but
> unfortunately, setting the mentioned module parameter doesn't
> have any effect on the problems I'm seeing.
It doesn't do anything over here, too. However downgrading the
kernel to 3.0.7-1 helps. There are a few interesting things:
% pacman -Q linux xorg-xbacklight
linux 3.0.7-1
xorg-xbacklight 1.1.2-2
% xbacklight -get
No outputs have backlight property
% cat /sys/class/backlight/intel_backlight/{,max_}brightness
cat: /sys/class/backlight/intel_backlight/brightness: No such file or directory
cat: /sys/class/backlight/intel_backlight/max_brightness: No such file or directory
So 3.1 has probably some new backlight code, which does not
work as it should. And indeed there are two threads on the
linux.kernel newsgroup[1, 2] and one on the LKML[3] with an
explanation, a patch[4] and a couple of bug reports[5, 6, 7].
I have not tested the patch yet, but is seems to deal with the
issue at hand.
[1]: https://groups.google.com/group/linux.kernel/browse_thread/thread/28296924e8888e31
[2]: https://groups.google.com/group/linux.kernel/browse_thread/thread/fde3079d9ff7b77c
[3]: https://lkml.org/lkml/2011/9/19/381
[4]: https://lkml.org/lkml/diff/2011/10/25/473/1
[5]: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/872652
[6]: https://launchpad.net/~kamalmostafa/+archive/stuck-backlight
[7]: https://bugs.freedesktop.org/show_bug.cgi?id=41926
