[Kde-hardware-devel] Powerdevil: brightness keys change values in incorrect file

[Àlex Fiestas]

On Wednesday, May 08, 2013 02:07:59 PM Sparhawk wrote:
> Hi, I've been chatting with bcooksley at the KDE forums [1], and he/she
> suggested that I post here with my question.
> 
> I have a Dell XPS 17 L702X, running Kubuntu 13.04 (KDE 4.10.2). In the
> update from 12.10 to 13.04, changing the brightness no longer works as
> expected. I get the same results using the brightness keys (which shows
> feedback with the OSD) or using xbacklight.
> 
> When `/etc/default/grub` contains
>     GRUB_CMDLINE_LINUX_DEFAULT="acpi_backlight=vendor"
> 
> `/sys/class/backlight/` contains `dell_backlight` and `intel_backlight`.
> Pressing the brightness keys modifies the value in
> `dell_backlight/brightness`, but does nothing to the display . However,
> if I manually modify the values in `intel_backlight/brightness`, the
> backlight changes as expected.
> 
> OTOH, If I modify `/etc/default/grub` to contain
>     GRUB_CMDLINE_LINUX_DEFAULT=""
> 
> `/sys/class/backlight/` now contains `acpi_video0`, `acpi_video1` and
> `intel_backlight`. Now, the brightness keys modify
> `acpi_video1/brightness`, and the display's brightness alters.
> 
> Even though this does work somewhat, setting `acpi_backlight=vendor` in
> Kubuntu 12.10 gave me more control over the backlight. For example, I
> could reduce the backlight down to 0% and have the screen turn off.
> Without this grub option, I can reduce `acpi_video1/brightness` to 0
> with the brightness keys, but the backlight is still on (dimly). Hence,
> it seems that the ideal situation is for the brightness keys to modify
> intel_backlight instead of acpi_video1. Is this possible?
> 
> Cheers, and thanks in advance.
> Sparhawk.
> 
> [1] http://forum.kde.org/viewtopic.php?f=63&t=110984
Hi

There are a bunch of bug reports in bugs.kde.org about these kinds of issues, 
bugzilla is the right place to discuss these things, not the mailist :p

I just wrote this small wiki page with some information about these issues:

http://community.kde.org/Solid/PowerDevil/Brightness

In summary brightness situation in Kernel sucks, There is no guarantee that 0 
means "display turned off" (for that we should use DPMS) and in your case is 
probably the GPU driver deciding which file to modify, not us.