With kernel 2.6.34, I am unable to change the display brightness: The display backlight always stays 100% bright even when on battery or using a energy-saving power plan, sucking my battery dry in no time. I had a look at /proc/acpi/video to check the brightness information, but the only folders there are GFX0 and PEGP, none of them contains an LCD subfolder. :-( This is what acpi_listen tells me when using the brightness control FN keys: http://pastebin.com/bTewPC87 It seems that brightness information is stored in: * GFX0/DD02/brightness * PEGP/DD02/brightness Writing "50" into this files (for 50% brightness) does not change the brightness. Interesting detail: xbacklight now sais the brightness was 50% but its not. Setting the brightness directly with xbacklight also does not work. Xorg detects backlight controls: "(II) intel(0): found backlight control interface /sys/class/backlight/acpi_video1", but there is also a acpi_video0. I have written different values in the "brightness" file of both entries but nothing happened. I suspect this to be an issue of the switchable graphics or the BIOS: My notebook (HP Envy 14) comes with switchable graphics: A discrete ATI Mobility Radeon HD 5650 and a integrated Intel Arrandale adapter.
The problem is, that ACPI brightness control misses _BQC function and the i915 driver for my Intel Arrandale graphics adapter fails to handle it. There is an experimental kernel and driver patch for this: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/568611/ All testers reported it to work, also an owner of my laptop model. Any chances to get this into the kernel, soon?
Created attachment 27129 [details] acpi/video: acpi_brightness_hook API
Created attachment 27130 [details] drm/i915: override acpi brightness control
You may also contact Kamal Mostafa as he is currently working on a more polished version of his patch: https://wiki.ubuntu.com/KamalMostafa
no, I don't agree with this proposal. I'd like to write something like a backlight control manager. ACPI, i915, platform drivers all register to this manager, and the backlight control manger choose with driver to use and export a unique sysfs I/F to users. and we can add dmi quirk to disable ACPI video driver in the backlight control manager. I plan to finish this in Oct. :) *** This bug has been marked as a duplicate of bug 14216 ***