The behavior I see does not quite fit into the 4 categories described in the 
document.
Controls *seem* to work as function keys bring up the OSD and the bar can be 
moved up and down - only the screen's actual brightness does not change at all.

Okay, I tried the following:

 * added acpi_backlight=vendor to the kernel parameters => no changes in
behavior and result.

 * looked at the /proc/acpi => no video there

 * looked at sys/class/backlight, found dell_backlight there with max
brightness 15, actual_brightness can be changed with the fn-keys.

Then I did:
zordid@Dell-XPS:/sys/class/backlight/dell_backlight$ dmesg|grep 'ACPI: Video'
[   14.193426] ACPI: Video Device [PEGP] (multi-head: yes  rom: no  post: no)
zordid@Dell-XPS:/sys/class/backlight/dell_backlight$ lspci | grep VGA
01:00.0 VGA compatible controller: nVidia Corporation Device 0dcd (rev a1)

What else do you need? I do not quite understand the next "steps"... sorry!
Can you give me a hint of what else is important?

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https://bugs.launchpad.net/bugs/762670

Title:
  Laptop brightness on Dell XPS not controllable

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