[Bug 76276] Backlight control broken on Dell XPS13 w/ kernel 3.14-rc7

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=76276

--- Comment #18 from Martin <bugs at mrvanes.com> ---
Created attachment 97111
  --> https://bugs.freedesktop.org/attachment.cgi?id=97111&action=edit
As requested

Some answers and output:

Let me start by saying that the backlight is ON, so I can work.
The only thing that stopped working is backlight control in KDE powercontrol
interface.

1. Does acpi_backlight work after that?

The KDE powercontrol interface works again after that (my main concern).
Echo'ing values between 0 and 100 in
/sys/class/backlight/acpi_video0/brightness correctly sets the brightness.

Echoing values between 0 and
/sys/class/backlight/intel_backlight/max_brightness to
/sys/class/backlight/intel_backlight/brightness makes the backlight pulsate
(slowly) except for max_brightness which makes the
backlight max without pulse. acpi_video0 doesn't work then.

2. Please attach /sys/kernel/debug/dri/0/i915_opregion
See attached file.

3. Also please provide a reg dump *before* i915 gets loaded. Does
acpi_backlight work before i915 is loaded?
I think this is one of the things I can't easily help you with, i915 is
compiled into my kernel since it makes no sense booting without (to me).

4. Also, is there a difference between EFI vs. legacy boot?
I boot legacy and have no EFI partition. I don't  want to risk reformatting my
drive, since this is my daily workhorse.

5. Please do all of the above, and additionally try this hack
Done. This fixes my problem.

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