drm/i915: XPS13 backlight regression in v3.14

[James Hogan]

Hi,

I've noticed that v3.14 breaks the backlight on Dell XPS13. Reverting the 
following commit fixes the issue for me (i.e. the GUI brightness controls work 
again):

bc0bb9fd1c78 drm/i915: remove QUIRK_NO_PCH_PWM_ENABLE

It appears that in v3.14 (with the above patch):
* intel_backlight/brightness=4882 (max & default) causes 
acpi_video0/brightness to have no effect.
* intel_backlight/brightness=0 causes acpi_video0/brightness to behave 
corrrectly (the firmware (I assume) fades the brightness gradually when 
acpi_video0/brightness is set)
* intel_backlight/brightness=something else in between often causes 
acpi_video0 and intel_backlight to apparently "fight" over the backlight, 
resulting in brightness flickering up and down every second or so.

Reverting the above patch on v3.14:
* intel_backlight/brightness has no effect
* acpi_video0/brightness works normally

It doesn't seem quite as simple as them trying to use the same hardware since 
when intel_backlight/brightness=4882, acpi_video0/brightness has no effect at 
all, so I don't really get what's going on.

Can anybody shed some light or suggest a proper fix for this recurring issue?

Thanks
James
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