On Thu, 03 Apr 2014, James Hogan james@albanarts.com wrote:

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?

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

BR, Jani.

Thanks James