[Intel-gfx] [REGRESSION BISECTED] backlight control stops workin with 3.14 and later

[Hans de Goede]

Hi Jani et al,

A friend of mine Bertrik Sikken (in the Cc) his backlight control
stopped working for him on his Samsung N150Plus netbook.

I took a quick look, and the raw intel_backlight backlight interface
works under 3.14, but the firmware samsung_laptop backlight interface,
which is what most userspace apps will use by default, stops working
in 3.14 .

I've asked him to bisect this and the bisect points out this
commit as the culprit:

b35684b8fa94e04f55fd38bf672b737741d2f9e2 is the first bad commit
commit b35684b8fa94e04f55fd38bf672b737741d2f9e2
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Thu Nov 14 12:13:41 2013 +0200

    drm/i915: do full backlight setup at enable time

    We should now have all the information we need to do a full
    initialization of the backlight registers.

    v2: Keep QUIRK_NO_PCH_PWM_ENABLE for now (Imre).

    Signed-off-by: Jani Nikula <jani.nikula at intel.com>
    Reviewed-by: Imre Deak <imre.deak at intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Note that this laptop has an acpi_video backlight interface too,
but that has been broken from the start and gets disabled by
samsung-laptop based on dmi matching.

Regards,

Hans