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

[Chris Wilson]

On Tue, Jul 22, 2014 at 08:52:48AM +0200, Daniel Vetter wrote:
> On Tue, Jul 22, 2014 at 8:42 AM, Hans de Goede <hdegoede at redhat.com> wrote:
> > 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.
> 
> How does the intel backlight fare? Please test both 3.14 and 3.15 and
> also test what happens when you blacklist the samsung-laptop driver
> (if that's possible without wreaking the machine).
> 
> Also please grab latest intel-gpu-tools and record a register dump
> with intel_reg_dump, again for both broken and working kernels.

What would be useful here would be dumping the regs when we takeover (and
our interpretation for ease of reading). Basically the question here is
what does the samsung backlight driver do - and the suspicion is that it
uses the combination mode and we don't set that up...
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre