[Regression 3.14-rc2] drm/i915: Brightness adjustment is broken in 945GM

[Luis Ortega]

> I dare say either your bisect went sour or you don't have 945GM. Please
> verify your steps.

Well, what can I say? I was careful when testing and the last kernel I compiled
shows the problem. I can replay the bisection if needed but if I have to start
all over again it's gonna take some time. I'm compiling in the netbook and took
me day and a half to finish the process.

In all honesty, along the way I met commit messages that I thought were more
probable to introduce this regression. (backlight related)

Also, here's the output of lspci:
00:00.0 Host bridge: Intel Corporation Mobile 945GSE Express Memory Controller Hub (rev 03)
00:02.0 VGA compatible controller: Intel Corporation Mobile 945GSE Express Integrated Graphics Controller (rev 03)
00:02.1 Display controller: Intel Corporation Mobile 945GM/GMS/GME, 943/940GML Express Integrated Graphics Controller (rev 03)

> Please provide dmesg with drm.debug=0xe with 3.14-rc2.

Care to explain a bit more before I recompile 3.14-rc2? Do I need to
enable some particular debug option? How do I proceed for drm.debug=0xe?


Luis
> On Thu, Feb 13, 2014 at 05:04:23PM +0200, Jani Nikula wrote:
> On Thu, 13 Feb 2014, Luis Ortega <luiorpe1 at upv.es> wrote:
> > Hi, testing 3.14-rc2 I noticed I could not adjust the brightness of the
> > screen any longer. This problem is already present in 3.14-rc1. 3.13 works fine.
> >
> > My hardware is a netbook with intel atom and a 945GM graphics card.
> >
> > I bisected the problem down to the next commit:
> 
> I dare say either your bisect went sour or you don't have 945GM. Please
> verify your steps.
> 
> Please provide dmesg with drm.debug=0xe with 3.14-rc2.
> 
> BR,
> Jani.
> 
> >
> > bc0bb9fd1c7810407ab810d204bbaecb255fddde is the first bad commit
> > commit bc0bb9fd1c7810407ab810d204bbaecb255fddde
> > Author: Jani Nikula <jani.nikula at intel.com>
> > Date:   Thu Nov 14 12:14:29 2013 +0200
> >
> >     drm/i915: remove QUIRK_NO_PCH_PWM_ENABLE
> >     
> >     The quirk was added as what I'd say was a stopgap measure in
> >     
> >     commit e85843bec6c2ea7c10ec61238396891cc2b753a9
> >     Author: Kamal Mostafa <kamal at canonical.com>
> >     Date:   Fri Jul 19 15:02:01 2013 -0700
> >     
> >         drm/i915: quirk no PCH_PWM_ENABLE for Dell XPS13 backlight
> >     
> >     without really digging into what was going on.
> >     
> >     Also, as mentioned in the related bug [1], having the quirk regressed
> >     some of the machines it was supposed to fix to begin with, and there
> >     were patches posted to disable the quirk on such machines [2]!
> >     
> >     The fact is, we do need the BLM_PCH_PWM_ENABLE bit set to have
> >     backlight. With the quirk, we've relied on BIOS to have set it, and our
> >     save/restore code to retain it. With the full backlight setup at enable,
> >     we have no place for things that rely on previous state.
> >     
> >     With the per platform hooks, we've also made a change in the PCH
> >     platform enable order: setting the backlight duty cycle between CPU and
> >     PCH PWM enable. Some experimenting and
> >     
> >     commit 770c12312ad617172b1a65b911d3e6564fc5aca8
> >     Author: Takashi Iwai <tiwai at suse.de>
> >     Date:   Sat Aug 11 08:56:42 2012 +0200
> >     
> >         drm/i915: Fix blank panel at reopening lid
> >     
> >     indicate that we can't set the backlight before enabling CPU PWM; the
> >     value just won't stick. But AFAICT we should do it before enabling the
> >     PCH PWM.
> >     
> >     Finally, any fallout we should fix properly, preferrably without quirks,
> >     and absolutely without quirks that rely on existing state. With the per
> >     platform hooks have much more flexibility to adjust the sequence as
> >     required by platforms.
> >     
> >     [1] https://bugzilla.kernel.org/show_bug.cgi?id=47941
> >     [2] http://lkml.kernel.org/r/1378229848-29113-1-git-send-email-kamal@canonical.com
> >     
> >     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>
> >
> >
> > Additionally, when I had narrowed the problem to around 11 commits all the
> > resulting kernel I compiled made the screen blink and shake from left to right
> > non-stop. In the -rc kernels this doesn't happen. Thought it was worth
> > mentioning.
> 
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center