[Intel-gfx] Bad flicker on skylake HQD due to code in the 4.7 merge window

[Ville Syrjälä]

On Tue, Jun 21, 2016 at 09:53:15AM -0400, James Bottomley wrote:
> On Mon, 2016-06-20 at 11:03 +0300, Jani Nikula wrote:
> > Cc: Ville
> > 
> > On Mon, 20 Jun 2016, James Bottomley <
> > James.Bottomley at HansenPartnership.com> wrote:
> > > OK, my candidate bad commit is this one:
> > > 
> > > commit a05628195a0d9f3173dd9aa76f482aef692e46ee
> > > Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > Date:   Mon Apr 11 10:23:51 2016 +0300
> > > 
> > >     drm/i915: Get panel_type from OpRegion panel details
> > > 
> > > After being more careful about waiting to identify flicker, this 
> > > one seems to be the one the bisect finds.  I'm now running v4.7-rc3
> > > with this one reverted and am currently seeing no flicker problems.
> > >   It is, however, early days because the flicker can hide for long 
> > > periods, so I 'll wait until Monday evening and a few reboots 
> > > before declaring victory.
> > 
> > If that turns out to be the bad commit, it doesn't really surprise 
> > me, and that in itself is depressing.
> 
> As far as I can tell, after running for a day with this reverted, this
> is the problem.  The flicker hasn't appeared with it reverted.  It's
> pretty noticeable with this commit included.

Hmm. The only difference I can see is low vs. normal vswing. Panel 0 has
low, panel 2 has normal. So either the VBT or opregion is telling utter
lies, or there's some other bug in our low vswing support.

To confirm it's really a vswing issue, you should be able to run with
i915.edp_vswing=2 without flickers on the broken kernel.

-- 
Ville Syrjälä
Intel OTC