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

[James Bottomley]

On Tue, 2016-06-21 at 17:00 -0400, James Bottomley wrote:
> On Tue, 2016-06-21 at 18:44 +0300, Ville Syrjälä wrote:
> > 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.
> 
> Preliminary boot indicates no flicker with the bad commit included 
> and this option, but I'll have to run for quite a bit longer to 
> verify, since it can sometimes be elusive.

Two days of runtime seems to confirm this is the problem (still no
flicker issues).

James