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

[Ville Syrjälä]

On Tue, Jun 21, 2016 at 06:44:34PM +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.

I did a quick once over of out DDI vswing stuff and didn't find anything
too serious. There were some buglets in the iboost handling, but I'm not
very hopeful that fixing those would help with your machine.

Here's a branch anyway in case you want to give it a go:
git://github.com/vsyrjala/linux.git ddi_iboost_fixes

Actually, I think the only patch in there that might make a difference is
15d887855180 ("drm/i915: Fix iboost setting for DDI with 4 lanes on SKL")

-- 
Ville Syrjälä
Intel OTC