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

[James Bottomley]

On Thu, 2016-07-07 at 09:55 -0700, James Bottomley wrote:
> On Thu, 2016-07-07 at 19:14 +0300, Ville Syrjälä wrote:
> > 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")
> 
> Running with it now (the entire branch).  So far it looks OK, but 
> I'll give it a couple of days to see if anything manifests before
> declaring victory.

Bad news, I'm afraid: after a couple of hours of run time, there is now
noticeable flicker on the display, so although the iboost fixes may
have lessened it, it's still present.

James