[Intel-gfx] [PATCH igt] igt/kms_flip: Calibrate timestamp errors

[Ville Syrjälä]

On Wed, Oct 26, 2016 at 08:18:13AM +0200, Daniel Vetter wrote:
> On Mon, Oct 24, 2016 at 10:38:34AM +0100, Chris Wilson wrote:
> > On Mon, Oct 24, 2016 at 11:14:31AM +0200, Daniel Vetter wrote:
> > > On Mon, Oct 24, 2016 at 09:54:52AM +0100, Chris Wilson wrote:
> > > Also with this patch we should be able to throw out the hacks for tv-out.
> > > I only added those because the reported mode-timings are massively off
> > > (due to the magic tv scaler thing) from the real timestamps we receive.
> > > Auto-detecting this is much better.
> > 
> > Not quite just yet, we need to split the timing tests into a subgroup
> > with a subtest per output so that we can skip one without skipping the
> > others. At the moment, this check makes it bail out on my ctg/ilk who
> > have a difference of about 50us between measured and expected vblank
> > interval on LVDS (which is nigh on impossible given our confidence in the
> > measurement, i.e. about 7 sigma).
> 
> Hm, should we be a bit more sloppy in our acceptance? Iirc Ville has made
> changes to make it a bit more strict a while ago, and way, way back this
> stuff worked on my ctg. Haven't fired it up in a while ;-)
> 
> > > And another issue: Failing to match the reported mode timings is a driver
> > > bug.
> > 
> > Not quite, remember we override the user for fixed mode panels. But yes,
> > piglit also has a similar expectation that the dotclock we report (via
> > GetMscRate) in someway corresponds to actual vblank interval.
> 
> Yeah, I hope that DRRS would fix that, at least on newer stuff. At least I
> proposed just using the matching dotclock for manual DRRS (mostly to
> perfectly match with the refresh rate of a video). Didn't yet happen :(
> 
> But at least for the default mode we should try real hard to match.

The problem is the granularity of the PLL. For fixed mode panels we
could easily fix up what we report to userspace as the clock, which
would fix these tests. For external displays it's not quite so clear.

-- 
Ville Syrjälä
Intel OTC