Re: [PATCH igt] igt/kms_flip: Calibrate timestamp errors

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Oct 27, 2016 at 11:29:55AM +0100, Chris Wilson wrote:
> On Thu, Oct 27, 2016 at 01:13:59PM +0300, Ville Syrjälä wrote:
> > On Thu, Oct 27, 2016 at 08:43:44AM +0200, Daniel Vetter wrote:
> > > On Wed, Oct 26, 2016 at 12:17:25PM +0300, Ville Syrjälä wrote:
> > > > 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.
> > > 
> > > Over-the top idea would be to adjust the reported modlines to match what
> > > we can do with the PLL on each platform. Probably not worth the trouble,
> > > but I guess if we bother with this for panels it won't be more work
> > > really.
> > 
> > Yeah for panels it should be quite doable. For external displays we may
> > run into problems with cloning (maybe would affect the PLL limits and
> > whatnot, can't recall exactly). And at least HDMI 12bpc would still
> > affect the clock you can get so not clear if we should report the 8bpc
> > one or the 12bpc one, or maybe both?
> > 
> > Hmm, or do you mean just adjuting the mode blob on the crtc? I think
> > that might confuse userspace if it compares that against what it
> > thought it set, or against the thing it pulled off from the connector's
> > mode list. We could add some kind of "actual pixel clock" type of
> > property though. Or even an "actual mode" blob if the driver would feel
> > the need to fudge things just a little to get a picture on the screen.
> 
> So does the adjusted_mode match reality?

Actually not right now. I think everything else but the clock is correct
in there. We really should fix up the clock as well since we use it to
calculate further things like FDI bandwidth. So everything else depending
on the clock might come out a little wrong currently.

Last time I looked it seemed that we'd have to move the clock
computation up in the modeset sequence quite a bit (assuming it wasn't
calculated by the encoders .compute_config() hook) since right now we do
it way too late.

-- 
Ville Syrjälä
Intel OTC
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