Re: [PATCH] drm/i915: Enable scanline read for gen9 dsi

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

On Mon, Sep 11, 2017 at 10:52:27AM +0200, Maarten Lankhorst wrote:
> Op 08-09-17 om 21:55 schreef Chris Wilson:
> > Quoting Daniel Vetter (2017-09-08 20:45:11)
> >> On Fri, Sep 08, 2017 at 05:55:24PM +0300, Ville Syrjälä wrote:
> >>> Another thought that just occurred to me: Maybe we could use these
> >>> timestamps as a workaround for the DDI "scanline reads as 0 at the
> >>> wrong time" problem. What we could do is check of the scanline counter
> >>> reads as 0, and if it does we could switch over to checking the
> >>> timestamps instead. Not sure if we should just do the full timestamp
> >>> based scanline read like you do here, or we could just check that if the
> >>> timestamps look like they're close to vblank_start we just return
> >>> vblank_start-1. This could then remove the obnoxious retry loop from the
> >>> scanline counter read.
> >> Another concern I have on this is timeframe jitter. If the vblank
> >> timestamp stuff isnt' perfectly accurately spaced, or we have a mismatch
> >> in clocks, then we might think there's still plenty of time before vblank
> >> while we're already racing.
> > You are sort of getting to the point where you just use the ART cpu
> > clock, using an ewma seeded with the vrefresh and fed with the vblank
> > intervals as an estimator for how long you have left to the next vblank.
> Agreed, this seems to be the case.. In which case can't we use that for all of DDI to get a
> better than scanline resolution for last vblank time by replacing the get_vblank_timestamp hook?

Like I said that should be doable with just a simple timestamp check to
fix up the bogus 0.

-- 
Ville Syrjälä
Intel OTC
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