[Intel-gfx] [PATCH] drm/i915/selftests: Measure CS_TIMESTAMP

[Chris Wilson]

Quoting Ville Syrjälä (2020-05-19 11:42:45)
> On Sat, May 16, 2020 at 02:31:02PM +0100, Chris Wilson wrote:
> > Count the number of CS_TIMESTAMP ticks and check that it matches our
> > expectations.
> 
> Looks ok for everything except g4x/ilk. Those would need something
> like
> https://patchwork.freedesktop.org/patch/355944/?series=74145&rev=1
> + read TIMESTAMP_UDW instead of TIMESTAMP.
> 
> bw/cl still needs
> https://patchwork.freedesktop.org/patch/355946/?series=74145&rev=1
> though the test seems a bit flaky on my cl. Sometimes the cycle count
> comes up short. Never seen it exceed the expected value, but it can 
> come up significantly short. And curiously it does seem to have a
> tendency to come out as roughly some nice fraction (seen at least
> 1/2 and 1/4 quite a few times). Dunno if the tick rate actually
> changes due to some unknown circumstances, or if the counter just
> updates somehow lazily. Certainly polling the counter over a longer
> period does show it to tick at the expected rate.

Any guestimate at how short a period is long enough?
-Chris