[Intel-gfx] [PATCH v2] drm/i915/display: Record the plane update times for debugging

[Chris Wilson]

Quoting Chris Wilson (2020-11-27 16:18:41)
> Since we try and estimate how long we require to update the registers to
> perform a plane update, it is of vital importance that we measure the
> distribution of plane updates to better guide our estimate. If we
> underestimate how long it takes to perform the plane update, we may
> slip into the next scanout frame causing a tear. If we overestimate, we
> may unnecessarily delay the update to the next frame, causing visible
> jitter.
> 
> Replace the warning that we exceed some arbitrary threshold for the
> vblank update with a histogram for debugfs.
> 
> v2: Add a per-crtc debugfs entry so that the information is easier to
> extract when testing individual CRTC, and so that it can be reset before
> a test.

With the per-crtc reset functionality, we can more easily detect errors
under testing, e.g.

https://patchwork.freedesktop.org/series/84359/

and suppress the warnings when no one is watching (but are flooding the
CI results at present).

Any one in favour?
-Chris