[Intel-gfx] [PATCH v3] drm/i915/display: Record the plane update times for debugging

[Chris Wilson]

Quoting Chris Wilson (2020-12-02 21:28:14)
> 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.
> 
> v3: Flip the graph on its side; creates space to label the time axis.
> 
> Updates: 4684
>        |
>    1us |
>        |
>    4us |********
>        |**********
>   16us |***********
>        |*****
>   66us |
>        |
>  262us |
>        |
>    1ms |
>        |
>    4ms |
>        |
>   17ms |
>        |
> Min update: 5918ns
> Max update: 54781ns
> Average update: 16628ns
> Overruns > 250us: 0
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1982
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com> #v2
-Chris