[PATCH v2 3/3] drm/vc4: Add a load tracker to prevent HVS underflow errors

[Boris Brezillon]

On Tue, 30 Oct 2018 16:12:55 -0700
Eric Anholt <eric at anholt.net> wrote:

> > +		/* Even if the bandwidth/plane required for a single frame is
> > +		 *
> > +		 * vc4_state->src_w[i] * vc4_state->src_h[i] * cpp * vrefresh
> > +		 *
> > +		 * when downscaling, we have to read more pixels per line in
> > +		 * the time frame reserved for a single line, so the bandwidth
> > +		 * demand can be punctually higher. To account for that, we
> > +		 * calculate the down-scaling factor and multiply the plane
> > +		 * load by this number. We're likely over-estimating the read
> > +		 * demand, but that's better than under-estimating it.
> > +		 */

One more thing: because we sometimes over estimate the load, we might
reject configuration that were previously accepted, so I'm wondering if
we shouldn't make the 'reject on HVS/membus load too high' optional,
and disable it by default.