[Mesa-dev] [PATCH 4/6] i965: Use blorp+userptr for GPU readback

[Chris Wilson]

Quoting Chris Wilson (2017-10-13 10:34:54)
> The primary benefit for this is that we get format conversion for
> "free", along with detiling and cache flushing (most relevant for !llc).
> Using the GPU does impose a bandwidth cost that is presumably better
> used for rendering, hence we limit the use to readback into client
> memory (not pbo) where we would need to stall on the GPU anyway.
> (Uploads remain direct/staged to avoid the synchronisation cost.)
> And we only use the GPU path if a direct read into client memory from
> video memory is unavailable.
> 
> The ultimate user of this is Xorg/glamor! On byt, bsw, bxt (and
> presumably but not measured ilk), x11perf -shmget500 is improved by
> 15-fold. Though conversely the overhead of executing and waiting upon an
> additional blorp batch is shown by x11perf -shmget10 being reduced by a
> factor of 2. I think it is fair to presume that large copies will
> dominate (and that the overhead of a single batch is something that we
> can iteratively reduce, for the benefit of all.) llc machines continue to
> use direct access where there is no format changes (which one hopes is
> the typical use case).

Ah, this needs some improvements to the direct read path
(intel_gettexsubimage_tiled_memcpy) to handle subimages for llc + Xorg.
I have those in the older patches to enable userptr readback, I'll dig
those out again.
-Chris