Chris Wilson noted that u_default_texture_subdata's transfer path
sometimes results in wasteful double copies. This patch is based
on an earlier path he wrote, but updated now that we have staging
blits for busy or compressed textures.
Consider the case of idle, non-CCS-compressed, tiled images:
The transfer-based CPU path has to return a "linear" mapping, so upon
map, it mallocs a temporary buffer. u_default_texture_subdata then
copies the client memory to this malloc'd buffer, and transfer unmap
performs a tiled_memcpy to copy it back into the texture. By writing
a direct texture_subdata() implementation, we're able to directly do
a tiled_memcpy from the client memory into the destination texture,
resulting in only one copy.
For linear buffers, there is no advantage to doing things directly, so
we simply fall back to u_default_texture_subdata()'s transfer path to
avoid replicating those cases.
We still may want to use GPU staging buffers for busy destinations
(to avoid stalls) or CCS-compressed images (to compress the data),
at which point we also fall back to the existing path. We thought
to try and use a tiled temporary, but this didn't appear to help.
Improves performance in x11perf -shmput500 by 1.96x on my Icelake.
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2500
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3818>
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