i965: Use BDW_MOCS_PTE for renderbuffers

Graphics / Mesa 3D Graphics Library / Mesa - Kenneth Graunke [whitecape.org] - 9 October 2014 08:03 UTC

Write-back caching cannot be used for buffers being scanned out by the display engine; surfaces used for scan-out must be write-through or uncached. I originally chose WT for render targets because it works in all cases. However, we really want to use write-back caching where possible, as it is more efficient.

Most renderbuffers are not used for scanout - off-screen FBOs certainly are fine, and non-pageflipped backbuffers should be fine as well. So in most cases WB will work. However, we don't know what will be used for scan-out, so we instead simply use the PTE value specified by the kernel, as it knows these things.

This matches our MOCS choice on Haswell.

Fixes performance regressions since commit ee4484be3dc827cf15bcf109f5 in a microbenchmark (spotted by Eero Tamminen). Improves performance in GLBenchmark 2.7/EgyptHD by 7.44362% +/- 0.496939% (n=55) on a Broadwell GT2. Improves performance in a bunch of other microbenchmarks by ~15% or so.

b7844d1 i965: Use BDW_MOCS_PTE for renderbuffers.
src/mesa/drivers/dri/i965/gen8_surface_state.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Upstream: cgit.freedesktop.org

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