drm/i915: swizzling support for snb/ivb
We have to do this manually. Somebody had a Great Idea. I've measured speed-ups just a few percent above the noise level (below 5% for the best case), but no slowdows. Chris Wilson measured quite a bit more (10-20% above the usual snb variance) on a more recent and better tuned version of sna, but also recorded a few slow-downs on benchmarks know for uglier amounts of snb-induced variance. v2: Incorporate Ben Widawsky's preliminary review comments and elaborate a bit about the performance impact in the changelog. v3: Add a comment as to why we don't need to check the 3rd memory channel. v4: Fixup whitespace. Acked-by: NChris Wilson <chris@chris-wilson.co.uk> Reviewed-by: NBen Widawsky <ben@bwidawsk.net> Reviewed-by: NEric Anholt <eric@anholt.net> Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
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