path: root/include/uapi/drm/i915_drm.h
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authorImre Deak <>2016-07-01 16:40:05 +0300
committerImre Deak <>2016-07-19 20:35:17 +0300
commit6bee14ed1e1136d700e5290c080e1145982ce43e (patch)
tree1c38e24e09e07473699c4f9bc6798c5ff0eaf880 /include/uapi/drm/i915_drm.h
parente419899b7c19ef99e340e2b1ba585d82fd28c53b (diff)
drm/i915/bxt: Fix inadvertent CPU snooping due to incorrect MOCS config
Setting a write-back cache policy in the MOCS entry definition also implies snooping, which has a considerable overhead. This is unexpected for a few reasons: - From user-space's point of view since it didn't want a coherent surface (it didn't set the buffer as such via the set caching IOCTL). - There is a separate MOCS entry field for snooping (which we never set). - This MOCS table is about caching in (e)LLC and there is no (e)LLC on BXT. There is a separate table for L3 cache control. Considering the above the current behavior of snooping looks like an unintentional side-effect of the WB setting. Changing it to be LLC-UC gets rid of the snooping without any ill-effects. For a coherent surface the application would use a separate MOCS entry at index 1 and call the set caching IOCTL to setup the PTE entries for the corresponding buffer to be snooped. In the future we could also add a new MOCS entry for coherent surfaces. This resulted in 70% improvement in synthetic texturing benchmarks. Kudos to Valtteri Rantala, Eero Tamminen and Michael T Frederick and Ville who helped to narrow the source of problem to the kernel and to the snooping behaviour in particular. With a follow-up change to adjust the 3rd entry value igt/gem_mocs_settings is passing after this change. v2: - Rebase on v2 of patch 1/2. v3: - Set the entry as LLC uncached instead of PTE-passthrough. This way we also keep snooping disabled, but we also make the cacheability/ coherency setting indepent of the PTE which is managed by the kernel. (Chris) CC: Rong R Yang <> CC: Yakui Zhao <> CC: Valtteri Rantala <> CC: Eero Tamminen <> CC: Michael T Frederick <> CC: Ville Syrjälä <> CC: Chris Wilson <> Signed-off-by: Imre Deak <> Acked-by: Zhao Yakui <> Tested-by: Rong R Yang <> Reviewed-by: Ville Syrjälä <> Link:
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