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 <[email protected]>
CC: Yakui Zhao <[email protected]>
CC: Valtteri Rantala <[email protected]>
CC: Eero Tamminen <[email protected]>
CC: Michael T Frederick <[email protected]>
CC: Ville Syrjälä <[email protected]>
CC: Chris Wilson <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Acked-by: Zhao Yakui <[email protected]>
Tested-by: Rong R Yang <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit 6bee14ed1e1136d700e5290c080e1145982ce43e)
Signed-off-by: Daniel Vetter <[email protected]>

0 files changed, 0 insertions, 0 deletions