[Intel-gfx] [PATCH v2 2/2] drm/i915/bxt: Fix inadvertent CPU snooping due to incorrect MOCS config

[Imre Deak]

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 PTE
based cacheability 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.

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.

CC: Valtteri Rantala <valtteri.rantala at intel.com>
CC: Eero Tamminen <eero.t.tamminen at intel.com>
CC: Michael T Frederick <michael.t.frederick at intel.com>
CC: Ville Syrjälä <ville.syrjala at linux.intel.com>
CC: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/i915/intel_mocs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_mocs.c b/drivers/gpu/drm/i915/intel_mocs.c
index f391ad6..d38f65e 100644
--- a/drivers/gpu/drm/i915/intel_mocs.c
+++ b/drivers/gpu/drm/i915/intel_mocs.c
@@ -149,8 +149,8 @@ static const struct drm_i915_mocs_entry broxton_mocs_table[] = {
 	  .l3cc_value =    L3_ESC(0) | L3_SCC(0) | L3_CACHEABILITY(L3_WB),
 	},
 	{
-	  /* 0x0000003b */
-	  .control_value = LE_CACHEABILITY(LE_WB) |
+	  /* 0x00000038 */
+	  .control_value = LE_CACHEABILITY(LE_PAGETABLE) |
 			   LE_TGT_CACHE(LE_TC_LLC_ELLC) |
 			   LE_LRUM(3) | LE_AOM(0) | LE_RSC(0) | LE_SCC(0) |
 			   LE_PFM(0) | LE_SCF(0),
-- 
2.5.0