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authorImre Deak <imre.deak@intel.com>2016-07-01 16:40:05 +0300
committerDaniel Vetter <daniel.vetter@ffwll.ch>2016-07-25 08:14:48 +0200
commit1542f5d90a16375201f24c8a06e7b6637f054218 (patch)
parent6005085591a678f7193b98fe3c3d0464ac7dd5cf (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 <rong.r.yang@intel.com> CC: Yakui Zhao <yakui.zhao@intel.com> CC: Valtteri Rantala <valtteri.rantala@intel.com> CC: Eero Tamminen <eero.t.tamminen@intel.com> CC: Michael T Frederick <michael.t.frederick@intel.com> CC: Ville Syrjälä <ville.syrjala@linux.intel.com> CC: Chris Wilson <chris@chris-wilson.co.uk> Signed-off-by: Imre Deak <imre.deak@intel.com> Acked-by: Zhao Yakui <yakui.zhao@intel.com> Tested-by: Rong R Yang <rong.r.yang@intel.com> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/1467380406-11954-3-git-send-email-imre.deak@intel.com (cherry picked from commit 6bee14ed1e1136d700e5290c080e1145982ce43e) Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
1 files changed, 2 insertions, 2 deletions
diff --git a/drivers/gpu/drm/i915/intel_mocs.c b/drivers/gpu/drm/i915/intel_mocs.c
index d36e60981a92..927825f5b284 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) |
+ /* 0x00000039 */
+ .control_value = LE_CACHEABILITY(LE_UC) |
LE_LRUM(3) | LE_AOM(0) | LE_RSC(0) | LE_SCC(0) |
LE_PFM(0) | LE_SCF(0),

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