From: Imre Deak <imre.d...@intel.com> Cherry-picked from drm-intel tree
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.y...@intel.com> CC: Yakui Zhao <yakui.z...@intel.com> CC: Valtteri Rantala <valtteri.rant...@intel.com> CC: Eero Tamminen <eero.t.tammi...@intel.com> CC: Michael T Frederick <michael.t.freder...@intel.com> CC: Ville Syrjälä <ville.syrj...@linux.intel.com> CC: Chris Wilson <ch...@chris-wilson.co.uk> Signed-off-by: Imre Deak <imre.d...@intel.com> Acked-by: Zhao Yakui <yakui.z...@intel.com> Tested-by: Rong R Yang <rong.r.y...@intel.com> Reviewed-by: Ville Syrjälä <ville.syrj...@linux.intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/1467380406-11954-3-git-send-email-imre.d...@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..c047d68 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_TGT_CACHE(LE_TC_LLC_ELLC) | LE_LRUM(3) | LE_AOM(0) | LE_RSC(0) | LE_SCC(0) | LE_PFM(0) | LE_SCF(0), -- 2.7.4 --------------------------------------------------------------------- Intel Finland Oy Registered Address: PL 281, 00181 Helsinki Business Identity Code: 0357606 - 4 Domiciled in Helsinki This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. -- _______________________________________________ linux-yocto mailing list linux-yocto@yoctoproject.org https://lists.yoctoproject.org/listinfo/linux-yocto
