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

[Chris Wilson]

On Mon, Apr 25, 2016 at 03:23:21PM +0300, Imre Deak wrote:
> 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.
> 
> 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>
> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_mocs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_mocs.c b/drivers/gpu/drm/i915/intel_mocs.c
> index 5006a92..23c7dd1 100644
> --- a/drivers/gpu/drm/i915/intel_mocs.c
> +++ b/drivers/gpu/drm/i915/intel_mocs.c
> @@ -142,7 +142,7 @@ static const struct drm_i915_mocs_entry broxton_mocs_table[] = {
>  	  .l3cc_value =    L3_ESC(0) | L3_SCC(0) | L3_CACHEABILITY(L3_WB),
>  	},
>  	{
> -	  .control_value = LE_CACHEABILITY(LE_WB) |
> +	  .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),

This is index 2? This should *be* the snooping entry?

Index 0: uncached
Index 1: follow pte
Index 2: snoop

Aim I missing something? Why isn't this a userspce bug for requesting a
mocs setting it didn't wnat? In my kernel this makes mocs_table[1] ==
mocs_table[2].
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre