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

[Imre Deak <imre.deak@intel.com>]

On ma, 2016-04-25 at 13:37 +0100, Chris Wilson wrote:
> 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@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>
> > Signed-off-by: Imre Deak <imre.deak@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].

I don't think there is or can be any snooping entry. On CHV for example
we can only setup snooping via the PTE, so there we necessarily have to
use entry 1 + PTE setup (set caching IOCTL). This is also what both
Windows and Mesa assumes apparently.

--Imre
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