Hi Jonathan, > -----Original Message----- > From: Cavitt, Jonathan <jonathan.cavitt@xxxxxxxxx> > Sent: Friday, November 3, 2023 1:15 PM > To: Sripada, Radhakrishna <radhakrishna.sripada@xxxxxxxxx>; intel- > gfx@xxxxxxxxxxxxxxxxxxxxx > Cc: Gupta, saurabhg <saurabhg.gupta@xxxxxxxxx>; > chris.p.wilson@xxxxxxxxxxxxxxx > Subject: RE: [PATCH] drm/i915/gt: Temporarily disable CPU caching > into DMA for MTL > > -----Original Message----- > From: Sripada, Radhakrishna <radhakrishna.sripada@xxxxxxxxx> > Sent: Friday, November 3, 2023 1:02 PM > To: Cavitt, Jonathan <jonathan.cavitt@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx > Cc: Gupta, saurabhg <saurabhg.gupta@xxxxxxxxx>; Cavitt, Jonathan > <jonathan.cavitt@xxxxxxxxx>; chris.p.wilson@xxxxxxxxxxxxxxx > Subject: RE: [PATCH] drm/i915/gt: Temporarily disable CPU caching > into DMA for MTL > > > > Hi Jonathan, > > > > > -----Original Message----- > > > From: Intel-gfx <intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of > Jonathan > > > Cavitt > > > Sent: Thursday, November 2, 2023 10:59 AM > > > To: intel-gfx@xxxxxxxxxxxxxxxxxxxxx > > > Cc: Gupta, saurabhg <saurabhg.gupta@xxxxxxxxx>; Cavitt, Jonathan > > > <jonathan.cavitt@xxxxxxxxx>; chris.p.wilson@xxxxxxxxxxxxxxx > > > Subject: [PATCH] drm/i915/gt: Temporarily disable CPU caching into > > > DMA for MTL > > > > > > FIXME: It is suspected that some Address Translation Service (ATS) > > > issue on IOMMU is causing CAT errors to occur on some MTL workloads. > > > Applying a write barrier to the ppgtt set entry functions appeared > > > to have no effect, so we must temporarily use I915_MAP_WC in the > > > map_pt_dma class of functions on MTL until a proper ATS solution is > > > found. > > > > > What is the performance impact here? Are we disabling caching only > > for the pte changes/scratch pages or does it extend beyond? > > > I don't actually know what map_pt_dma is used for, but if the name is > indicative of its purpose, it should only impact mappings into the dma > page table. > As for the performance impact, I don't imagine it'll be much. Maybe > a single-digit percentage slowdown? It might actually improve > performance if we're avoiding enough cache misses, but the true > performance impact would have to be measured empirically. > -Jonathan Cavitt > Even I am assuming the performance impact to be low as only pte uncached would be uncached and hence Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@xxxxxxxxx> > > > > > Regards, > > Radhakrishna(RK) Sripada > > > Signed-off-by: Jonathan Cavitt <jonathan.cavitt@xxxxxxxxx> > > > CC: Chris Wilson <chris.p.wilson@xxxxxxxxxxxxxxx> > > > --- > > > drivers/gpu/drm/i915/gt/intel_gtt.c | 20 ++++++++++++++++++++ > > > 1 file changed, 20 insertions(+) > > > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.c > > > b/drivers/gpu/drm/i915/gt/intel_gtt.c > > > index 4fbed27ef0ecc..21719563a602a 100644 > > > --- a/drivers/gpu/drm/i915/gt/intel_gtt.c > > > +++ b/drivers/gpu/drm/i915/gt/intel_gtt.c > > > @@ -95,6 +95,16 @@ int map_pt_dma(struct i915_address_space *vm, > struct > > > drm_i915_gem_object *obj) > > > void *vaddr; > > > > > > type = intel_gt_coherent_map_type(vm->gt, obj, true); > > > + /* > > > + * FIXME: It is suspected that some Address Translation Service (ATS) > > > + * issue on IOMMU is causing CAT errors to occur on some MTL > > > workloads. > > > + * Applying a write barrier to the ppgtt set entry functions appeared > > > + * to have no effect, so we must temporarily use I915_MAP_WC here on > > > + * MTL until a proper ATS solution is found. > > > + */ > > > + if (IS_METEORLAKE(vm->i915)) > > > + type = I915_MAP_WC; > > > + > > > vaddr = i915_gem_object_pin_map_unlocked(obj, type); > > > if (IS_ERR(vaddr)) > > > return PTR_ERR(vaddr); > > > @@ -109,6 +119,16 @@ int map_pt_dma_locked(struct i915_address_space > > > *vm, struct drm_i915_gem_object > > > void *vaddr; > > > > > > type = intel_gt_coherent_map_type(vm->gt, obj, true); > > > + /* > > > + * FIXME: It is suspected that some Address Translation Service (ATS) > > > + * issue on IOMMU is causing CAT errors to occur on some MTL > > > workloads. > > > + * Applying a write barrier to the ppgtt set entry functions appeared > > > + * to have no effect, so we must temporarily use I915_MAP_WC here on > > > + * MTL until a proper ATS solution is found. > > > + */ > > > + if (IS_METEORLAKE(vm->i915)) > > > + type = I915_MAP_WC; > > > + > > > vaddr = i915_gem_object_pin_map(obj, type); > > > if (IS_ERR(vaddr)) > > > return PTR_ERR(vaddr); > > > -- > > > 2.25.1 > > > >
