[Intel-gfx] [PATCH] drm/i915: Apply VT-d scanout adjustment to the VMA

[Chris Wilson]

Quoting Ville Syrjälä (2021-02-03 09:00:54)
> On Wed, Feb 03, 2021 at 08:38:41AM +0000, Chris Wilson wrote:
> > Currently, we allocate exactly the VMA requested for the framebuffer and
> > rely on filling the whole of the GGTT with scratch pages to catch when
> > VT-d prefetches beyond the bounds of the surface. However, this means
> > that we have to scrub the GGTT on startup and resume, and on recent HW
> > this is made even slower as the only access to GSM is uncached.
> > 
> > Since we do fill the pad-to-size PTE with scratch pages, and this is
> > also reapplied on resume, we can forgo the overzealous clearing of the
> > entire GGTT and instead pad the VMA to avoid the VT-d prefetches going
> > outside of the VMA.
> 
> We have a lot of these "allocate a huge pile of extra PTEs
> after (or before for rotation) the scanout surface" workarounds
> we're not handling at all. Not sure ths is sufficient to cover
> those.

I am being optimistic in that we should have a lot of weird cases
covered in CI and with iommu enabled, we will get splats.

But if it's pages/after before regardless of alignment, then this is
insufficient.

Also, the case of remapped pages we don't want obj->size here, but
vma->size.
-Chris