[Intel-gfx] [PATCH 1/2] drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping

[Chris Wilson]

On Tue, Dec 20, 2016 at 11:33:27AM +0000, Chris Wilson wrote:
> On Tue, Dec 20, 2016 at 11:13:43AM +0000, Tvrtko Ursulin wrote:
> > How much is the cost of freeing and re-acquiring pages in the fall
> > back case? It could be avoidable by using the table and adding
> > something like sgt = i915_sg_copy(sgt, table_max_segment). But it
> > depends on how likely is this path to be hit on swiotlb platforms. I
> > have no idea. Our datasets are much bigger than the swiotlb space -
> > if that is true on such platforms?
> 
> It's below my level of care (atm). Platforms hitting this are using
> swiotlb *bounce* buffers. They will not be able to support a full gfx
> workload and be going through a copy. We could avoid the additional
> work, the sg_table is large enough for a 1:1 copy if we do it before the
> trim, but more importantly we need a simple fix for 4.10.

Pushed this pair as I think this is the safe course of action. Creating
i915_sg_expand() is a job for a rainy day.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre