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

[Chris Wilson]

On Tue, Dec 20, 2016 at 02:14:21PM +0000, Tvrtko Ursulin wrote:
> 
> On 20/12/2016 13:56, Chris Wilson wrote:
> >On Tue, Dec 20, 2016 at 01:38:16PM +0000, Tvrtko Ursulin wrote:
> >>
> >>On 20/12/2016 12:36, Chris Wilson wrote:
> >>>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.
> >>
> >>It would have been very simple and much more elegant in my opinion.
> >
> >I'm ready to be impressed, in my head to do an inplace rewrite was tricky.
> >:)
> 
> Maybe I've missed something then. We'll see. :)
> 
> >>But I understand Tested-by tag was precious to keep. I'll send a
> >>patch shortly but it won't be very tested due to time constraints.
> >>
> >>Also I don't know why you changed page_count and i to unsigned long
> >>when the sg API can only handle unsigned int for that.
> >
> >Primary concern was moving them out of the way and worrying about our
> >own 64bit object size issues. Hmm, can we reuse
> >
> >if (overflows_type(pgcount, unsigned int))
> >	return -E2BIG;
> >
> >to catch the mismatch?
> 
> You have already added that to i915_gem_object_create some time ago! :)

I know, I'm thinking ahead of documenting the types around the place so
that we can start actually preparing for huge objects.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre