[Intel-gfx] [PATCH v2] drm/i915: Allow compaction upto SWIOTLB max segment size

[Chris Wilson]

On Wed, Oct 12, 2016 at 05:19:14PM -0400, Konrad Rzeszutek Wilk wrote:
> On Mon, Oct 10, 2016 at 11:27:00PM +0100, Chris Wilson wrote:
> > commit 1625e7e549c5 ("drm/i915: make compact dma scatter lists creation
> > work with SWIOTLB backend") took a heavy handed approach to undo the
> > scatterlist compaction in the face of SWIOTLB. (The compaction hit a bug
> > whereby we tried to pass a segment larger than SWIOTLB could handle.) We
> > can be a little more intelligent and try compacting the scatterlist up
> > to the maximum SWIOTLB segment size (when using SWIOTLB).
> > 
> 
> Won't this cause a bigger usage of the SWIOTLB bounce buffer ?

It won't change the frequency of the usage of the bounce buffer, if that
is what you mean. Either you have intel-iommu and so will not go through
swiotlb, or you are forced to use swiotlb even though the hw doesn't
require it (swiotlb config is byzantium and always enabled unless you
hack it out and can rejoice at the lower cpu usage).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre