drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping

drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping

If we at first do not succeed with attempting to remap our physical
pages using a coalesced scattergather list, try again with one
scattergather entry per page. This should help with swiotlb as it uses a
limited buffer size and only searches for contiguous chunks within its
buffer aligned up to the next boundary - i.e. we may prematurely cause a
failure as we are unable to utilize the unused space between large
chunks and trigger an error such as:

	 i915 0000:00:02.0: swiotlb buffer is full (sz: 1630208 bytes)

Reported-by: Juergen Gross <[email protected]>
Tested-by: Juergen Gross <[email protected]>
Fixes: 871dfbd67d4e ("drm/i915: Allow compaction upto SWIOTLB max segment size")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Imre Deak <[email protected]>
Cc: <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Daniel Vetter <[email protected]>