[PATCH] drm/i915: make compact dma scatter lists creation work with SWIOTLB backend.

[Daniel Vetter]

On Mon, Jun 24, 2013 at 11:47:48AM -0400, Konrad Rzeszutek Wilk wrote:
> Git commit 90797e6d1ec0dfde6ba62a48b9ee3803887d6ed4
> ("drm/i915: create compact dma scatter lists for gem objects") makes
> certain assumptions about the under laying DMA API that are not always
> correct.
> 
> On a ThinkPad X230 with an Intel HD 4000 with Xen during the bootup
> I see:
> 
> [drm:intel_pipe_set_base] *ERROR* pin & fence failed
> [drm:intel_crtc_set_config] *ERROR* failed to set mode on [CRTC:3], err = -28
> 
> Bit of debugging traced it down to dma_map_sg failing (in
> i915_gem_gtt_prepare_object) as some of the SG entries were huge (3MB).
> 
> That unfortunately are sizes that the SWIOTLB is incapable of handling -
> the maximum it can handle is a an entry of 512KB of virtual contiguous
> memory for its bounce buffer. (See IO_TLB_SEGSIZE).
> 
> Previous to the above mention git commit the SG entries were of 4KB, and
> the code introduced by above git commit squashed the CPU contiguous PFNs
> in one big virtual address provided to DMA API.
> 
> This patch is a simple semi-revert - were we emulate the old behavior
> if we detect that SWIOTLB is online. If it is not online then we continue
> on with the new compact scatter gather mechanism.
> 
> An alternative solution would be for the the '.get_pages' and the
> i915_gem_gtt_prepare_object to retry with smaller max gap of the
> amount of PFNs that can be combined together - but with this issue
> discovered during rc7 that might be too risky.
> 
> Reported-and-Tested-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
> CC: Chris Wilson <chris at chris-wilson.co.uk>
> CC: Imre Deak <imre.deak at intel.com>
> CC: Daniel Vetter <daniel.vetter at ffwll.ch>
> CC: David Airlie <airlied at linux.ie>
> CC: <dri-devel at lists.freedesktop.org>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>

Queued for -next (with cc: stable), thanks for the patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch