i915 regression in kernel 4.10

[Chris Wilson]

On Mon, Dec 19, 2016 at 12:39:16PM +0100, Juergen Gross wrote:
> With recent 4.10 kernel the graphics isn't coming up under Xen. First
> failure message is:
> 
> [   46.656649] i915 0000:00:02.0: swiotlb buffer is full (sz: 1630208 bytes)

Do we get a silent failure? i915_gem_gtt_prepare_pages() is where we
call dma_map_sg() and pass the sg to swiotlb (in this case) for
remapping, and we do check for an error value of 0. After that error,
SWIOTLB_MAP_ERROR is propagated back and converted to 0 for
dma_map_sg(). That looks valid, and we should report ENOMEM back to the
caller.

> Later I see splats like:
> 
> [   49.393583] general protection fault: 0000 [#1] SMP

What was the faulting address? RAX is particularly non-pointer-like so I
wonder if we walked onto an uninitialised portion of the sgtable. We may
have tripped over a bug in our sg_page iterator.

The attached patch should prevent an early ENOMEM following the swiotlb
allocation failure. But I suspect that we will still be tripping up the
failure in the sg walker when binding to the GPU.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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