[PATCH v2] arm64/dma-mapping: fix DMA_ATTR_FORCE_CONTIGUOUS mmaping code

[Russell King - ARM Linux]

On Fri, Mar 31, 2017 at 01:02:51PM +0200, Andrzej Hajda wrote:
> In this version of the patch I have replaced temporal pages and
> iommu_dma_mmap with remap_pfn_range or rather its simplified version
> vm_iomap_memory.
> Unfortunately I have not find nice helper for sgtable creation, so I
> left sg allocation inside _iommu_mmap_attrs.
> 
> As the fixing of DMA_ATTR_FORCE_CONTIGUOUS related crashes has higher
> priority I have focused only on it in this patch.

As I mentioned elsewhere, I don't believe that fudging around in this way
is a proper fix.

DMA coherent memory was never, ever, intended to be passed back into the
streaming APIs - it was designed that the two would be mutually exclusive.

The problem is that we now have DMA coherent allocations being passed
through the dma-buf API using this dma_get_sgtable() thing, which is quite
broken.  I regard dma_get_sgtable() as very broken, and had I realised at
the time that this was what it was trying to do, I would have NAK'd it.

Rather than bodging around this brokenness, trying to get dma_get_sgtable()
to work, I believe we need to address the root cause - which is proper
support for passing DMA coherent allocations through dma-buf, which does
not involve scatterlists and calling dma_map_sg() on it.

That's going to need to be addressed in any case, because of the
dma_declare_coherent_memory() issue, where we may not have struct pages
backing a coherent allocation.  Such a case can come up on ARM64 via
DT's "shared-dma-pool" reserved memory stuff.

Right now, I have a "fix" for ARM queued which causes dma_get_sgtable()
to fail when used with reserved memory, but we have one user who needs
this to work.  So, dma-buf needs to be fixed for this one way or another,
and I don't think that bending the current broken stuff to suit it by
trying these vmalloc_to_page() hacks is acceptable.

dma_get_sgtable() is fundamentally broken IMHO.

-- 
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