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SubjectRe: [PATCH v4 0/3] Apple M1 DART IOMMU driver
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On 2021-07-16 07:24, Christoph Hellwig wrote:
> On Wed, Jul 14, 2021 at 07:19:50PM +0100, Robin Murphy wrote:
>> Even at the DMA API level you could hide *some* of it (at the cost of
>> effectively only having 1/4 of the usable address space), but there are
>> still cases like where v4l2 has a hard requirement that a page-aligned
>> scatterlist can be mapped into a contiguous region of DMA addresses.
>
> Where does v4l2 make that broken assumption? Plenty of dma mapping
> implementations including dma-direct do not support that.

See vb2_dc_get_contiguous_size() and its callers. I still remember
spending an entire work day on writing one email at the culmination of
this discussion:

https://lore.kernel.org/linux-iommu/56409B6D.5090903@arm.com/

809eac54cdd6 was framed as an efficiency improvement because it
technically was one (and something I had wanted to implement anyway),
but it was also very much to save myself from any further email debates
or customer calls about "regressing" code ported from 32-bit platforms...

Robin.

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