On 2021-07-16 07:24, Christoph Hellwig wrote:

On Wed, Jul 14, 2021 at 07:19:50PM +0100, Robin Murphy wrote:

quoted

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/ (local)

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.