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.

Drivers need to call dma_get_merge_boundary() to check for that kind of
behavior.