Messages in this thread | | | Subject | Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver | From | Robin Murphy <> | Date | Fri, 16 Jul 2021 16:32:50 +0100 |
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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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