Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver

Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver

[Christoph Hellwig]

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.

Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver

[Robin Murphy]

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.