On Thu, Aug 20, 2020 at 6:52 PM Christoph Hellwig hch@lst.de wrote:
On Thu, Aug 20, 2020 at 12:24:31PM +0200, Tomasz Figa wrote:
Of course this still uses the scatterlist structure with its annoying mix of input and output parametes, so I'd rather not expose it as an official API at the DMA layer.
The problem with the above open coded approach is that it requires explicit handling of the non-IOMMU and IOMMU cases and this is exactly what we don't want to have in vb2 and what was actually the job of the DMA API to hide. Is the plan to actually move the IOMMU handling out of the DMA API?
Do you think we could instead turn it into a dma_alloc_noncoherent() helper, which has similar semantics as dma_alloc_attrs() and handles the various corner cases (e.g. invalidate_kernel_vmap_range and flush_kernel_vmap_range) to achieve the desired functionality without delegating the "hell", as you called it, to the users?
Yes, I guess I could do something in that direction. At least for dma-iommu, which thanks to Robin should be all you'll need in the foreseeable future.
That would be really great. Let me know if we can help by testing with V4L2/vb2 or in any other way.
Best regards, Tomasz