Messages in this thread | | | Subject | Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API | From | Robin Murphy <> | Date | Wed, 9 Dec 2020 13:05:11 +0000 |
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On 2020-12-09 11:12, Christoph Hellwig wrote: > On Tue, Dec 08, 2020 at 01:54:00PM +0900, Tomasz Figa wrote: >> >From the media perspective, it would be good to have the vmap >> optional, similarly to the DMA_ATTR_NO_KERNEL_MAPPING attribute for >> coherent allocations. Actually, in the media drivers, the need to have >> a kernel mapping of the DMA buffers corresponds to a minority of the >> drivers. Most of them only need to map them to the userspace. >> >> Nevertheless, that minority actually happens to be quite widely used, >> e.g. the uvcvideo driver, so we can't go to the other extreme and just >> drop the vmap at all. > > My main problem is that the DMA_ATTR_NO_KERNEL_MAPPING makes a mess > of an API. I'd much rather have low-level API that returns the > discontiguous allocations and another one that vmaps them rather > than starting to overload arguments like in dma_alloc_attrs with > DMA_ATTR_NO_KERNEL_MAPPING.
Agreed - if iommu-dma's dma_alloc_coherent() ends up as little more than a thin wrapper around those two functions I think that would be a good sign. It also seems like it might be a good idea for this API to use scatterlists rather than page arrays as it's fundamental format, to help reduce impedance with dma-buf - if we can end up with a wider redesign that also gets rid of dma_get_sgtable(), all the better!
Robin.
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