Hi Russell, and Marek, On 04/14/2017 03:46 AM, Russell King - ARM Linux wrote: > On Fri, Apr 14, 2017 at 09:56:07AM +0200, Marek Szyprowski wrote: >>>> This would be however quite large task, especially taking into account >>>> all current users of DMA-buf framework... >>> Yeah it will be a large task. >> >> Maybe once scatterlist are switched to pfns, changing dmabuf internal >> memory representation to pfn array might be much easier. > > Switching to a PFN array won't work either as we have no cross-arch > way to translate PFNs to a DMA address and vice versa. Yes, we have > them in ARM, but they are an _implementation detail_ of ARM's > DMA API support, they are not for use by drivers. > > So, the very first problem that needs solving is this: > > How do we go from a coherent DMA allocation for device X to a set > of DMA addresses for device Y. > > Essentially, we need a way of remapping the DMA buffer for use with > another device, and returning a DMA address suitable for that device. > This could well mean that we need to deal with setting up an IOMMU > mapping. My guess is that this needs to happen at the DMA coherent > API level - the DMA coherent API needs to be augmented with support > for this. I'll call this "DMA coherent remap". > > We then need to think about how to pass this through the dma-buf API. > dma_map_sg() is done by the exporter, who should know what kind of > memory is being exported. The exporter can avoid calling dma_map_sg() > if it knows in advance that it is exporting DMA coherent memory. > Instead, the exporter can simply create a scatterlist with the DMA > address and DMA length prepopulated with the results of the DMA > coherent remap operation above. As Russell pointed to armama-drm case, I looked at that closely. armada-drm is creating sg_table and populating it with DMA-address in its map_dma_buf ops and unmap_dma_buf ops handles the special case and doesn't call dma_unmap_sg(). In the case of drm, gem_prime_map_dma_buf interfaces and the common drm_gem_map_dma_buf() will need modification to not do dma_map_sg() and create scatterlist with the DMA address and DMA length instead. We have to get drm_gem_map_dma_buf() info. to have it not do dma_map_sg() and create scatterlist. Focusing on drm for now, looks like there are probably about 15 or so map_dma_buf interfaces will need to handle coherent memory case. > > What the scatterlist can't carry in this case is a set of valid > struct page pointers, and an importer must not walk the scatterlist > expecting to get at the virtual address parameters or struct page > pointers. Right - importers need handling to not walk the sg_list and handle it differently. Is there a good example drm you can point me to for this? aramda-drm seems to special case this in armada_gem_map_import() if I am not mistaken. > > On the mmap() side of things, remember that DMA coherent allocations > may require special mapping into userspace, and which can only be > mapped by the DMA coherent mmap support. kmap etc will also need to > be different. So it probably makes sense for DMA coherent dma-buf > exports to use a completely separate set of dma_buf_ops from the > streaming version. > I agree. It would make is easier and also limits the scope of changes. > I think this is the easiest approach to solving the problem without > needing massive driver changes all over the kernel. > Anyway this is a quick note to say that I am looking into this and haven't drooped it :) thanks, -- Shuah
