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. The only way to conclusively say that it is coming from coherent area is at the time it is getting allocated in dma_alloc_from_coherent(). Since dma_alloc_attrs() will go on to find memory from other areas if dma_alloc_from_coherent() doesn't allocate memory. dma_get_sgtable_attrs() is what is used by the exporter to create the sg_table. One way to do this cleanly without needing to check buffer type flags would be to add a set of sg_table ops: get_sgtable, map_sg, and unmap_sg. Sounds like sg_table interfaces need to be in dma_buf_ops level. More below. > > 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. > > 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. How about adding get_sgtable, map_sg, unmap_sg to dma_buf_ops. The right ops need to be installed based on buffer type. As I mentioned before, we don't know which memory we got until dma_alloc_from_coherent() finds memory in dev->mem area. So how about using the dma_check_dev_coherent() to determine which ops we need. These could be set based on buffer type. vb2_dc_get_dmabuf() can do that. I think this will work. thanks, -- Shuah
