Hi Russel,
On 31.03.2017 13:16, Russell King - ARM Linux wrote: > On Fri, Mar 31, 2017 at 01:02:51PM +0200, Andrzej Hajda wrote: >> In this version of the patch I have replaced temporal pages and >> iommu_dma_mmap with remap_pfn_range or rather its simplified version >> vm_iomap_memory. >> Unfortunately I have not find nice helper for sgtable creation, so I >> left sg allocation inside _iommu_mmap_attrs. >> >> As the fixing of DMA_ATTR_FORCE_CONTIGUOUS related crashes has higher >> priority I have focused only on it in this patch. > As I mentioned elsewhere, I don't believe that fudging around in this way > is a proper fix. > > DMA coherent memory was never, ever, intended to be passed back into the > streaming APIs - it was designed that the two would be mutually exclusive. > > The problem is that we now have DMA coherent allocations being passed > through the dma-buf API using this dma_get_sgtable() thing, which is quite > broken. This patch just fixes current implementation of the API by re-using existing techniques, nothing more. It prevents code from using some pointer (area->pages) not because of lack backing struct pages, but because this pointer is just not initialized and apparently not necessary in case of contiguous allocations. I think, the problem of fixing DMA-API should be addressed in separate set of patches, as it is a different issue and requires much more experience in MM and DMA-API than I have. Regards Andrzej > I regard dma_get_sgtable() as very broken, and had I realised at > the time that this was what it was trying to do, I would have NAK'd it. > > Rather than bodging around this brokenness, trying to get dma_get_sgtable() > to work, I believe we need to address the root cause - which is proper > support for passing DMA coherent allocations through dma-buf, which does > not involve scatterlists and calling dma_map_sg() on it. > > That's going to need to be addressed in any case, because of the > dma_declare_coherent_memory() issue, where we may not have struct pages > backing a coherent allocation. Such a case can come up on ARM64 via > DT's "shared-dma-pool" reserved memory stuff. > > Right now, I have a "fix" for ARM queued which causes dma_get_sgtable() > to fail when used with reserved memory, but we have one user who needs > this to work. So, dma-buf needs to be fixed for this one way or another, > and I don't think that bending the current broken stuff to suit it by > trying these vmalloc_to_page() hacks is acceptable. > > dma_get_sgtable() is fundamentally broken IMHO. > _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
