On Tuesday 03 February 2015 09:04:03 Rob Clark wrote: > On Tue, Feb 3, 2015 at 2:48 AM, Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote: > >> On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <daniel@xxxxxxxx> wrote: > >> >> My initial thought is for dma-buf to not try to prevent something than > >> >> an exporter can actually do.. I think the scenario you describe could > >> >> be handled by two sg-lists, if the exporter was clever enough. > >> > > >> > That's already needed, each attachment has it's own sg-list. After all > >> > there's no array of dma_addr_t in the sg tables, so you can't use one sg > >> > for more than one mapping. And due to different iommu different devices > >> > can easily end up with different addresses. > >> > >> > >> Well, to be fair it may not be explicitly stated, but currently one > >> should assume the dma_addr_t's in the dmabuf sglist are bogus. With > >> gpu's that implement per-process/context page tables, I'm not really > >> sure that there is a sane way to actually do anything else.. > > > > Hm, what does per-process/context page tables have to do here? At least on > > i915 we have a two levels of page tables: > > - first level for vm/device isolation, used through dma api > > - 2nd level for per-gpu-context isolation and context switching, handled > > internally. > > > > Since atm the dma api doesn't have any context of contexts or different > > pagetables, I don't see who you could use that at all. > > Since I'm stuck w/ an iommu, instead of built in mmu, my plan was to > drop use of dma-mapping entirely (incl the current call to dma_map_sg, > which I just need until we can use drm_cflush on arm), and > attach/detach iommu domains directly to implement context switches. > At that point, dma_addr_t really has no sensible meaning for me. I think what you see here is a quite common hardware setup and we really lack the right abstraction for it at the moment. Everybody seems to work around it with a mix of the dma-mapping API and the iommu API. These are doing different things, and even though the dma-mapping API can be implemented on top of the iommu API, they are not really compatible. The drm_clflush helpers don't seem like the right solution to me, because all other devices outside of drm will face the same issue, and I suspect we should fill the missing gaps in the API in a more generic way. Arnd -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>