On Wed, Apr 18, 2012 at 9:06 AM, Arnd Bergmann <arnd at arndb.de> wrote: > On Wednesday 18 April 2012, Daniel Vetter wrote: >> + ? Because existing importing subsystems might presume coherent mappings for >> + ? userspace, the exporter needs to set up a coherent mapping. If that's not >> + ? possible, it needs to fake coherency by manually shooting down ptes when >> + ? leaving the cpu domain and flushing caches at fault time. Note that all >> the >> + ? dma_buf files share the same anon inode, hence the exporter needs to >> replace >> + ? the dma_buf file stored in vma->vm_file with it's own if pte shootdown is >> + ? requred. This is because the kernel uses the underlying inode's >> address_space >> + ? for vma tracking (and hence pte tracking at shootdown time with >> + ? unmap_mapping_range). >> + >> + ? If the above shootdown dance turns out to be too expensive in certain >> + ? scenarios, we can extend dma-buf with a more explicit cache tracking >> scheme >> + ? for userspace mappings. But the current assumption is that using mmap is >> + ? always a slower path, so some inefficiencies should be acceptable. >> + >> + ? Exporters that shoot down mappings (for any reasons) shall not do any >> + ? synchronization at fault time with outstanding device operations. >> + ? Synchronization is an orthogonal issue to sharing the backing storage of >> a >> + ? buffer and hence should not be handled by dma-buf itself. This is >> explictly >> + ? mentioned here because many people seem to want something like this, but >> if >> + ? different exporters handle this differently, buffer sharing can fail in >> + ? interesting ways depending upong the exporter (if userspace starts >> depending >> + ? upon this implicit synchronization). > > How do you ensure that no device can do DMA on the buffer while it's mapped > into user space in a noncoherent manner?
you do unmap_mapping_range() before DMA.. if you have userspace accessing buffer simultaneously with DMA then the results are undefined, as they always have been (even w/ uncached mappings) BR, -R > > ? ? ? ?Arnd > -- > To unsubscribe from this list: send the line "unsubscribe linux-media" in > the body of a message to majordomo at vger.kernel.org > More majordomo info at ?http://vger.kernel.org/majordomo-info.html
