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? Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
