On Wed, Apr 18, 2012 at 9:06 AM, Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- 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
