18 Apr
2012
18 Apr
'12
2:20 p.m.
On Wed, Apr 18, 2012 at 9:06 AM, Arnd Bergmann arnd@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
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