[PATCH] dma-buf: mmap support
rob.clark at linaro.org
Wed Apr 18 07:20:26 PDT 2012
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
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