Re: [PATCH] dma-buf: mmap support

[Arnd Bergmann <arnd@arndb.de>]

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