On 22/05/2019 14:34, Christoph Hellwig wrote:
On Wed, May 22, 2019 at 02:25:38PM +0100, Robin Murphy wrote:
Sure, but that should be irrelevant since the effective problem here is in
the sync_*_for_cpu direction, and it's the unmap which nobbles the buffer.
If the driver does this:
dma_map_single(whole buffer);
<device writes to part of buffer>
dma_unmap_single(whole buffer);
<contents of rest of buffer now undefined>
then it could instead do this and be happy:
dma_map_single(whole buffer, SKIP_CPU_SYNC);
<device writes to part of buffer>
dma_sync_single_for_cpu(updated part of buffer);
dma_unmap_single(whole buffer, SKIP_CPU_SYNC);
<contents of rest of buffer still valid>
Assuming the driver knows how much was actually DMAed this would
solve the issue. Horia, does this work for you?
Ohhh, and now I've just twigged what you were suggesting - your
DMA_ATTR_PARTIAL flag would mean "treat this as a read-modify-write of
the buffer because we *don't* know exactly which parts the device may
write to". So indeed if we did go down that route we wouldn't need any
of the sync stuff I was worrying about (but I might suggest naming it
DMA_ATTR_UPDATE instead). Apologies for being slow :)
Robin.
