Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE

[Christoph Hellwig]

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?