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

[Christoph Hellwig]

On Wed, May 22, 2019 at 01:50:47PM +0100, Robin Murphy wrote:
> Would that work out any different from the existing DMA_ATTR_SKIP_CPU_SYNC? 
>
> If drivers are prepared to handle this issue from their end, they can 
> already do so for single mappings by using that attr along with explicit 
> partial syncs via dma_sync_single(). For page/sg mappings we'd still have 
> the problem of identifying what part of "partial" actually matters, and 
> probably having to add some additional new sync operations to cope.

Except that the same optimization we are tripping over here is also
present in dma_sync_* - dma_sync_*_to_device with DMA_FROM_DEVICE is a
no-op in swiotlb.