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.
