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

[Christoph Hellwig]

I'm a little worried about this.  While it looks functionally correct
we have surived without it, and doing another copy for every swiotlb
dma mapping from the device looks extremely painful for the typical use
cases where we expect the device to transfer the whole mapping.

I'd be tempted to instead properl document the current behavior and
introduce a new DMA_ATTR_PARTIAL flag to allow for partial mappings.