On Thu, Jan 20, 2022 at 02:56:02PM +0100, Christoph Hellwig wrote:

 - on the input side to dma mapping the bio_vecs (or phyrs) are chained
   as bios or whatever the containing structure is.  These already exist
   and have infrastructure at least in the block layer
 - on the output side I plan for two options:

	1) we have a sane IOMMU and everyting will be coalesced into a
	   single dma_range.  This requires setting the block layer
	   merge boundary to match the IOMMU page size, but that is
	   a very good thing to do anyway.

It doesn't look like IOMMU page sizes are exported, or even necessarily
consistently sized on at least one arch (power).