On Fri, Jun 05, 2015 at 05:19:55PM -0400, Dan Williams wrote:

As long as a dma_map_sg() implementation avoids sg_page() conversions it
can support scatterlists that carry "page-less" __pfn_t entries.
However, a couple implementations require that __pfn_t_has_page() is
always true. The Xen swiotlb implementation's entanglements with ARM and
the Calgary MMUs requirement to have a pre-existing virtual mapping make
them unable to support this conversion (i.e. these now have 'depends on
!HAVE_DMA_PFN').

That's why we really need a whole kernel conversion and not just a piecemail
one.  Given how trivial this patch is that doesn't look like a too big
task ayway.