[ofa-general] iommu dma mapping alignment requirements

[Roland Dreier]

 > It appears that my problem boils down to a single host page of memory
 > that is mapped for dma, and the dma address returned by dma_map_sg()
 > is _not_ 64KB aligned.  Here is an example:

 > My first question is: Is there an assumption or requirement in linux
 > that dma_addressess should have the same alignment as the host address
 > they are mapped to?  IE the rdma core is mapping the entire 64KB page,
 > but the mapping doesn't begin on a 64KB page boundary.

I don't think this is explicitly documented anywhere, but it certainly
seems that we want the bus address to be page-aligned in this case.
For mthca/mlx4 at least, we tell the adapter what the host page size
is (so that it knows how to align doorbell pages etc) and I think this
sort of thing would confuse the HW.

 - R.