Re: [ofa-general] iommu dma mapping alignment requirements

Roland Dreier
2007-12-20T18:17:27Z

> 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.