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

Steve Wise
2007-12-20T19:11:57Z

Roland Dreier wrote:
> > 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.

In arch/powerpc/kernel/iommu.c:iommu_map_sg() I see that it calls
iommu_range_alloc() with a alignment_order of 0:

> vaddr = (unsigned long)page_address(s->page) + s->offset;
> npages = iommu_num_pages(vaddr, slen);
> entry = iommu_range_alloc(tbl, npages, &handle, mask >> IOMMU_PAGE_SHIFT, 0);

But perhaps the alignment order needs to be based on the host page size?

Steve.