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

[Steve Wise <swise@opengridcomputing.com>]

Steve Wise wrote:
> 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?

Or based on the alignment of vaddr actually...

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