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

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Date:	Thu, 20 Dec 2007 13:11:28 -0600
From:	Steve Wise <swise@...ngridcomputing.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	Benjamin Herrenschmidt <bherren@....ibm.com>,
	linux-kernel@...r.kernel.org,
	OpenFabrics General <general@...ts.openfabrics.org>,
	Wen Xiong <wenxiong@...ibm.com>
Subject: Re: [ofa-general] iommu dma mapping alignment requirements

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.
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