To: Steve Wise <swise@opengridcomputing.com>
Cc: Benjamin Herrenschmidt <bherren@au1.ibm.com>, linux-kernel@vger.kernel.org,
       OpenFabrics General <general@lists.openfabrics.org>,
       Wen Xiong <wenxiong@us.ibm.com>
Subject: Re: [ofa-general] iommu dma mapping alignment requirements
References: <476AA2E2.5010007@opengridcomputing.com>
From: Roland Dreier <rdreier@cisco.com>
Date: Thu, 20 Dec 2007 10:07:39 -0800
In-Reply-To: <476AA2E2.5010007@opengridcomputing.com> (Steve Wise's message of "Thu, 20 Dec 2007 11:14:10 -0600")
Message-ID: <adalk7pz1ck.fsf@cisco.com>
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 > 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.
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