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

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Message-Id: <1198182061.6779.7.camel@pasglop>
Date:	Fri, 21 Dec 2007 07:21:01 +1100
From:	Benjamin Herrenschmidt <benh@....ibm.com>
To:	Steve Wise <swise@...ngridcomputing.com>
Cc:	Roland Dreier <rdreier@...co.com>,
	Benjamin Herrenschmidt <bherren@....ibm.com>,
	linux-kernel@...r.kernel.org,
	OpenFabrics General <general@...ts.openfabrics.org>,
	Wen Xiong <wenxiong@...ibm.com>,
	Olof Johansson <olof@...om.net>,
	Paul Mackerras <pmac@....ibm.com>
Subject: Re: [ofa-general] iommu dma mapping alignment requirements

On Thu, 2007-12-20 at 13:29 -0600, Steve Wise wrote:

> Or based on the alignment of vaddr actually...

The later wouldn't be realistic. What I think might be necessay, though
it would definitely cause us problems with running out of iommu space
(which is the reason we did the switch down to 4K), is to provide
alignment to the real page size, and alignement to the allocation order
for dma_map_consistent.

It might be possible to -tweak- and only provide alignment to the page
size for allocations that are larger than IOMMU_PAGE_SIZE. That would
solve the problem with small network packets eating up too much iommu
space though.

What do you think ?

Ben.

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