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

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Message-ID: <476ADD05.5090801@opengridcomputing.com>
Date:	Thu, 20 Dec 2007 15:22:13 -0600
From:	Steve Wise <swise@...ngridcomputing.com>
To:	benh@....ibm.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

Benjamin Herrenschmidt wrote:
> 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 ?

That might work.

If you gimme a patch, i'll try it out!

Steve.
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