Date: Thu, 17 Jun 2004 14:19:03 +0100
From: Christoph Hellwig <hch@infradead.org>
To: Matthew Wilcox <willy@debian.org>
Cc: "Salyzyn, Mark" <mark_salyzyn@adaptec.com>, Alan Cox <alan@redhat.com>,
       linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org
Subject: Re: PATCH: Further aacraid work
Message-ID: <20040617131903.GA31775@infradead.org>
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	Matthew Wilcox <willy@debian.org>,
	"Salyzyn, Mark" <mark_salyzyn@adaptec.com>,
	Alan Cox <alan@redhat.com>, linux-kernel@vger.kernel.org,
	linux-scsi@vger.kernel.org
References: <547AF3BD0F3F0B4CBDC379BAC7E4189FD23FF9@otce2k03.adaptec.com> <20040617130713.GX20511@parcelfarce.linux.theplanet.co.uk>
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On Thu, Jun 17, 2004 at 02:07:13PM +0100, Matthew Wilcox wrote:
> It's a hard problem.  Where's the right tradeoff?  I have a system that
> has a memory map that puts the first 3.75GB of memory at 0, then the
> next 256MB at 64GB, then continues from 4GB.  If there's only 4GB of
> RAM in that system, I'm sure you'd rather use 32-bit descriptors and
> anything in that 256MB gets bounce-buffered.

With the SGI iommu it doesn't matter how much memory you have anyway,
if you're using DAC you'll always get high bits set in your dma address.
I suspect many other iommus work the same.

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