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Date: Fri, 6 Jul 2001 23:58:10 -0400 (EDT)
From: Ben LaHaise <bcrl@redhat.com>
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To: "David S. Miller" <davem@redhat.com>
cc: Jes Sorensen <jes@sunsite.dk>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
        "MEHTA,HIREN (A-SanJose,ex1)" <hiren_mehta@agilent.com>,
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Subject: Re: (reposting) how to get DMA'able memory within 4GB on 64-bit m
 achi ne
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On Fri, 6 Jul 2001, David S. Miller wrote:

> What about for drivers of SAC-only devices, they eat the overhead
> when highmem is enabled too?

Yes.  It's not an unreasonable overhead considering that it's configured
out for all the non-highmem kernels that will be shipped.  Keep in mind
that the expected lifespan for 32 bit systems is now less than 3 years, so
elaborate planning that delays implementation buys us nothing more than a
smaller window of usefulness.


> This says nothing about the real reason the IA64 solution is
> unacceptable, the inputs to the mapping functions which must
> be "page+offset+len" triplets as there is no logical "virtual
> address" to pass into the mapping routines on 32-bit systems.

On x86 a 64 bit DMA address cookie is fine.  If you've got concerns, tell
us what you have in mind for a design.


> Face it, the ia64 stuff is not what we can use across the board.  It
> simply doesn't deal with all the necessary issues.  Therefore,
> encouraging driver author's to use this ia64 hacked up scheme is
> not such a hot idea until we have a real API implemented.

So what's the API?

		-ben

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