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From: "David S. Miller" <davem@redhat.com>
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Date: Wed, 11 Jul 2001 16:17:02 -0700 (PDT)
To: Chris Wedgwood <cw@f00f.org>
Cc: Jes Sorensen <jes@sunsite.dk>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Jeff Garzik <jgarzik@mandrakesoft.com>, Ben LaHaise <bcrl@redhat.com>,
        "\"MEHTA,HIREN (A-SanJose,ex1)\"" <hiren_mehta@agilent.com>,
        "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>
Subject: Re: (reposting) how to get DMA'able memory within 4GB on 64-bit m
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Chris Wedgwood writes:
 > What kind of packing makes a 32-bit value take 8-bytes on any
 > currently supported archicture? The worst-case I can think of is
 > 7-bytes in the case of misaligned by 3 (e.g. __attribute__((packed))
 > struct blah { char foo[3]; long bar }; sort of thing).

If you have this:

struct {
	u32	foo;
	void	*bar;
};

"bar" will be at offset 8 on a 64-bit platforms since it must be
aligned on a 64-byte boundary, so what Jes is saying is that for:

struct {
	dma_addr_t	foo;
	void		*bar;
};

the "dma_addr_t" is already consuming 8 bytes of space on 64-bit
systems.

Later,
David S. Miller
davem@redhat.com

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