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Subject: Re: RFC: being more anal about iospace accesses..
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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Date: Thu, 16 Sep 2004 14:57:49 +1000
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To: Linus Torvalds <torvalds@osdl.org>
Cc: Matthew Wilcox <willy@debian.org>, James Bottomley <James.Bottomley@steeleye.com>, Geert Uytterhoeven <geert@linux-m68k.org>, Linux Arch list <linux-arch@vger.kernel.org>, Al Viro <viro@parcelfarce.linux.theplanet.co.uk>, Andrew Morton <akpm@osdl.org>, Alan Cox <alan@lxorguk.ukuu.org.uk>, "David S. Miller" <davem@redhat.com>, Jeff Garzik <jgarzik@pobox.com>
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On Thu, 2004-09-16 at 14:28, Linus Torvalds wrote:

> (Note that 64-bit accesses aren't necessarily atomic even on 64-bit
> architectures. BenH says that the IO architecture on a G5 means that when
> you do a 64-bit load, it will be done as two 32-bit bus cycles
> regardless).

Yup, at least with the current models, due to the fact that the U3
northbridge basically has a 32 bits path to IO devices. Note that this
isn't the case with IBM machines who do proper 64 bits PCI IOs.

Ben.