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From: Roland Dreier <rdreier@cisco.com>
To: Jeff Garzik <jeff@garzik.org>
Cc: Ingo Molnar <mingo@elte.hu>, David Miller <davem@davemloft.net>,
       Linus Torvalds <torvalds@linux-foundation.org>, hpa@zytor.com,
       tglx@linutronix.de, h.mitake@gmail.com, rpjday@crashcourse.ca,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: Remove readq()/writeq() on 32-bit
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	<20090429115654.GC11586@elte.hu> <49F843BC.7020902@garzik.org>
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Date: Wed, 29 Apr 2009 10:25:55 -0700
In-Reply-To: <49F843BC.7020902@garzik.org> (Jeff Garzik's message of "Wed, 29
	Apr 2009 08:10:36 -0400")
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 > This removal patch is completely pointless, because it moves us
 > backwards to the point where we had a bunch of drivers defining it.

No, the current kernel still requires drivers to define it anyway,
because there are tons of 32-bit architectures that are not x86.

And more than that, centralizing the definition makes the API much more
dangerous for driver authors.

 > At least the networking drivers I messed with (until 11/2008) were
 > always fine with a non-atomic readq.

The commit to niu I keep citing (e23a59e1, "niu: Fix readq
implementation when architecture does not provide one.") shows that
drivers need to take care.  Now, the x86 implementation would happen to
work for that hardware, but eg drivers/infiniband/hw/amso1100 defines
readq with the opposite order -- whether that's required or just an
arbitrary choice, I don't know.  And drivers/infiniband/hw/mthca has
some uses of __raw_writeq() that only work if no other CPU accesses to
the same page can happen between the two halves, so it adds a per-page
spinlock for 32-bit architectures.  etc.

 - R.