linux-kernel - Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

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Date:	Wed, 29 Apr 2009 10:25:55 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	tglx@...utronix.de, h.mitake@...il.com, rpjday@...shcourse.ca,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

 > 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.
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