linux-kernel - Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE

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Message-ID: <49EBCDC0.1020001@zytor.com>
Date:	Sun, 19 Apr 2009 18:20:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"Robert P. J. Day" <rpjday@...shcourse.ca>,
	Hitoshi Mitake <h.mitake@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars

Roland Dreier wrote:
> 
> Notice that it reads from addr+4 *before* it reads from addr, rather
> than after as in your example (and in fact your example depends on
> undefined compiler semantics, since there is no sequence point between
> the two operands of the | operator).  Now, I don't know that hardware,
> so I don't know if it makes a difference, but the niu example I gave in
> my original email shows that given hardware with clear-on-read
> registers, the order does very much matter.
> 

At least for x86, the order should be low-high, because that is the
order that those two transactions would be seen on a 32-bit bus
downstream from the CPU if the CPU issued a 64-bit transaction.

The only sane way to handle this as something other than per-driver
hacks would be something like:

#include <linux/io64.h>		/* Any 64-bit I/O OK */

#include <linux/io64lh.h>	/* Low-high splitting OK */

#include <linux/io64hl.h>	/* High-low splitting OK */

#include <linux/io64atomic.h>	/* 64-bit I/O must be atomic */

... i.e. letting the driver choose what fallback method it will accept.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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