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

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Message-ID: <20090420105304.GC6670@elte.hu>
Date:	Mon, 20 Apr 2009 12:53:04 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Roland Dreier <rdreier@...co.com>,
	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


* H. Peter Anvin <hpa@...or.com> wrote:

> 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.

Yeah - with the default being the natural low-high order.

The other argument is that if a driver really wants some rare, oddly 
different order it should better define its own method that is not 
named in the same (or in a similar) way as an existing generic API. 
Otherwise, confusion will ensue.

	Ingo
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