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

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Date:	Mon, 20 Apr 2009 18:03:32 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Hitoshi Mitake <h.mitake@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Roland Dreier <rdreier@...co.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Robert P. J. Day" <rpjday@...shcourse.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars


* Hitoshi Mitake <h.mitake@...il.com> wrote:

> On Mon, Apr 20, 2009 at 19:53, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * 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.
> I think this is a good way.
> readq/writeq are already in Linus's tree, removing these is not a good idea.
> 
> And I've sent the patch to fix a little problem of Kconfig about
> readq/writeq to you.
> http://marc.info/?l=linux-kernel&m=123521109218008&w=2
> Did you notice?
> 
> Adding cautions about accessing order or non-atomic to Kconfig's help
> part may be benefit.

It's better to add add such non-interactive help text as Makefile 
comments:

#
# This option ...
#

and they should be invisible in make menuconfig. This is a facility 
provided by architectures.

Note, the whole patchset is still incomplete - readq/writeq wrappers 
should be provided on all 32-bit architectures. Are those in the 
works?

	Ingo
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