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

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Date:	Mon, 20 Apr 2009 23:47:10 +0900
From:	Hitoshi Mitake <h.mitake@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

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.
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