linux-kernel - Re: MMIO and gcc re-ordering issue

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Message-ID: <Pine.LNX.4.64.0806031253180.3242@t2.domain.actdsltmp>
Date:	Tue, 3 Jun 2008 12:57:56 -0700 (PDT)
From:	Trent Piepho <tpiepho@...escale.com>
To:	Matthew Wilcox <matthew@....cx>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	scottwood@...escale.com, linuxppc-dev@...abs.org,
	alan@...rguk.ukuu.org.uk, linux-kernel@...r.kernel.org
Subject: Re: MMIO and gcc re-ordering issue

On Tue, 3 Jun 2008, Matthew Wilcox wrote:
> On Tue, Jun 03, 2008 at 11:47:00AM -0700, Trent Piepho wrote:
>> On Tue, 3 Jun 2008, Linus Torvalds wrote:
>>> On Tue, 3 Jun 2008, Nick Piggin wrote:
>>>>
>>>> Linus: on x86, memory operations to wc and wc+ memory are not ordered
>>>> with one another, or operations to other memory types (ie. load/load
>>>> and store/store reordering is allowed). Also, as you know, store/load
>>>> reordering is explicitly allowed as well, which covers all memory
>>>> types. So perhaps it is not quite true to say readl/writel is strongly
>>>> ordered by default even on x86. You would have to put in some
>>>> mfence instructions in them to make it so.
>>
>> So on x86, these could be re-ordered?
>>
>> writel(START_OPERATION, CONTROL_REGISTER);
>> status = readl(STATUS_REGISTER);
>
> You wouldn't ask for write-combining memory mapping for control or
> status registers.

But Nick said, "store/load reordering is explicitly allowed as well, which
covers *all* memory types."
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