Date: Tue, 3 Jun 2008 07:47:26 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
cc: Russell King <rmk+lkml@arm.linux.org.uk>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       David Miller <davem@davemloft.net>, linux-arch@vger.kernel.org,
       scottwood@freescale.com, linuxppc-dev@ozlabs.org,
       alan@lxorguk.ukuu.org.uk, linux-kernel@vger.kernel.org,
       tpiepho@freescale.com
Subject: Re: MMIO and gcc re-ordering issue
In-Reply-To: <200806031416.18195.nickpiggin@yahoo.com.au>
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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.

Well, you have to ask for WC/WC+ anyway, so it's immaterial. A driver that 
does that needs to be aware of it. IOW, it's a non-issue, imnsho.

			Linus
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