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

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Message-ID: <alpine.LFD.1.10.0806030746190.3473@woody.linux-foundation.org>
Date:	Tue, 3 Jun 2008 07:47:26 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	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,
	tpiepho@...escale.com
Subject: Re: MMIO and gcc re-ordering issue



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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