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

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Date:	Mon, 26 May 2008 18:40:47 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	benh@...nel.crashing.org
Cc:	linux-arch@...r.kernel.org, scottwood@...escale.com,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	tpiepho@...escale.com
Subject: Re: MMIO and gcc re-ordering issue

From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Date: Tue, 27 May 2008 11:33:46 +1000

> Quick summary: gcc is happily re-ordering readl/writel vs. surrounding
> memory accesses (and thus accesses to DMA coherent memory) which is
> obviously a _BAD_THING_.
> 
> This is on all archs. Quick fix is to stick a "memory" clobber in all arch
> implementations of readl/writel/... (ie, making them a barrier()).
> 
> However, I'm using that as an excuse to bring back my pet subject, which
> is basically, should we instead just finally mandate the use of explicit
> rmb/wmb/mb's (which boils down to barrier() on x86) to drivers who want
> to order memory consistent accesses vs. MMIO ?

This is basically what drivers are effectively doing.
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