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

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Date:	Thu, 29 May 2008 14:53:38 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	benh@...nel.crashing.org
Cc:	Jes Sorensen <jes@....com>, Roland Dreier <rdreier@...co.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	tpiepho@...escale.com, linuxppc-dev@...abs.org,
	scottwood@...escale.com, torvalds@...ux-foundation.org,
	David Miller <davem@...emloft.net>, alan@...rguk.ukuu.org.uk
Subject: Re: MMIO and gcc re-ordering issue

On Thursday, May 29, 2008 2:40 pm Benjamin Herrenschmidt wrote:
> On Thu, 2008-05-29 at 10:47 -0400, Jes Sorensen wrote:
> > The only way to guarantee ordering in the above setup, is to either
> > make writel() fully ordered or adding the mmiowb()'s inbetween the two
> > writel's. On Altix you have to go and read from the PCI brige to
> > ensure all writes to it have been flushed, which is also what mmiowb()
> > is doing. If writel() was to guarantee this ordering, it would make
> > every writel() call extremely expensive :-(
>
> Interesting. I've always been taught by ia64 people that mmiowb() was
> intended to be used solely between writel() and spin_unlock().

Well, that *was* true, afaik, but maybe these days multipath isn't just for 
fail-over.  If that's true, then yeah making every single writeX ordered 
would be the only way to go...

> If this is a performance problem, then provide relaxed variants and
> use them in selected drivers.

Sounds reasonable.  That way drivers "just work" and important drivers can be 
optimized.

Jesse
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