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Date: Fri, 30 May 2008 11:36:20 +0200
From: Jes Sorensen <jes@sgi.com>
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To: James Bottomley <James.Bottomley@HansenPartnership.com>
Subject: Re: MMIO and gcc re-ordering issue
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Cc: linux-arch@vger.kernel.org, Roland Dreier <rdreier@cisco.com>,
	linux-kernel@vger.kernel.org, Jeremy Higdon <jeremy@sgi.com>,
	David Miller <davem@davemloft.net>, linuxppc-dev@ozlabs.org,
	scottwood@freescale.com, torvalds@linux-foundation.org,
	tpiepho@freescale.com, alan@lxorguk.ukuu.org.uk,
	Arjan van de Ven <arjan@infradead.org>
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James Bottomley 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 :-(
> 
> So if a read from the bridge achieves the same effect, can't we just put
> one after the writes within the spinlock (an unrelaxed one).  That way
> this whole sequence will look like a well understood PCI posting flush
> rather than have to muck around with little understood (at least by most
> driver writers) io barriers?

Hmmm,

I think mmiowb() does some sort of status read from the bridge, I am not
sure if it's enough to just do a regular readl().

I'm adding Jeremy to the list, he should know for sure.

Cheers,
Jes