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

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Date:	Fri, 30 May 2008 07:40:23 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jes Sorensen <jes@....com>
Cc:	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 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().

I think in the above case, you really should make writel() ordered.
Anything else is asking for trouble, for the exact same reasons that I
made it fully ordered on powerpc at least vs. previous stores. I only
kept it relaxed vs. subsequent cacheable stores (ie, spin_unlock), for
which I use the trick mentioned before.

Yes, this has some cost (can be fairly significant on powerpc too) but
I think it's a very basic assumption from drivers that consecutive
writel's, especially issued by the same CPU, will get to the device
in order.

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

Cheers,
Ben.

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