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

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Message-ID: <483FCD5C.208@sgi.com>
Date:	Fri, 30 May 2008 11:48:12 +0200
From:	Jes Sorensen <jes@....com>
To:	benh@...nel.crashing.org
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,
	Jeremy Higdon <jeremy@....com>
Subject: Re: MMIO and gcc re-ordering issue

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().
> 
> 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.

Hmmm I hope I didn't mess up the description of this and added to the
confusion.

The net result of that would be to kill performance completely, I really
don't like that idea.... Having each writel() go out and poll the PCI
bridge is going to make every driver used on Altix slow as a dog.

In addition it's still not going to solve the problem for userland
mapped stuff such as infinibug.

> 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.

In this case the cost is more than just significant, it's pretty
crippling.

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

We'd have to make major changes to drivers like e1000, tg3, mptsas, the
qla2/3/4xxx and a bunch of others to get performance back. I really
think the code maintenance issue there will get far worse than what we
have today :(

Cheers,
Jes
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