linux-kernel - Re: Opinion on ordering of writel vs. stores to RAM

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Date:	Mon, 11 Sep 2006 11:04:04 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Miller <davem@...emloft.net>, jeff@...zik.org,
	paulus@...ba.org, torvalds@...l.org, linux-kernel@...r.kernel.org,
	akpm@...l.org
Subject: Re: Opinion on ordering of writel vs. stores to RAM

> If we go this route though, can I request that we don't introduce any 
> performance regressions in drivers currently using mmiowb()?  I.e. 
> they'll be converted over to the new accessor routines when they become 
> available along with the new barrier macros?

There are few enough of them, I've grep'ed, so that should be doable.
The segher mentioned in favor of his approach (option B -> ioremap
flags) that doing a test in writeX/readX is very cheap compared to the
cost of IOs in general and would make driver conversion easier: you
don't have to change a single occurence of writel/readl : just add the
necessary barriers and change the ioremap call. Thus I tend to agree
that his approach makes it easier from a driver writer point of view.

Now, I don't have a strong preference myself, which is why I asked for a
vote here. So far, I could your vote for A :)

Ben.

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