MMIO and gcc re-ordering issue

David Miller davem at
Tue May 27 12:28:12 EST 2008

From: Benjamin Herrenschmidt <benh at>
Date: Tue, 27 May 2008 12:15:40 +1000

> Some of them. USB comes to mind. I'd be happy to make it "the rule" and
> document that MMIO vs. coherent access aren't implicitely ordered. I
> would still keep them ordered on powerpc for a little while tho until
> I'm happy enough with driver auditing.
> But heh, it's you who was telling me that it would be a bad engineering
> decision and we had to make everybody look like x86 & fully ordered :-)
> I decided to agree back then and stuck all those nasty heavy barriers
> in the powerpc variants of readl/writel/...

I still believe this.

It's just another complicated thing for driver authors to get wrong.
The other side of the coin is of course the cost.

The only thing I am absolutely sure of is that we should make a
decision fast, document it, and just stick to it.

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