Subject: Re: MMIO and gcc re-ordering issue

From: Roland Dreier <[email-masked]>
Date: Tue, 27 May 2008 08:50:17 -0700

> Though it's my understanding that at least ia64 does require the
 > explicit barriers anyway, so we are still in a dodgy situation here
 > where it's not clear what drivers should do and we end up with
 > possibly excessive barriers on powerpc where I end up with both
 > the wmb/rmb/mb that were added for ia64 -and- the ones I have in
 > readl/writel to make them look synchronous... Not nice.

ia64 is a disaster with a slightly different ordering problem -- the
mmiowb() issue.  I know Ben knows far too much about this, but for big
SGI boxes, you sometimes need mmiowb() to avoid problems with driver
code that does totally sane stuff like

	spin_lock(&mmio_lock);
	writel(val1, reg1);
	writel(val2, reg2);
	spin_unlock(&mmio_lock);

If that snippet is called on two CPUs at the same time, then the device
might see a sequence like

	CPU1 -- write reg1
	CPU2 -- write reg1
	CPU1 -- write reg2
	CPU2 -- write reg2

in spite of the fact that everything is totally ordered on the CPUs by
the spin lock.

The reason this is such a disaster is because the code looks right,
makes sense, and works fine on 99.99% of all systems out there.  So I
would bet that 99% of our drivers have missing mmiowb() "bugs" -- no one
has plugged the hardware into an Altix box and cared enough to stress
test it.

However for the issue at hand, my expectation as a driver writer is that
readl()/writel() are ordered with respect to MMIO operations, but not
necessarily with respect to normal writes to coherent CPU memory.  And
I've included explicit wmb()s in code I've written like
drivers/infiniband/hw/mthca.

 - R.