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

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Message-Id: <1158010331.3879.56.camel@localhost.localdomain>
Date:	Tue, 12 Sep 2006 07:32:10 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	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, segher@...nel.crashing.org
Subject: Re: Opinion on ordering of writel vs. stores to RAM

On Mon, 2006-09-11 at 11:08 -0700, Jesse Barnes wrote:
> On Sunday, September 10, 2006 6:00 pm, Benjamin Herrenschmidt wrote:
> > > If we accept this, I don't think we're much better off than we are
> > > currently (not that I have a problem with that).  That is, many
> > > drivers
> > > would still need to be fixed up.
> >
> > Not necessarily if you introduce the trick of doing the mmiowb() in
> > spin_unlock when a per-cpu flag has been set previously by writel... not
> > sure if it's worth tho.
> 
> True, though again this would add a branch to writeX.

No, it adds a cacheable store to writeX and a branch to spin_unlock

> Sure, that's where one would typically use it, but it really is a memory 
> barrier...

I prefer having separate semantics for it so people understand it better
but I may be wrong :)

> That's because it *is* a barrier.  I don't think it's any harder to understand 
> then regular memory barriers for example.  It's just that you'd typically use 
> it in conjunction with locks to ensure proper device access.

That's why I prefer defining it as a MMIO + lock barrier.

> Ok, that's fine, though I think you'd only want the very weak semantics (as 
> provided by your __raw* routines) on write combined memory typically?

Well, that and memory with no side effects (like frame buffers)

> > I'm very much against your terminology. It's -not- an IO to IO barrier.
> > It's an IO to lock barrier. Really. IO to IO is something else. ordering
> > of IOs between CPUs has absolutely no meaning outside of the context of
> > locked regions in any case.
> 
> But it *is* MMIO vs. MMIO.  There's confusion because your __raw* routines 
> don't even guarantee same CPU ordering, while mmiowb() is solely intended for 
> inter-CPU ordering.
> 
> But as you say, the most common (maybe only) use model for it is to make sure 
> critical sections protecting device access behave correctly, so I don't have 
> a problem tying it to locks somehow.

Ben.


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