Re: [PATCH 00/18] Cross-architecture definitions of relaxed MMIO accessors

[Will Deacon]

Hi Ben,

On Thu, Apr 17, 2014 at 10:36:58PM +0100, Benjamin Herrenschmidt wrote:
> On Thu, 2014-04-17 at 16:00 +0200, Peter Zijlstra wrote:
> 
> > So the non-relaxed ops already imply the expensive I/O barrier (mmiowb?)
> > and therefore, PPC can drop it from spin_unlock()?
> 
> We play a trick. We set a per-cpu flag in writeX and test it in unlock
> before doing the barrier. Still better than having the barrier in every
> MMIO at this stage for us.
> 
> Whether we want to change that with then new scheme ... we'll see.
> 
> > Also, I read mmiowb() as MMIO-write-barrier(), what do we have to
> > order/contain mmio-reads?
> > 
> > I have _0_ experience with MMIO, so I've no idea if ordering/containing
> > reads is silly or not. 
> 
> I will review the rest when I'm back from vacation (or maybe this
> week-end).

Did you get a chance to look at this? I've got a handful of Acks from other
architectures, and there's a bug to fix in the x86 patch but it seems daft
to send a v2 without talking about the fundamental rules of the accessors.

Cheers,

Will
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