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

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Date:	Thu, 1 May 2014 12:10:43 +0100
From:	Will Deacon <will.deacon@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"monstr@...str.eu" <monstr@...str.eu>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"broonie@...aro.org" <broonie@...aro.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 00/18] Cross-architecture definitions of relaxed MMIO
 accessors

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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