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

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Date:	Fri, 18 Apr 2014 07:36:58 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Will Deacon <will.deacon@....com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, arnd@...db.de, monstr@...str.eu,
	dhowells@...hat.com, broonie@...aro.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH 00/18] Cross-architecture definitions of relaxed MMIO
 accessors

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).

Thanks Will for picking that up, it's long overdue :)

Cheers,
Ben.


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