linux-kernel - Re: [PATCH v2 00/18] Cross-architecture definitions of relaxed MMIO accessors

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Date:	Tue, 27 May 2014 20:32:19 +0100
From:	Will Deacon <will.deacon@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	"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>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 00/18] Cross-architecture definitions of relaxed MMIO
 accessors

Hi Ben,

On Sun, May 25, 2014 at 10:46:03PM +0100, Benjamin Herrenschmidt wrote:
> On Thu, 2014-05-22 at 17:47 +0100, Will Deacon wrote:
> > A corollary to this is that mmiowb() probably needs rethinking. As it currently
> > stands, an mmiowb() is required to order MMIO writes to a device from multiple
> > CPUs, even if that device is protected by a lock. However, this isn't often used
> > in practice, leading to PowerPC implementing both mmiowb() *and* synchronising
> > I/O in spin_unlock.
> > 
> > I would propose making the non-relaxed I/O accessors ordered with respect to
> > LOCK/UNLOCK, leaving mmiowb() to be used with the relaxed accessors, if
> > required, but would welcome thoughts/suggestions on this topic.
> 
> I agree on the proposed semantics, though for us that does mean we still need
> that per-cpu flag tracking non-relaxed MMIO stores and corresponding added barrier
> in unlock. Eventually, if the use of the relaxed accessors becomes pervasive
> enough I suppose I can just make the ordered ones unconditionally do 2 barriers.

Why would you need two barriers? I would have though an mmiowb() inlined
into writel after the store operation would be sufficient. Or is this to
ensure a non-relaxed write is ordered with respect to a relaxed write?

Anyway, we may need something similar for other architectures with mmiowb
implementations:

  blackfin
  frv
  ia64
  mips
  sh

so I'm anticipating some more discussion when I try to push that patch :)

Cheers,

Will
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