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

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Date:	Wed, 28 May 2014 06:21:38 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Will Deacon <will.deacon@....com>
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

On Tue, 2014-05-27 at 20:32 +0100, Will Deacon wrote:

> 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?

Well, so the non-relaxed writel would have to do:

	sync
	store
	sync

The first sync is to synchronize with DMAs, so that a sequence of

	store to mem
	writel

Remains ordered vs. the device (ie, when the writel causes the device
to do a DMA, it will see the previous store to mem).

The second sync is needed as mmiowb, to order with unlocks.
 
At this point, I'm keen on keeping my per-cpu trick to avoid that
second one in most cases.

> 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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