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

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Date:	Tue, 27 May 2014 21:32:49 +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

On Tue, May 27, 2014 at 09:21:38PM +0100, Benjamin Herrenschmidt wrote:
> 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.

Ah yeah, thanks. I was so hung up on the ordering against locks that I
completely forgot about DMA!

> At this point, I'm keen on keeping my per-cpu trick to avoid that
> second one in most cases.

Makes sense. The alternative is dropping that requirement and instead
relying on drivers to use mmiowb() even with the non-relaxed accessors,
but I think that's going to be fairly painful (and hence why you have the
trick to start with).

Will
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