linux-kernel - Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

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Date:	Wed, 04 Jun 2014 16:01:15 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Will Deacon <will.deacon@....com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	Rob Herring <robherring2@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Cho KyongHo <pullip.cho@...sung.com>,
	Grant Grundler <grundler@...omium.org>,
	Dave P Martin <Dave.Martin@....com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Joerg Roedel <joro@...tes.org>,
	Stephen Warren <swarren@...dotorg.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

On Wednesday 04 June 2014 14:56:01 Will Deacon wrote:
> On Wed, Jun 04, 2014 at 02:44:03PM +0100, Thierry Reding wrote:
> > On Fri, May 30, 2014 at 09:54:37PM +0200, Arnd Bergmann wrote:
> > > On Friday 30 May 2014 22:29:13 Hiroshi Doyu wrote:
> > > > The disadvantage of this is that this limits the max number of streamIDs
> > > > to support. If # of streamID is increased later more than 64, this
> > > > format cannot cover any more. You have to predict the max # of streamIDs
> > > > in advance if steamID is statically assigned.
> > > > 
> > > 
> > > Well, the iommu specific binding could allow a variable #address-cells.
> > > That way, you just need to know the number of stream IDs for that instance
> > > of the iommu.
> > 
> > That sounds fairly complicated to me. I don't see what that buys us over
> > the clarity and simplicity that the above explicit notation gives us. Is
> > it not more common for a device to have a single master rather than a
> > whole bunch of them?
> 
> I've never seen a device upstream of an ARM SMMU with a single stream-id;
> they always seem to have a whole bunch of them. Calxeda's SATA controller
> had 10 IDs, for example, and a PL330 DMA controller tends to have at least
> 3.

What are those good for? Would we just always use the first one?

	Arnd
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