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

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Date:	Fri, 20 Jun 2014 18:50:51 +0100
From:	Will Deacon <will.deacon@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Mark Rutland <Mark.Rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Grant Grundler <grundler@...omium.org>,
	Joerg Roedel <joro@...tes.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Cho KyongHo <pullip.cho@...sung.com>,
	Dave P Martin <Dave.Martin@....com>,
	Hiroshi Doyu <hdoyu@...dia.com>
Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

On Fri, Jun 20, 2014 at 04:53:08PM +0100, Arnd Bergmann wrote:
> On Wednesday 18 June 2014 11:14:39 Will Deacon wrote:
> > On Wed, Jun 18, 2014 at 12:37:16AM +0100, Thierry Reding wrote:
> >   - Each master has a set of fixed StreamIDs
> >   - StreamIDs can be remastered by adding a constant offset (this could also
> >     be used to describe RequesterID -> StreamID mapping)
> > 
> > I'd hope this would be sufficient for most people. Dynamic ID assignment can
> > be worked out later (I'm not even sure it belongs in this binding) and any
> > mappings other than `add a constant offset' can be treated on a case-by-case
> > basis. We don't want to throw the kitchen sink at a language for describing
> > arbitrary transformations!
> > 
> > > We've had similar discussions before (power sequences anyone?) where we
> > > tried to come up with a generic way to describe something in device tree
> > > that just didn't work out too well. Some things are better done in code,
> > > so I think we should at least consider that possibility rather than
> > > blindly try and force everything into device tree.
> > 
> > If we can support 90% of SoCs with a simple DT-based description, we can
> > address the corner cases as they arise. I'm not ruling our hardcoding
> > topology if we have no choice, but I don't think that's a healthy place to
> > start from.
> 
> So we could use the "arm,gicv3" comaptible string for all those that
> have a relatively simple mapping, and describe that mapping entirely
> in DT properties, but use a different compatible string for those
> SoCs that have a mapping which we can't easily describe, and then
> put that into code?

That doesn't sound unreasonable, but I don't think we should commit to
putting things into code until they come along and we can't describe them.

Will
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