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

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Message-ID: <20140617233714.GD25525@mithrandir>
Date:	Wed, 18 Jun 2014 01:37:16 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Dave P Martin <Dave.Martin@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Arnd Bergmann <arnd@...db.de>,
	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>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Joerg Roedel <joro@...tes.org>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <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 Tue, Jun 17, 2014 at 01:18:11PM +0100, Will Deacon wrote:
> On Tue, Jun 17, 2014 at 12:58:30PM +0100, Thierry Reding wrote:
> > On Mon, Jun 16, 2014 at 01:57:04PM +0100, Will Deacon wrote:
> > > On Wed, Jun 04, 2014 at 10:12:38PM +0100, Thierry Reding wrote:
> > > > It can easily be argued that if the algorithm used to remap the ID
> > > > varies, the compatibility of the device changes. Therefore I would
> > > > expect any variant of the GICv3 that deviates from the "standard"
> > > > mapping (if there is such a thing) to have its own compatible string.
> > > 
> > > There is no standard mapping; it's a property defined at system integration
> > > time. I fully expect different SoCs to do different things here.
> > 
> > My point was that the mapping itself seems to be fundamental enough to
> > make devices with different mappings "incompatible". Therefore I think
> > this could probably be handled by using different compatible values,
> > something along the lines of this:
> > 
> > 	compatible = "vendor,soc-gicv3", "arm,gicv3";
> > 
> > Then the mapping can be described in code, which should be a whole lot
> > easier and more flexible than a more or less generic notation in device
> > tree.
> 
> I don't think that scales well beyond a handful of unique mappings, and I
> really anticipate everybody doing something different based on their
> integration constraints.
> 
> You'd very quickly end up with sets of tables for each SoC, describing the
> topology and associated IDs in the kernel source, which feels like a giant
> step backwards from where we are today with device tree.

Well, today we don't have a generic binding at all, so anything will
really be a giant step forward in my opinion.

But seriously, from what you said earlier I got the impression that some
of the mappings may not be easy or possible to represent in DT, which is
why I proposed to encode it into the compatible property so that it can
be handled in code instead.

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.

Thierry

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