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

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Message-ID: <20140617115829.GB18816@ulmo>
Date:	Tue, 17 Jun 2014 13:58:30 +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 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:
> > On Fri, May 30, 2014 at 12:27:28PM +0100, Dave Martin wrote:
> > > On Fri, May 30, 2014 at 08:30:08AM +0100, Thierry Reding wrote:
> > [...]
> > > > Arnd, can you take another look at this binding and see if there's
> > > > anything else missing? If not I'll go through the document again and
> > > > update all #address-cells/#size-cells references with #iommu-cells as
> > > > appropriate and submit v3.
> > > 
> > > How do you envisage propagation of the master ID bits downstream of the
> > > IOMMU would be described?
> > > 
> > > We will definitely need a way to describe this for GICv3.  How those
> > > values are propagated is likely to vary between related SoCs and doesn't
> > > feel like it should be baked into a driver, especially for the ARM SMMU
> > > which may get reused in radically different SoC families from different
> > > vendors.
> > 
> > Well, we've had cases like these in the past (power sequences come to
> > mind). Some concepts are just too difficult or unwieldy to be put into
> > device tree. I think that this is one of them.
> > 
> > > The most likely types of remapping are the adding of a base offset or
> > > some extra bits to the ID -- because not all MSIs to the GIC will
> > > necessarily pass through the IOMMU.  It's also possible that we might
> > > see ID squashing or folding in some systems.
> > 
> > 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.

Thierry

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