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

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Date:	Mon, 16 Jun 2014 13:57:04 +0100
From:	Will Deacon <will.deacon@....com>
To:	Thierry Reding <thierry.reding@...il.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 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.

Will
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