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

[Thierry Reding <thierry.reding@gmail.com>]

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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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