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

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

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