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

[Will Deacon <will.deacon@arm.com>]

On Fri, Jun 20, 2014 at 04:53:08PM +0100, Arnd Bergmann wrote:
> On Wednesday 18 June 2014 11:14:39 Will Deacon wrote:
> > On Wed, Jun 18, 2014 at 12:37:16AM +0100, Thierry Reding wrote:
> >   - Each master has a set of fixed StreamIDs
> >   - StreamIDs can be remastered by adding a constant offset (this could also
> >     be used to describe RequesterID -> StreamID mapping)
> > 
> > I'd hope this would be sufficient for most people. Dynamic ID assignment can
> > be worked out later (I'm not even sure it belongs in this binding) and any
> > mappings other than `add a constant offset' can be treated on a case-by-case
> > basis. We don't want to throw the kitchen sink at a language for describing
> > arbitrary transformations!
> > 
> > > 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.
> > 
> > If we can support 90% of SoCs with a simple DT-based description, we can
> > address the corner cases as they arise. I'm not ruling our hardcoding
> > topology if we have no choice, but I don't think that's a healthy place to
> > start from.
> 
> So we could use the "arm,gicv3" comaptible string for all those that
> have a relatively simple mapping, and describe that mapping entirely
> in DT properties, but use a different compatible string for those
> SoCs that have a mapping which we can't easily describe, and then
> put that into code?

That doesn't sound unreasonable, but I don't think we should commit to
putting things into code until they come along and we can't describe them.

Will