Subject: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

From: [email-masked] (Mark Rutland)
Date: Thu, 31 Jul 2014 10:22:41 +0100

[...]

> > > +Examples:
> > > +=========
> > > +
> > > +Single-master IOMMU:
> > > +--------------------
> > > +
> > > +	iommu {
> > > +		#iommu-cells = <0>;
> > > +	};
> > > +
> > > +	master {
> > > +		iommus = <&/iommu>;
> > 
> > Nit: this should be iommus = <&{/iommu}>, or it's not valid dts syntax.
> 
> Done.

Cheers. I take it that was done for the other occurrences too?

> 
> > > +	};
> > > +
> > > +Multiple-master IOMMU with fixed associations:
> > > +----------------------------------------------
> > > +
> > > +	/* multiple-master IOMMU */
> > > +	iommu {
> > > +		/*
> > > +		 * Masters are statically associated with this IOMMU and
> > > +		 * address translation is always enabled.
> > > +		 */
> > > +		#iommu-cells = <0>;
> > 
> > I don't follow why translation being always enabled is relevant to the
> > example; that would seem to be independent from the binding.
> > 
> > Surely the key point is that with no way to distinguish devices, they
> > presumably share the same translations?
> 
> Both aspects are important I think. For #iommu-cells = <0> there is no
> way for the IOMMU driver to know how to enable translation for a given
> device. So it must be either always on or always off.

Sure. But "always on or off" is not the same as "always enabled", which
was what confused me.

> I guess one could say that this is implicit if all masters share the
> same translations. And I guess translations don't always have to be on
> or off technically. Let me try to rephrase this:
> 
> 		/*
> 		 * Masters are statically associated with this IOMMU and share
> 		 * the same address translations because the IOMMU does not
> 		 * have sufficient information to distinguish between masters.
> 		 *
> 		 * Consequently address translation is always on or off for
> 		 * all masters at any given point in time.
> 		 */
> 
> Does that sound better?

That addresses my concern, so yes.

Given these are minor and everyone wants this in now, I'm happy for
these to go through in a fixup patch later.

Cheers,
Mark.