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

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

On Sat, Jun 21, 2014 at 12:16:25AM +0100, Olav Haugan wrote:
> On 5/30/2014 12:06 PM, Arnd Bergmann wrote:
> > On Friday 30 May 2014 08:16:05 Rob Herring wrote:
> >> Presumably the ID would be the streamID on ARM's SMMU. How would a
> >> master with 8 streamIDs be described? This is what Calxeda midway has
> >> for SATA and I would expect that to be somewhat common. Either you
> >> need some ID masking or you'll have lots of duplication when you have
> >> windows.
> > 
> > I don't understand the problem. If you have stream IDs 0 through 7,
> > you would have
> > 
> > 	master@a {
> > 		...
> > 		iommus = <&smmu 0>;
> > 	};
> > 
> > 	master@b {
> > 		...
> > 		iommus = <&smmu 1;
> > 	};
> > 
> > 	...
> > 
> > 	master@12 {
> > 		...
> > 		iommus = <&smmu 7;
> > 	};
> > 
> > and you don't need a window at all. Why would you need a mask of
> > some sort?
> 
> We have multiple-master SMMUs and each master emits a variable number of
> StreamIDs. However, we have to apply a mask (the ARM SMMU spec allows
> for this) to the StreamIDs due to limited number of StreamID 2 Context
> Bank entries in the SMMU. If my understanding is correct we would
> represent this in the DT like this:
> 
> 	iommu {
> 		#address-cells = <2>;
> 		#size-cells = <0>;
> 	};
> 
> 	master@a {
> 		...
> 		iommus = <&iommu StreamID0 MASK0>,
> 			 <&iommu StreamID1 MASK1>,
> 			 <&iommu StreamID2 MASK2>;
> 	};

Stupid question, but why not simply describe the masked IDs? What use does
the `raw' ID have to Linux?

Will