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

[Hiroshi Doyu]

Arnd Bergmann <arnd@xxxxxxxx> writes:

>> > +Multiple-master IOMMU:
>> > +----------------------
>> > +
>> > +       iommu {
>> > +               /* the specifier represents the ID of the master */
>> > +               #address-cells = <1>;
>> > +               #size-cells = <0>;
>> > +       };
>> > +
>> > +       master {
>> > +               /* device has master ID 42 in the IOMMU */
>> > +               iommus = <&/iommu 42>;
>> > +       };
>> 
>> 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?

IIUC the original problem, "a master with 8 streamIDs" means something
like below, where some devices have multiple IDs but some have a
single. A sinle #address-cells cannot afford those 2 masters at once.

       iommu {
               /* the specifier represents the ID of the master */
               #address-cells = <1>;
               #size-cells = <0>;
       };

 	master@a {
 		...
 		iommus = <&smmu 1 2 3>; # 3 IDs
 	};

 	master@b {
 		...
 		iommus = <&smmu 4>;     # 1 ID
 	};

Tegra,SMMU has a similar problem and we have used a fixed size bitmap(64
bit) to afford 64 stream IDs so that a single device can hold multiple
IDs. If we apply the same bitmap to the above exmaple:

       iommu {
               /* the specifier represents the ID of the master */
               #address-cells = <1>;
               #size-cells = <0>;
       };

 	master@a {
 		...
 		iommus = <&smmu (BIT(1) | BIT(2) | BIT(3))>; # IDs 1 2 3
 	};

 	master@b {
 		...
 		iommus = <&smmu BIT(4)>;     # ID 4
 	};

The disadvantage of this is that this limits the max number of streamIDs
to support. If # of streamID is increased later more than 64, this
format cannot cover any more. You have to predict the max # of streamIDs
in advance if steamID is statically assigned.
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