Thierry Reding [off-list ref] writes:

+Multiple-master IOMMU:
+----------------------
+
+       iommu {
+               /* the specifier represents the ID of the master */
+               #iommu-cells = <1>;
+       };
+
+       master@1 {
+               /* device has master ID 42 in the IOMMU */
+               iommus = <&{/iommu} 42>;
+       };
+
+       master@2 {
+               /* device has master IDs 23 and 24 in the IOMMU */
+               iommus = <&{/iommu} 23>, <&{/iommu} 24>;
+       };

I think that this "master ID" will be parsed in IOMMU driver. For
example, ARM,SMMU expects "streamID" as "master ID", right?

If a SoC has a feature to configure to assign streamID to devices at
runtime, "streamID" is not equal to "master ID".

  iommus = <&{/smmu} "soc specific master ID">;

"soc master ID" needs to be translated into "streamID" by SoC SW. It
seems that ARM,SMMU kernel driver doesn't expect this kind of ID
translation. If ARM,SMMU kernel driver is used as is, "soc master ID"
would be incompatible? ARM,SMMU needs such translation before
parsing. Is this my understanding right?

If so I think that this master ID configuration/translation may be quite
reasonable requirment for SoC using ARM,SMMU.

Can we consider this ID translation within ARM,SMMU compatibility?

IOW, is it possible to implement some SoC specific hook for ID
translation/configuration in ARM,SMMU kernel driver?