Varun Sethi [off-list ref] writes:

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Also, for dynamic stream ID allocation we would need to represent
the specific master register (to store the stream ID) in the device tree.

I assmue that the above means that iMX has such configuration
register to map steramID and a device dynamically.

We have per master registers for setting the stream ID on the
Layerscape platforms. My point was that we would need the iommu master
node to include a reference to the master id register.

master@1 {
               /* device has master ID 42 in the IOMMU */
             iommus = <&{/iommu} 42>;
             master-id-reg = <phandle offset> };

In the above, for "iommus=" bindings, you wouldn't need to break
ARM,SMMU compatibility at all if you set "streamID" exactly as below.

  master@1 {
                 /* device has master ID 42 in the IOMMU */
               iommus = <&{/iommu} 'any given streamID'>;
               master-id-reg = <phandle offset>
  };

And your SoC needs to register bus_notifier and ADD_DEVICE should configure
to map 'any given streamID' to a device via the above register. This wouldn't
need any modification from ARM,SMMU driver and keep the iommus bindings
as it is.

IOW, SoC only needs to register ADD_DEVICE in bus_notifier to map StreamID
to a device. This needs to be executed earlier than IOMMU bus's ADD_DEVICE,
though.

Is my understanding right?

I don't think that SOC specific code needs a bus notifier for setting
the stream ID. It can be done as a part of SOC specific
initialization. The device tree can be updated to reflect the correct
stream ID (SMMU driver can get the updated stream ID from device
tree). 

That's possible.

I was thinking more on the lines of updating the device stream id
while attaching a device to the domain.

I thought the same but this would break the ARM,SMMU /compatibility/
since the 1st param of "iommus=" is always expected as "streamID".

If streamID can be assigned dynamically like PCIe, not like streamID
statically set in DT, how should we describe this dynmaic steramID
shifting/assignment in DT?