How use UDPs RDLImporter for RDL generation.

[engdoreis]

I'm developing a SystemRDL exporter for Opentitan reggen. Since some reggen properties lack direct SystemRDL equivalents, I'm using UDPs. However, RDLImporter::assign_property throws an exception when called with a UDP (raise ValueError(f"Unrecognized property '{prop_name}'")).
I couldn't find a way to register the UDP with the compiler. Is this feature supported?

[amykyta3]

Hi Douglas,
There are a few things at play here.

UDP declarations in RDL

For some background - First I'll explain how UDPs work a pure RDL design, without any importers in-play.
The SystemRDL standard mandates that any user-defined properties be explicitly declared in the code ahead of their use. For example:

property my_udp {
    type = longint;
    component = field;
};

This allows one to declare a property, alongside some very very basic semantics: The property called my_udp takes an integer type, and can only be applied to fields. Once declared, this unlocks the ability to assign the my_udp property to field components. If not declared, the compiler will reject it as an unrecognized property.
My interpretation is that this is RDL's way to allow organizations to formally declare some basic semantics of custom extensions, in a cross-compatible and portable way that lives within your RDL code.

In your case, the importer API is likely rejecting the assignment because there was no declaration of the UDP yet. One way to fix this is to compile an RDL file that contains this UDP declaration prior to any importer activities.

The UDPDefinition Descriptor

If you're working with UDPs in the RDL compiler API, pretty quickly, you'll probably realize that the purely textual declaration in RDL has some shortfalls in practice:

• The RDL declaration has no mechanism to provide additional constraints or validation on semantics of the property (what if the my_udp property is only allowed to be assigned even numbers?)
• If a project does not pre-declare the UDP in RDL, then calling the API's Node.get_property("my_udp") will cause an error since the compiler does not know of the existence of the UDP.
• What if a user's declaration accidentally contradicts what your tool expects (Did they accidentally set component = reg instead of field?)

To solve these problems, the RDL compiler lets you "pre-register" a UDP.
This is a mechanism that allows you to tell the compiler to expect the possibility of a specific UDP, and further enforce some expectations.
I have a pretty good write-up about this mechanism here: https://systemrdl-compiler.readthedocs.io/en/stable/api/udp.html

Why you should use "soft" UDP registrations

When reading through the UDP docs, you'll notice that the default behavior will still require one to explicitly declare the UDP in RDL code. Despite this seeming like a silly extra step, I would still recommend enforcing this since it improves the portability of your RDL in case you use other RDL-compatible tools in the future.

Hope this helps get you in the right direction. I'd be happy to clarify anything else if needed

[engdoreis]

Thanks for the quick reply.
It worked Thanks!