Implement RFC 16 (CSR register API) #40

Before this commit, CSR elements were added with `Multiplexer.add()`. Now, a memory map of elements is populated beforehand, then given as argument to `Multiplexer.__init__()`. This decouples the definition of a group of neighboring registers from their implementation by a CSR Multiplexer.

Also, fix FieldPort signature direction.

See amaranth-lang/amaranth#909.

Before this commit, a Register whose fields were variable annotations couldn't be instantiated more than once, as the latter belonged to the global scope. The Field class is now a factory for user-defined field components, instead of a base class. Every FieldMap or FieldArray sharing a Field will use a different instance of its component, obtained from Field.create(). Also: * update to follow RFCs 37 and 38 * docstring and diagnostics improvements.

This allows csr.Register subclasses that use variable annotations to define their access mode without overriding __init__(). In addition, the default value of access= (which was "rw") is removed. The access mode of a register has an influence on the rest of the SoC and should therefore be explicitly set. As a consequence of access= being a required __init_subclass__() kwarg, the csr.Register class can no longer be directly instantiated and must be subclassed first.

The access= parameter can now be set in either __init_subclass__() or __init__(). It has a default value of None, and __init__() will error out if set in both sites or none of them.

Also, csr.Multiplexer now requires the signature of its memory map resources to contain a csr.Element.Signature member, named "element" and oriented as output.

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Implement RFC 16 (CSR register API) #40

Implement RFC 16 (CSR register API) #40

Commits on Feb 9, 2024