Add support for user-extending cpuif for PeakRDL command-line

docs/cpuif/customizing.rst

@@ -76,3 +76,39 @@ you can define your own.
 
 3. Use your new CPUIF definition when exporting.
 4. If you think the CPUIF protocol is something others might find useful, let me know and I can add it to PeakRDL!
+
+
+Entry point for the PeakRDL command line tool
+---------------------------------------------
+To make your custom CPUIF class visible to the `PeakRDL command-line tool <https://peakrdl.readthedocs.io>`_,
+provide an entry point linkage in your package's ``setup.py``. This advertises
+your custom CPUIF class to the PeakRDL-regblock tool as a plugin that should be
+loaded, and made available as a command-line option in PeakRDL.
+
+
+.. code-block:: python
+    :emphasize-lines: 7-11
+
+    import setuptools
+
+    setuptools.setup(
+        name="my_package",
+        packages=["my_package"],
+        # ...
+        entry_points = {
+            "peakrdl_regblock.cpuif": [
+                'my-cpuif = my_package.__peakrdl_regblock__:MyCPUIF'
+            ]
+        }
+    )
+
+
+*   ``my_package``: The name of your installable Python module
+*   ``peakrdl-regblock.cpuif``: This is the namespace that PeakRDL-regblock will
+    search. Any cpuif plugins you create must be enclosed in this namespace in
+    order to be discovered.
+*   ``my_package.__peakrdl_regblock__:MyCPUIF``: This is the import path that
+    points to your CPUIF class definition.
+*   ``my-cpuif``: The lefthand side of the assignment is your cpuif's name. This
+    text is what the end-user uses in the command line interface to select your
+    CPUIF implementation.

src/peakrdl_regblock/__peakrdl__.py

@@ -1,15 +1,15 @@
 from typing import TYPE_CHECKING
 
 from .exporter import RegblockExporter
-from .cpuif import apb3, apb4, axi4lite, passthrough
+from .cpuif import apb3, apb4, axi4lite, passthrough, CpuifBase
 from .udps import ALL_UDPS
+from . import entry_points
 
 if TYPE_CHECKING:
     import argparse
     from systemrdl.node import AddrmapNode
 
 
-# TODO: make this user-extensible
 CPUIF_DICT = {
     "apb3": apb3.APB3_Cpuif,
     "apb3-flat": apb3.APB3_Cpuif_flattened,
@@ -20,6 +20,16 @@
     "passthrough": passthrough.PassthroughCpuif
 }
 
+# Load any user-plugins
+for ep in entry_points.get_entry_points("peakrdl_regblock.cpuif"): # type: ignore
+    name = ep.name
+    cpuif = ep.load()
+    if name in CPUIF_DICT:
+        raise RuntimeError(f"A plugin for 'peakrdl-regblock' tried to load cpuif '{name}' but it already exists")
+    if not issubclass(cpuif, CpuifBase):
+        raise RuntimeError(f"A plugin for 'peakrdl-regblock' tried to load cpuif '{name}' but it not a CpuifBase class")
+    CPUIF_DICT[name] = cpuif
+
 
 class Exporter:
     short_desc = "Generate a SystemVerilog control/status register (CSR) block"

src/peakrdl_regblock/entry_points.py

@@ -0,0 +1,19 @@
+# type: ignore
+import sys
+
+if sys.version_info >= (3,10,0):
+    from importlib import metadata
+
+    def get_entry_points(group_name):
+        return metadata.entry_points().select(group=group_name)
+
+elif sys.version_info >= (3,8,0):
+    from importlib import metadata
+
+    def get_entry_points(group_name):
+        return metadata.entry_points().get(group_name, tuple())
+
+else:
+    import pkg_resources
+    def get_entry_points(group_name):
+        return pkg_resources.iter_entry_points(group_name)