Raise an error when registering a lowering for an unknown platform

jax/_src/interpreters/mlir.py

@@ -834,6 +834,12 @@ def register_lowering(prim: core.Primitive, rule: LoweringRule,
   if platform is None:
     _lowerings[prim] = rule
   else:
+    if not xb.is_known_platform(platform):
+      known_platforms = sorted(xb.known_platforms())
+      raise NotImplementedError(
+          f"Registering an MLIR lowering rule for primitive {prim}"
+          f" for an unknown platform {platform}. Known platforms are:"
+          f" {', '.join(known_platforms)}.")
     # For backward compatibility reasons, we allow rules to be registered
     # under "gpu" even though the platforms are now called "cuda" and "rocm".
     # TODO(phawkins): fix up users to specify either "cuda" or "rocm" and remove

jax/_src/xla_bridge.py

@@ -202,6 +202,9 @@ class BackendRegistration:
 # for unimplemented features. Wrong outputs are not acceptable.
 _nonexperimental_plugins: set[str] = {'cuda', 'rocm'}
 
+# The set of known experimental plugins that have registrations in JAX codebase.
+_experimental_plugins: set[str] = {"METAL"}
+
 def register_backend_factory(name: str, factory: BackendFactory, *,
                              priority: int = 0,
                              fail_quietly: bool = True,
@@ -774,12 +777,20 @@ def _discover_and_register_pjrt_plugins():
   _alias_to_platforms.setdefault(_alias, []).append(_platform)
 
 
+def known_platforms() -> set[str]:
+  platforms = set()
+  platforms |= set(_nonexperimental_plugins)
+  platforms |= set(_experimental_plugins)
+  platforms |= set(_backend_factories.keys())
+  platforms |= set(_platform_aliases.values())
+  return platforms
+
+
 def is_known_platform(platform: str) -> bool:
   # A platform is valid if there is a registered factory for it. It does not
   # matter if we were unable to initialize that platform; we only care that
   # we've heard of it and it isn't, e.g., a typo.
-  return (platform in _backend_factories.keys() or
-          platform in _platform_aliases.keys())
+  return platform in known_platforms()
 
 
 def canonicalize_platform(platform: str) -> str:

tests/extend_test.py

@@ -291,5 +291,15 @@ def ffi_call_lu_pivots_to_permutation(pivots, permutation_size, **kwargs):
   )
 
 
+class MlirRegisterLoweringTest(jtu.JaxTestCase):
+
+  def test_unknown_platform_error(self):
+    with self.assertRaisesRegex(
+        NotImplementedError,
+        "Registering an MLIR lowering rule for primitive .+ for an unknown "
+        "platform foo. Known platforms are: .+."):
+      mlir.register_lowering(prim=None, rule=None, platform="foo")
+
+
 if __name__ == "__main__":
   absltest.main(testLoader=jtu.JaxTestLoader())