build/generic_toolchain/add_false_path: Only add keep attribute if from_/to are Signals.

litex/build/generic_toolchain.py

@@ -8,7 +8,7 @@
 import os
 import math
 
-from migen.fhdl.structure import _Fragment
+from migen.fhdl.structure import Signal, _Fragment
 
 from litex.gen import LiteXContext
 
@@ -178,7 +178,9 @@ class GenericToolchain:
 
     def add_false_path_constraint(self, platform, from_, to, keep=True):
         if keep:
+            if isinstance(from_, Signal):
                 from_.attr.add("keep")
+            if isinstance(to, Signal):
                 to.attr.add("keep")
         if (to, from_) not in self.false_paths:
             self.false_paths.add((from_, to))