python: Always throw TypeError on slave-slave connections

params.py checks the validity of memory port-port connections before
they are instantiated in C++. This commit ensures that attempting to
connect two slave ports together will cause a TypeError.

Change-Id: Ia7d0a15df28b96c7bf5e568c4f4917d21a19b824
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15896
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/python/m5/params.py

@@ -1883,12 +1883,9 @@ class PortRef(object):
     def ccConnect(self):
         from _m5.pyobject import connectPorts
 
-        if self.role == 'SLAVE':
-            # do nothing and let the master take care of it
-            return
-
         if self.ccConnected: # already done this
             return
+
         peer = self.peer
         if not self.peer: # nothing to connect to
             return
@@ -1899,6 +1896,10 @@ class PortRef(object):
                 "cannot connect '%s' and '%s' due to identical role '%s'" \
                 % (peer, self, self.role)
 
+        if self.role == 'SLAVE':
+            # do nothing and let the master take care of it
+            return
+
         try:
             # self is always the master and peer the slave
             connectPorts(self.simobj.getCCObject(), self.name, self.index,