Clock: Inherit the clock from parent by default

This patch changes the default 1 Tick clock period to a proxy that
resolves the parents clock. As a result of this, the caches and
L1-to-L2 bus, for example, will automatically use the clock period of
the CPU unless explicitly overridden.

To ensure backwards compatibility, the System class overrides the
proxy and specifies a 1 Tick clock. We could change this to something
more reasonable in a follow-on patch, perhaps 1 GHz or something
similar.

With this patch applied, all clocked objects should have a reasonable
clock period set, and could start specifying delays in Cycles instead
of absolute time.

src/sim/ClockedObject.py

@@ -37,9 +37,13 @@
 
 from m5.SimObject import SimObject
 from m5.params import *
+from m5.proxy import *
 
 class ClockedObject(SimObject):
     type = 'ClockedObject'
     abstract = True
 
-    clock = Param.Clock('1t', "Clock speed")
+    # Clock period of this object, with the default value being the
+    # clock period of the parent object, unproxied at instantiation
+    # time
+    clock = Param.Clock(Parent.clock, "Clock speed")

src/sim/System.py

@@ -41,6 +41,13 @@ class System(MemObject):
     type = 'System'
     system_port = MasterPort("System port")
 
+    # Override the clock from the ClockedObject which looks at the
+    # parent clock by default
+    clock = '1t'
+    # @todo Either make this value 0 and treat it as an error if it is
+    # not overridden, or choose a more sensible value in the range of
+    # 1GHz
+
     @classmethod
     def export_method_cxx_predecls(cls, code):
         code('#include "sim/system.hh"')