Bus: Split the bus into separate request/response layers

This patch splits the existing buses into multiple layers. The
non-coherent bus is split into a request and a response layer, and the
coherent bus adds an additional layer for the snoop responses. The
layer is modified to be templatised on the port type, such that the
different layers can have retryLists with either master or slave
ports. This patch also removes the dynamic cast from the retry, as
previously promised when moving the recvRetry from the port base class
to the master/slave port respectively.

Overall, the split bus more closely reflects any modern on-chip bus
and should be at step in the right direction. From this point, it
would be reasonable straight forward to add separate layers (and thus
contention points and arbitration) for each port and thus create a
true crossbar.

The regressions all produce the correct output, but have varying
degrees of changes to their statistics. A separate patch will be
pushed with the updates to the reference statistics.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/mem/noncoherent_bus.hh b/src/mem/noncoherent_bus.hh
index dd43d8c19..e8c1ab57a 100644
--- a/src/mem/noncoherent_bus.hh
+++ b/src/mem/noncoherent_bus.hh
@@ -73,9 +73,11 @@ class NoncoherentBus : public BaseBus
   protected:
 
     /**
-     * Declare the single layer of this bus.
+     * Declare the two layers of this bus, one for requests and one
+     * for responses.
      */
-    Layer layer;
+    Layer<SlavePort> reqLayer;
+    Layer<MasterPort> respLayer;
 
     /**
      * Declaration of the non-coherent bus slave port type, one will