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, 16 insertions, 12 deletions

diff --git a/src/mem/coherent_bus.cc b/src/mem/coherent_bus.cc
index 956654981..5bcb2f14f 100644
--- a/src/mem/coherent_bus.cc
+++ b/src/mem/coherent_bus.cc
@@ -54,7 +54,9 @@
 #include "mem/coherent_bus.hh"
 
 CoherentBus::CoherentBus(const CoherentBusParams *p)
-    : BaseBus(p), layer(*this, ".layer", p->clock)
+    : BaseBus(p), reqLayer(*this, ".reqLayer", p->clock),
+      respLayer(*this, ".respLayer", p->clock),
+      snoopRespLayer(*this, ".snoopRespLayer", p->clock)
 {
     // create the ports based on the size of the master and slave
     // vector ports, and the presence of the default port, the ports
@@ -115,7 +117,7 @@ CoherentBus::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
 
     // test if the bus should be considered occupied for the current
     // port, and exclude express snoops from the check
-    if (!is_express_snoop && !layer.tryTiming(src_port)) {
+    if (!is_express_snoop && !reqLayer.tryTiming(src_port)) {
         DPRINTF(CoherentBus, "recvTimingReq: src %s %s 0x%x BUSY\n",
                 src_port->name(), pkt->cmdString(), pkt->getAddr());
         return false;
@@ -176,10 +178,10 @@ CoherentBus::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
                     src_port->name(), pkt->cmdString(), pkt->getAddr());
 
             // update the bus state and schedule an idle event
-            layer.failedTiming(src_port, headerFinishTime);
+            reqLayer.failedTiming(src_port, headerFinishTime);
         } else {
             // update the bus state and schedule an idle event
-            layer.succeededTiming(packetFinishTime);
+            reqLayer.succeededTiming(packetFinishTime);
         }
     }
 
@@ -194,7 +196,7 @@ CoherentBus::recvTimingResp(PacketPtr pkt, PortID master_port_id)
 
     // test if the bus should be considered occupied for the current
     // port
-    if (!layer.tryTiming(src_port)) {
+    if (!respLayer.tryTiming(src_port)) {
         DPRINTF(CoherentBus, "recvTimingResp: src %s %s 0x%x BUSY\n",
                 src_port->name(), pkt->cmdString(), pkt->getAddr());
         return false;
@@ -221,7 +223,7 @@ CoherentBus::recvTimingResp(PacketPtr pkt, PortID master_port_id)
     // deadlock
     assert(success);
 
-    layer.succeededTiming(packetFinishTime);
+    respLayer.succeededTiming(packetFinishTime);
 
     return true;
 }
@@ -258,7 +260,7 @@ CoherentBus::recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id)
 
     // test if the bus should be considered occupied for the current
     // port
-    if (!layer.tryTiming(src_port)) {
+    if (!snoopRespLayer.tryTiming(src_port)) {
         DPRINTF(CoherentBus, "recvTimingSnoopResp: src %s %s 0x%x BUSY\n",
                 src_port->name(), pkt->cmdString(), pkt->getAddr());
         return false;
@@ -309,7 +311,7 @@ CoherentBus::recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id)
         assert(success);
     }
 
-    layer.succeededTiming(packetFinishTime);
+    snoopRespLayer.succeededTiming(packetFinishTime);
 
     return true;
 }
@@ -335,8 +337,10 @@ CoherentBus::forwardTiming(PacketPtr pkt, PortID exclude_slave_port_id)
 void
 CoherentBus::recvRetry()
 {
-    // only one layer that can deal with it
-    layer.recvRetry();
+    // responses and snoop responses never block on forwarding them,
+    // so the retry will always be coming from a port to which we
+    // tried to forward a request
+    reqLayer.recvRetry();
 }
 
 Tick
@@ -503,8 +507,8 @@ CoherentBus::forwardFunctional(PacketPtr pkt, PortID exclude_slave_port_id)
 unsigned int
 CoherentBus::drain(Event *de)
 {
-    // only one layer to worry about at the moment
-    return layer.drain(de);
+    // sum up the individual layers
+    return reqLayer.drain(de) + respLayer.drain(de) + snoopRespLayer.drain(de);
 }
 
 CoherentBus *