mem: Make packet bus-related time accounting relative

This patch changes the bus-related time accounting done in the packet
to be relative. Besides making it easier to align the cache timing to
cache clock cycles, it also makes it possible to create a Last-Level
Cache (LLC) directly to a memory controller without a bus inbetween.

The bus is unique in that it does not ever make the packets wait to
reflect the time spent forwarding them. Instead, the cache is
currently responsible for making the packets wait. Thus, the bus
annotates the packets with the time needed for the first word to
appear, and also the last word. The cache then delays the packets in
its queues before passing them on. It is worth noting that every
object attached to a bus (devices, memories, bridges, etc) should be
doing this if we opt for keeping this way of accounting for the bus
timing.

1 files changed, 7 insertions, 5 deletions

diff --git a/src/mem/noncoherent_bus.cc b/src/mem/noncoherent_bus.cc
index ae5344425..4f6751512 100644
--- a/src/mem/noncoherent_bus.cc
+++ b/src/mem/noncoherent_bus.cc
@@ -110,8 +110,8 @@ NoncoherentBus::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
     // set the source port for routing of the response
     pkt->setSrc(slave_port_id);
 
-    Tick headerFinishTime = calcPacketTiming(pkt);
-    Tick packetFinishTime = pkt->finishTime;
+    calcPacketTiming(pkt);
+    Tick packetFinishTime = pkt->busLastWordDelay + curTick();
 
     // since it is a normal request, determine the destination
     // based on the address and attempt to send the packet
@@ -124,7 +124,8 @@ NoncoherentBus::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
         DPRINTF(NoncoherentBus, "recvTimingReq: src %s %s 0x%x RETRY\n",
                 src_port->name(), pkt->cmdString(), pkt->getAddr());
 
-        reqLayer.failedTiming(src_port, headerFinishTime);
+        // occupy until the header is sent
+        reqLayer.failedTiming(src_port, clockEdge(Cycles(headerCycles)));
 
         return false;
     }
@@ -152,7 +153,7 @@ NoncoherentBus::recvTimingResp(PacketPtr pkt, PortID master_port_id)
             src_port->name(), pkt->cmdString(), pkt->getAddr());
 
     calcPacketTiming(pkt);
-    Tick packetFinishTime = pkt->finishTime;
+    Tick packetFinishTime = pkt->busLastWordDelay + curTick();
 
     // send the packet to the destination through one of our slave
     // ports, as determined by the destination field
@@ -189,7 +190,8 @@ NoncoherentBus::recvAtomic(PacketPtr pkt, PortID slave_port_id)
     // forward the request to the appropriate destination
     Tick response_latency = masterPorts[dest_id]->sendAtomic(pkt);
 
-    pkt->finishTime = curTick() + response_latency;
+    // @todo: Not setting first-word time
+    pkt->busLastWordDelay = response_latency;
     return response_latency;
 }