mem: Avoid unecessary retries when bus peer is not ready

This patch removes unecessary retries that happened when the bus layer
itself was no longer busy, but the the peer was not yet ready. Instead
of sending a retry that will inevitably not succeed, the bus now
silenty waits until the peer sends a retry.

src/mem/bus.cc

@@ -136,8 +136,7 @@ template <typename SrcType, typename DstType>
 BaseBus::Layer<SrcType,DstType>::Layer(DstType& _port, BaseBus& _bus,
                                        const std::string& _name) :
     port(_port), bus(_bus), _name(_name), state(IDLE), drainManager(NULL),
-    retryingPort(NULL), waitingForPeer(NULL),
-    releaseEvent(this)
+    waitingForPeer(NULL), releaseEvent(this)
 {
 }
 
@@ -190,9 +189,6 @@ BaseBus::Layer<SrcType,DstType>::tryTiming(SrcType* src_port)
     // update the state to busy
     state = BUSY;
 
-    // reset the retrying port
-    retryingPort = NULL;
-
     return true;
 }
 
@@ -243,7 +239,10 @@ BaseBus::Layer<SrcType,DstType>::releaseLayer()
 
     // bus layer is now idle, so if someone is waiting we can retry
     if (!waitingForLayer.empty()) {
-        retryWaiting();
+        // there is no point in sending a retry if someone is still
+        // waiting for the peer
+        if (waitingForPeer == NULL)
+            retryWaiting();
     } else if (waitingForPeer == NULL && drainManager) {
         DPRINTF(Drain, "Bus done draining, signaling drain manager\n");
         //If we weren't able to drain before, do it now.
@@ -268,8 +267,7 @@ BaseBus::Layer<SrcType,DstType>::retryWaiting()
 
     // set the retrying port to the front of the retry list and pop it
     // off the list
-    assert(retryingPort == NULL);
-    retryingPort = waitingForLayer.front();
+    SrcType* retryingPort = waitingForLayer.front();
     waitingForLayer.pop_front();
 
     // tell the port to retry, which in some cases ends up calling the
@@ -282,7 +280,6 @@ BaseBus::Layer<SrcType,DstType>::retryWaiting()
         // update the state to busy and reset the retrying port, we
         // have done our bit and sent the retry
         state = BUSY;
-        retryingPort = NULL;
 
         // occupy the bus layer until the next cycle ends
         occupyLayer(bus.clockEdge(Cycles(1)));

src/mem/bus.hh

@@ -220,14 +220,6 @@ class BaseBus : public MemObject
          */
         std::deque<SrcType*> waitingForLayer;
 
-        /**
-         * Port that we are currently in the process of telling to
-         * retry a previously failed attempt to perform a timing
-         * transaction. This is a valid port when in the retry state,
-         * and NULL when in busy or idle.
-         */
-        SrcType* retryingPort;
-
         /**
          * Track who is waiting for the retry when receiving it from a
          * peer. If no port is waiting NULL is stored.