Bridge: Remove NACKs in the bridge and unify with packet queue

This patch removes the NACKing in the bridge, as the split
request/response busses now ensure that protocol deadlocks do not
occur, i.e. the message-dependency chain is broken by always allowing
responses to make progress without being stalled by requests. The
NACKs had limited support in the system with most components ignoring
their use (with a suitable call to panic), and as the NACKs are no
longer needed to avoid protocol deadlocks, the cleanest way is to
simply remove them.

The bridge is the starting point as this is the only place where the
NACKs are created. A follow-up patch will remove the code that deals
with NACKs in the endpoints, e.g. the X86 table walker and DMA
port. Ultimately the type of packet can be complete removed (until
someone sees a need for modelling more complex protocols, which can
now be done in parts of the system since the port and interface is
split).

As a consequence of the NACK removal, the bridge now has to send a
retry to a master if the request or response queue was full on the
first attempt. This change also makes the bridge ports very similar to
QueuedPorts, and a later patch will change the bridge to use these. A
first step in this direction is taken by aligning the name of the
member functions, as done by this patch.

A bit of tidying up has also been done as part of the simplifications.

Surprisingly, this patch has no impact on any of the
regressions. Hence, there was never any NACKs issued. In a follow-up
patch I would suggest changing the size of the bridge buffers set in
FSConfig.py to also test the situation where the bridge fills up.

1 files changed, 49 insertions, 89 deletions

diff --git a/src/mem/bridge.hh b/src/mem/bridge.hh
index cc428e44d..cf7673c47 100644
--- a/src/mem/bridge.hh
+++ b/src/mem/bridge.hh
@@ -52,15 +52,10 @@
 #define __MEM_BRIDGE_HH__
 
 #include <list>
-#include <queue>
-#include <string>
 
 #include "base/types.hh"
 #include "mem/mem_object.hh"
-#include "mem/packet.hh"
-#include "mem/port.hh"
 #include "params/Bridge.hh"
-#include "sim/eventq.hh"
 
 /**
  * A bridge is used to interface two different busses (or in general a
@@ -71,9 +66,9 @@
  * The bridge comprises a slave port and a master port, that buffer
  * outgoing responses and requests respectively. Buffer space is
  * reserved when a request arrives, also reserving response space
- * before forwarding the request. An incoming request is always
- * accepted (recvTiming returns true), but is potentially NACKed if
- * there is no request space or response space.
+ * before forwarding the request. If there is no space present, then
+ * the bridge will delay accepting the packet until space becomes
+ * available.
  */
 class Bridge : public MemObject
 {
@@ -106,40 +101,18 @@ class Bridge : public MemObject
     };
 
     /**
-     * A deferred request stores a packet along with its scheduled
-     * transmission time, and whether we can expect to see a response
-     * or not.
+     * A deferred packet stores a packet along with its scheduled
+     * transmission time
      */
-    class DeferredRequest
+    class DeferredPacket
     {
 
       public:
 
-        Tick ready;
+        Tick tick;
         PacketPtr pkt;
-        bool expectResponse;
 
-        DeferredRequest(PacketPtr _pkt, Tick t)
-            : ready(t), pkt(_pkt), expectResponse(_pkt->needsResponse())
-        { }
-    };
-
-    /**
-     * A deferred response stores a packet along with its scheduled
-     * transmission time. It also contains information of whether the
-     * bridge NACKed the packet to be able to correctly maintain
-     * counters of outstanding responses.
-     */
-    class DeferredResponse {
-
-      public:
-
-        Tick ready;
-        PacketPtr pkt;
-        bool nackedHere;
-
-        DeferredResponse(PacketPtr _pkt, Tick t, bool nack = false)
-            : ready(t), pkt(_pkt), nackedHere(nack)
+        DeferredPacket(PacketPtr _pkt, Tick _tick) : tick(_tick), pkt(_pkt)
         { }
     };
 
@@ -157,21 +130,18 @@ class Bridge : public MemObject
 
       private:
 
-        /** A pointer to the bridge to which this port belongs. */
-        Bridge *bridge;
+        /** The bridge to which this port belongs. */
+        Bridge& bridge;
 
         /**
-         * Master port on the other side of the bridge
-         * (connected to the other bus).
+         * Master port on the other side of the bridge (connected to
+         * the other bus).
          */
         BridgeMasterPort& masterPort;
 
         /** Minimum request delay though this bridge. */
         Tick delay;
 
-        /** Min delay to respond with a nack. */
-        Tick nackDelay;
-
         /** Address ranges to pass through the bridge */
         AddrRangeList ranges;
 
@@ -180,13 +150,13 @@ class Bridge : public MemObject
          * queue for a specified delay to model the processing delay
          * of the bridge.
          */
-        std::list<DeferredResponse> responseQueue;
+        std::list<DeferredPacket> transmitList;
 
         /** Counter to track the outstanding responses. */
         unsigned int outstandingResponses;
 
-        /** If we're waiting for a retry to happen. */
-        bool inRetry;
+        /** If we should send a retry when space becomes available. */
+        bool retryReq;
 
         /** Max queue size for reserved responses. */
         unsigned int respQueueLimit;
@@ -199,22 +169,15 @@ class Bridge : public MemObject
         bool respQueueFull();
 
         /**
-         * Turn the request packet into a NACK response and put it in
-         * the response queue and schedule its transmission.
-         *
-         * @param pkt the request packet to NACK
-         */
-        void nackRequest(PacketPtr pkt);
-
-        /**
          * Handle send event, scheduled when the packet at the head of
          * the response queue is ready to transmit (for timing
          * accesses only).
          */
-        void trySend();
+        void trySendTiming();
 
         /** Send event for the response queue. */
-        EventWrapper<BridgeSlavePort, &BridgeSlavePort::trySend> sendEvent;
+        EventWrapper<BridgeSlavePort,
+                     &BridgeSlavePort::trySendTiming> sendEvent;
 
       public:
 
@@ -225,44 +188,50 @@ class Bridge : public MemObject
          * @param _bridge the structural owner
          * @param _masterPort the master port on the other side of the bridge
          * @param _delay the delay from seeing a response to sending it
-         * @param _nack_delay the delay from a NACK to sending the response
          * @param _resp_limit the size of the response queue
          * @param _ranges a number of address ranges to forward
          */
-        BridgeSlavePort(const std::string &_name, Bridge *_bridge,
+        BridgeSlavePort(const std::string& _name, Bridge& _bridge,
                         BridgeMasterPort& _masterPort, int _delay,
-                        int _nack_delay, int _resp_limit,
-                        std::vector<Range<Addr> > _ranges);
+                        int _resp_limit, std::vector<Range<Addr> > _ranges);
 
         /**
          * Queue a response packet to be sent out later and also schedule
          * a send if necessary.
          *
          * @param pkt a response to send out after a delay
+         * @param when tick when response packet should be sent
+         */
+        void schedTimingResp(PacketPtr pkt, Tick when);
+
+        /**
+         * Retry any stalled request that we have failed to accept at
+         * an earlier point in time. This call will do nothing if no
+         * request is waiting.
          */
-        void queueForSendTiming(PacketPtr pkt);
+        void retryStalledReq();
 
       protected:
 
         /** When receiving a timing request from the peer port,
             pass it to the bridge. */
-        virtual bool recvTimingReq(PacketPtr pkt);
+        bool recvTimingReq(PacketPtr pkt);
 
         /** When receiving a retry request from the peer port,
             pass it to the bridge. */
-        virtual void recvRetry();
+        void recvRetry();
 
         /** When receiving a Atomic requestfrom the peer port,
             pass it to the bridge. */
-        virtual Tick recvAtomic(PacketPtr pkt);
+        Tick recvAtomic(PacketPtr pkt);
 
         /** When receiving a Functional request from the peer port,
             pass it to the bridge. */
-        virtual void recvFunctional(PacketPtr pkt);
+        void recvFunctional(PacketPtr pkt);
 
         /** When receiving a address range request the peer port,
             pass it to the bridge. */
-        virtual AddrRangeList getAddrRanges() const;
+        AddrRangeList getAddrRanges() const;
     };
 
 
@@ -276,12 +245,12 @@ class Bridge : public MemObject
 
       private:
 
-        /** A pointer to the bridge to which this port belongs. */
-        Bridge* bridge;
+        /** The bridge to which this port belongs. */
+        Bridge& bridge;
 
         /**
-         * Pointer to the slave port on the other side of the bridge
-         * (connected to the other bus).
+         * The slave port on the other side of the bridge (connected
+         * to the other bus).
          */
         BridgeSlavePort& slavePort;
 
@@ -293,10 +262,7 @@ class Bridge : public MemObject
          * queue for a specified delay to model the processing delay
          * of the bridge.
          */
-        std::list<DeferredRequest> requestQueue;
-
-        /** If we're waiting for a retry to happen. */
-        bool inRetry;
+        std::list<DeferredPacket> transmitList;
 
         /** Max queue size for request packets */
         unsigned int reqQueueLimit;
@@ -306,10 +272,11 @@ class Bridge : public MemObject
          * the outbound queue is ready to transmit (for timing
          * accesses only).
          */
-        void trySend();
+        void trySendTiming();
 
         /** Send event for the request queue. */
-        EventWrapper<BridgeMasterPort, &BridgeMasterPort::trySend> sendEvent;
+        EventWrapper<BridgeMasterPort,
+                     &BridgeMasterPort::trySendTiming> sendEvent;
 
       public:
 
@@ -322,7 +289,7 @@ class Bridge : public MemObject
          * @param _delay the delay from seeing a request to sending it
          * @param _req_limit the size of the request queue
          */
-        BridgeMasterPort(const std::string &_name, Bridge *_bridge,
+        BridgeMasterPort(const std::string& _name, Bridge& _bridge,
                          BridgeSlavePort& _slavePort, int _delay,
                          int _req_limit);
 
@@ -338,8 +305,9 @@ class Bridge : public MemObject
          * a send if necessary.
          *
          * @param pkt a request to send out after a delay
+         * @param when tick when response packet should be sent
          */
-        void queueForSendTiming(PacketPtr pkt);
+        void schedTimingReq(PacketPtr pkt, Tick when);
 
         /**
          * Check a functional request against the packets in our
@@ -355,11 +323,11 @@ class Bridge : public MemObject
 
         /** When receiving a timing request from the peer port,
             pass it to the bridge. */
-        virtual bool recvTimingResp(PacketPtr pkt);
+        bool recvTimingResp(PacketPtr pkt);
 
         /** When receiving a retry request from the peer port,
             pass it to the bridge. */
-        virtual void recvRetry();
+        void recvRetry();
     };
 
     /** Slave port of the bridge. */
@@ -368,17 +336,7 @@ class Bridge : public MemObject
     /** Master port of the bridge. */
     BridgeMasterPort masterPort;
 
-    /** If this bridge should acknowledge writes. */
-    bool ackWrites;
-
   public:
-    typedef BridgeParams Params;
-
-  protected:
-    Params *_params;
-
-  public:
-    const Params *params() const { return _params; }
 
     virtual MasterPort& getMasterPort(const std::string& if_name,
                                       int idx = -1);
@@ -386,6 +344,8 @@ class Bridge : public MemObject
 
     virtual void init();
 
+    typedef BridgeParams Params;
+
     Bridge(Params *p);
 };