MEM: Remove the Broadcast destination from the packet

This patch simplifies the packet by removing the broadcast flag and
instead more firmly relying on (and enforcing) the semantics of
transactions in the classic memory system, i.e. request packets are
routed from a master to a slave based on the address, and when they
are created they have neither a valid source, nor destination. On
their way to the slave, the request packet is updated with a source
field for all modules that multiplex packets from multiple master
(e.g. a bus). When a request packet is turned into a response packet
(at the final slave), it moves the potentially populated source field
to the destination field, and the response packet is routed through
any multiplexing components back to the master based on the
destination field.

Modules that connect multiplexing components, such as caches and
bridges store any existing source and destination field in the sender
state as a stack (just as before).

The packet constructor is simplified in that there is no longer a need
to pass the Packet::Broadcast as the destination (this was always the
case for the classic memory system). In the case of Ruby, rather than
using the parameter to the constructor we now rely on setDest, as
there is already another three-argument constructor in the packet
class.

In many places where the packet information was printed as part of
DPRINTFs, request packets would be printed with a numeric "dest" that
would always be -1 (Broadcast) and that field is now removed from the
printing.

1 files changed, 21 insertions, 11 deletions

diff --git a/src/mem/packet.hh b/src/mem/packet.hh
index ce5748c24..e49fa67b8 100644
--- a/src/mem/packet.hh
+++ b/src/mem/packet.hh
@@ -1,4 +1,16 @@
 /*
+ * Copyright (c) 2012 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
  * Copyright (c) 2006 The Regents of The University of Michigan
  * Copyright (c) 2010 Advanced Micro Devices, Inc.
  * All rights reserved.
@@ -317,10 +329,6 @@ class Packet : public FastAlloc, public Printable
     /// The time at which the first chunk of the packet will be transmitted
     Tick firstWordTime;
 
-    /// The special destination address indicating that the packet
-    /// should be routed based on its address.
-    static const NodeID Broadcast = -1;
-
     /**
      * A virtual base opaque structure used to hold state associated
      * with the packet but specific to the sending device (e.g., an
@@ -478,6 +486,8 @@ class Packet : public FastAlloc, public Printable
     NodeID getDest() const     { assert(flags.isSet(VALID_DST)); return dest; }
     /// Accessor function to set the destination index of the packet.
     void setDest(NodeID _dest) { dest = _dest; flags.set(VALID_DST); }
+    /// Reset destination field, e.g. to turn a response into a request again.
+    void clearDest() { flags.clear(VALID_DST); }
 
     Addr getAddr() const { assert(flags.isSet(VALID_ADDR)); return addr; }
     unsigned getSize() const  { assert(flags.isSet(VALID_SIZE)); return size; }
@@ -513,9 +523,9 @@ class Packet : public FastAlloc, public Printable
      * not be valid. The command and destination addresses must be
      * supplied.
      */
-    Packet(Request *_req, MemCmd _cmd, NodeID _dest)
-        :  flags(VALID_DST), cmd(_cmd), req(_req), data(NULL),
-           dest(_dest), bytesValidStart(0), bytesValidEnd(0),
+    Packet(Request *_req, MemCmd _cmd)
+        :  cmd(_cmd), req(_req), data(NULL),
+           bytesValidStart(0), bytesValidEnd(0),
            time(curTick()), senderState(NULL)
     {
         if (req->hasPaddr()) {
@@ -533,9 +543,9 @@ class Packet : public FastAlloc, public Printable
      * a request that is for a whole block, not the address from the
      * req.  this allows for overriding the size/addr of the req.
      */
-    Packet(Request *_req, MemCmd _cmd, NodeID _dest, int _blkSize)
-        :  flags(VALID_DST), cmd(_cmd), req(_req), data(NULL),
-           dest(_dest), bytesValidStart(0), bytesValidEnd(0),
+    Packet(Request *_req, MemCmd _cmd, int _blkSize)
+        :  cmd(_cmd), req(_req), data(NULL),
+           bytesValidStart(0), bytesValidEnd(0),
            time(curTick()), senderState(NULL)
     {
         if (req->hasPaddr()) {
@@ -659,7 +669,7 @@ class Packet : public FastAlloc, public Printable
         assert(wasNacked());
         cmd = origCmd;
         assert(needsResponse());
-        setDest(Broadcast);
+        clearDest();
     }
 
     void