Cache: better comments particularly regarding writeback situation.

--HG--
extra : convert_revision : 59ff9ee63ee0fec5a7dfc27b485b737455ccf362

src/mem/cache/cache.hh

@@ -270,12 +270,32 @@ class Cache : public BaseCache
     void squash(int threadNum);
 
     /**
-     * Selects a outstanding request to service.
-     * @return The request to service, NULL if none found.
+     * Generate an appropriate downstream bus request packet for the
+     * given parameters.
+     * @param cpu_pkt  The upstream request that needs to be satisfied.
+     * @param blk The block currently in the cache corresponding to
+     * cpu_pkt (NULL if none).
+     * @param needsExclusive  Indicates that an exclusive copy is required
+     * even if the request in cpu_pkt doesn't indicate that.
+     * @return A new Packet containing the request, or NULL if the
+     * current request in cpu_pkt should just be forwarded on.
      */
     PacketPtr getBusPacket(PacketPtr cpu_pkt, BlkType *blk,
                            bool needsExclusive);
+
+    /**
+     * Return the next MSHR to service, either a pending miss from the
+     * mshrQueue, a buffered write from the write buffer, or something
+     * from the prefetcher.  This function is responsible for
+     * prioritizing among those sources on the fly.
+     */
     MSHR *getNextMSHR();
+
+    /**
+     * Selects an outstanding request to service.  Called when the
+     * cache gets granted the downstream bus in timing mode.
+     * @return The request to service, NULL if none found.
+     */
     PacketPtr getTimingPacket();
 
     /**

src/mem/cache/cache_impl.hh

@@ -439,12 +439,12 @@ Cache<TagStore>::timingAccess(PacketPtr pkt)
     }
 
 #if 0
+    /** @todo make the fast write alloc (wh64) work with coherence. */
+
     PacketList writebacks;
 
     // If this is a block size write/hint (WH64) allocate the block here
     // if the coherence protocol allows it.
-    /** @todo make the fast write alloc (wh64) work with coherence. */
-    /** @todo Do we want to do fast writes for writebacks as well? */
     if (!blk && pkt->getSize() >= blkSize && coherence->allowFastWrites() &&
         (pkt->cmd == MemCmd::WriteReq
          || pkt->cmd == MemCmd::WriteInvalidateReq) ) {
@@ -517,6 +517,7 @@ Cache<TagStore>::timingAccess(PacketPtr pkt)
 }
 
 
+// See comment in cache.hh.
 template<class TagStore>
 PacketPtr
 Cache<TagStore>::getBusPacket(PacketPtr cpu_pkt, BlkType *blk,
@@ -529,14 +530,11 @@ Cache<TagStore>::getBusPacket(PacketPtr cpu_pkt, BlkType *blk,
         return NULL;
     }
 
-    if (!blkValid &&
-        (cpu_pkt->cmd == MemCmd::Writeback ||
-         cpu_pkt->cmd == MemCmd::UpgradeReq)) {
-            // For now, writebacks from upper-level caches that
-            // completely miss in the cache just go through. If we had
-            // "fast write" support (where we could write the whole
-            // block w/o fetching new data) we might want to allocate
-            // on writeback misses instead.
+    if (!blkValid && (cpu_pkt->cmd == MemCmd::Writeback ||
+                      cpu_pkt->cmd == MemCmd::UpgradeReq)) {
+        // Writebacks that weren't allocated in access() and upgrades
+        // from upper-level caches that missed completely just go
+        // through.
         return NULL;
     }