Patch from Eric Norum <eric@cls.usask.ca> to address these issues:

1) Coalesce outgoing packet into a single mbuf when the packet is spread
      over more mbufs than configured transmit buffer descriptors.
   2) Add dianostic counters for successful and failed coalesce attempts.
   3) Add diagnostic counter for transmit retries.

NOTE: (1) lead to deadlock and the same design based on underlying
hardware characteristics is currently also in the Sonic and
i386ex/network driver.

c/src/lib/libbsp/m68k/gen68360/network/network.c

@@ -99,6 +99,9 @@ struct scc_softc {
 	unsigned long	txUnderrun;
 	unsigned long	txLostCarrier;
 	unsigned long	txRawWait;
+	unsigned long	txCoalesced;
+	unsigned long	txCoalesceFailed;
+	unsigned long	txRetry;
 };
 static struct scc_softc scc_softc[NSCCDRIVER];
 
@@ -344,20 +347,13 @@ m360Enet_retire_tx_bd (struct scc_softc *sc)
 	int i;
 	int nRetired;
 	struct mbuf *m, *n;
+	int retries = 0;
+	int saveStatus = 0;
 
 	i = sc->txBdTail;
 	nRetired = 0;
 	while ((sc->txBdActiveCount != 0)
 	   &&  (((status = (sc->txBdBase + i)->status) & M360_BD_READY) == 0)) {
-		/*
-		 * See if anything went wrong
-		 */
-		if (status & (M360_BD_DEFER |
-				M360_BD_HEARTBEAT |
-				M360_BD_LATE_COLLISION |
-				M360_BD_RETRY_LIMIT |
-				M360_BD_UNDERRUN |
-				M360_BD_CARRIER_LOST)) {
 		/*
 		 * Check for errors which stop the transmitter.
 		 */
@@ -367,11 +363,11 @@ m360Enet_retire_tx_bd (struct scc_softc *sc)
 			int j;
 
 			if (status & M360_BD_LATE_COLLISION)
-					scc_softc[0].txLateCollision++;
+				sc->txLateCollision++;
 			if (status & M360_BD_RETRY_LIMIT)
-					scc_softc[0].txRetryLimit++;
+				sc->txRetryLimit++;
 			if (status & M360_BD_UNDERRUN)
-					scc_softc[0].txUnderrun++;
+				sc->txUnderrun++;
 
 			/*
 			 * Reenable buffer descriptors
@@ -406,19 +402,23 @@ m360Enet_retire_tx_bd (struct scc_softc *sc)
 			M360ExecuteRISC (M360_CR_OP_RESTART_TX | M360_CR_CHAN_SCC1);
 			continue;
 		}
-			if (status & M360_BD_DEFER)
+		saveStatus |= status;
-				scc_softc[0].txDeferred++;
+		retries += (status >> 2) & 0xF;
-			if (status & M360_BD_HEARTBEAT)
-				scc_softc[0].txHeartbeat++;
-			if (status & M360_BD_CARRIER_LOST)
-				scc_softc[0].txLostCarrier++;
-		}
 		nRetired++;
 		if (status & M360_BD_LAST) {
 			/*
 			 * A full frame has been transmitted.
 			 * Free all the associated buffer descriptors.
 			 */
+			if (saveStatus & M360_BD_DEFER)
+				sc->txDeferred++;
+			if (saveStatus & M360_BD_HEARTBEAT)
+				sc->txHeartbeat++;
+			if (saveStatus & M360_BD_CARRIER_LOST)
+				sc->txLostCarrier++;
+			saveStatus = 0;
+			sc->txRetry += retries;
+			retries = 0;
 			sc->txBdActiveCount -= nRetired;
 			while (nRetired) {
 				nRetired--;
@@ -600,6 +600,74 @@ sendpacket (struct ifnet *ifp, struct mbuf *m)
 	nAdded = 0;
 	txBd = firstTxBd = sc->txBdBase + sc->txBdHead;
 	while (m) {
+		/*
+		 * There are more mbufs in the packet than there
+		 * are transmit buffer descriptors.
+		 * Coalesce into a single buffer.
+		 */
+		if (nAdded == sc->txBdCount) {
+			struct mbuf *nm;
+			int j;
+			char *dest;
+
+			/*
+			 * Get the pointer to the first mbuf of the packet
+			 */
+			if (sc->txBdTail != sc->txBdHead)
+				rtems_panic ("sendpacket coalesce");
+			m = sc->txMbuf[sc->txBdTail];
+
+			/*
+			 * Rescind the buffer descriptor READY bits
+			 */
+			for (j = 0 ; j < sc->txBdCount ; j++)
+				(sc->txBdBase + j)->status = 0;
+
+			/*
+			 * Allocate an mbuf cluster
+			 * Toss the packet if allocation fails
+			 */
+			MGETHDR (nm, M_DONTWAIT, MT_DATA);
+			if (nm == NULL) {
+				sc->txCoalesceFailed++;
+				m_freem (m);
+				return;
+			}
+			MCLGET (nm, M_DONTWAIT);
+			if (nm->m_ext.ext_buf == NULL) {
+				sc->txCoalesceFailed++;
+				m_freem (m);
+				m_free (nm);
+				return;
+			}
+			nm->m_pkthdr = m->m_pkthdr;
+			nm->m_len = nm->m_pkthdr.len;
+
+			/*
+			 * Copy data from packet chain to mbuf cluster
+			 */
+			sc->txCoalesced++;
+			dest = nm->m_ext.ext_buf;
+			while (m) {
+				struct mbuf *n;
+
+				if (m->m_len) {
+					memcpy (dest, mtod(m, caddr_t), m->m_len);
+					dest += m->m_len;
+				}
+				MFREE (m, n);
+				m = n;
+			}
+			
+			/*
+			 * Redo the send with the new mbuf cluster
+			 */
+			m = nm;
+			nAdded = 0;
+			status = 0;
+			continue;
+		}
+
 		/*
 		 * Wait for buffer descriptor to become available.
 		 */
@@ -815,7 +883,10 @@ scc_stats (struct scc_softc *sc)
 	printf ("Retransmit Limit:%-8lu", sc->txRetryLimit);
 	printf ("  Late Collision:%-8lu\n", sc->txLateCollision);
 	printf ("           Underrun:%-8lu", sc->txUnderrun);
-	printf (" Raw output wait:%-8lu\n", sc->txRawWait);
+	printf (" Raw output wait:%-8lu", sc->txRawWait);
+	printf ("       Coalesced:%-8lu\n", sc->txCoalesced);
+	printf ("    Coalesce failed:%-8lu", sc->txCoalesceFailed);
+	printf ("         Retries:%-8lu\n", sc->txRetry);
 }
 
 /*