This fixes a out-of-order problem with several

of the newer drivers. The basic problem was
that the driver was pulling the mbuf off the
drbr ring and then when sending with xmit(), encounting
a full transmit ring. Thus the lower layer
xmit() function would return an error, and the
drivers would then append the data back on to the ring.
For TCP this is a horrible scenario sure to bring
on a fast-retransmit.

The fix is to use drbr_peek() to pull the data pointer
but not remove it from the ring. If it fails then
we either call the new drbr_putback or drbr_advance
method. Advance moves it forward (we do this sometimes
when the xmit() function frees the mbuf). When
we succeed we always call advance. The
putback will always copy the mbuf back to the top
of the ring. Note that the putback *cannot* be used
with a drbr_dequeue() only with drbr_peek(). We most
of the time, in putback, would not need to copy it
back since most likey the mbuf is still the same, but
sometimes xmit() functions will change the mbuf via
a pullup or other call. So the optimial case for
the single consumer is to always copy it back. If
we ever do a multiple_consumer (for lagg?) we
will  need a test and atomic in the put back possibly
a seperate putback_mc() in the ring buf.

Reviewed by:	jhb@freebsd.org, jlv@freebsd.org

1 files changed, 27 insertions, 6 deletions

diff --git a/sys/dev/e1000/if_igb.c b/sys/dev/e1000/if_igb.c
index 2921a91..8c6b429 100644
--- a/sys/dev/e1000/if_igb.c
+++ b/sys/dev/e1000/if_igb.c
@@ -350,6 +350,16 @@ TUNABLE_INT("hw.igb.max_interrupt_rate", &igb_max_interrupt_rate);
 SYSCTL_INT(_hw_igb, OID_AUTO, max_interrupt_rate, CTLFLAG_RDTUN,
     &igb_max_interrupt_rate, 0, "Maximum interrupts per second");
 
+#if __FreeBSD_version >= 800000
+/*
+** Tuneable number of buffers in the buf-ring (drbr_xxx)
+*/
+static int igb_buf_ring_size = IGB_BR_SIZE;
+TUNABLE_INT("hw.igb.buf_ring_size", &igb_buf_ring_size);
+SYSCTL_INT(_hw_igb, OID_AUTO, buf_ring_size, CTLFLAG_RDTUN,
+    &igb_buf_ring_size, 0, "Size of the bufring");
+#endif
+
 /*
 ** Header split causes the packet header to
 ** be dma'd to a seperate mbuf from the payload.
@@ -965,12 +975,13 @@ igb_mq_start(struct ifnet *ifp, struct mbuf *m)
 		** out-of-order delivery, but 
 		** settle for it if that fails
 		*/
-		if (m)
+		if (m != NULL)
 			drbr_enqueue(ifp, txr->br, m);
 		err = igb_mq_start_locked(ifp, txr);
 		IGB_TX_UNLOCK(txr);
 	} else {
-		err = drbr_enqueue(ifp, txr->br, m);
+		if (m != NULL)
+			err = drbr_enqueue(ifp, txr->br, m);
 		taskqueue_enqueue(que->tq, &txr->txq_task);
 	}
 
@@ -994,12 +1005,22 @@ igb_mq_start_locked(struct ifnet *ifp, struct tx_ring *txr)
 	enq = 0;
 
 	/* Process the queue */
-	while ((next = drbr_dequeue(ifp, txr->br)) != NULL) {
+	while ((next = drbr_peek(ifp, txr->br)) != NULL) {
 		if ((err = igb_xmit(txr, &next)) != 0) {
-			if (next != NULL)
-				err = drbr_enqueue(ifp, txr->br, next);
+			if (next == NULL) {
+				/* It was freed, move forward */
+				drbr_advance(ifp, txr->br);
+			} else {
+				/* 
+				 * Still have one left, it may not be
+				 * the same since the transmit function
+				 * may have changed it.
+				 */
+				drbr_putback(ifp, txr->br, next);
+			}
 			break;
 		}
+		drbr_advance(ifp, txr->br);
 		enq++;
 		ifp->if_obytes += next->m_pkthdr.len;
 		if (next->m_flags & M_MCAST)
@@ -3301,7 +3322,7 @@ igb_allocate_queues(struct adapter *adapter)
         	}
 #if __FreeBSD_version >= 800000
 		/* Allocate a buf ring */
-		txr->br = buf_ring_alloc(IGB_BR_SIZE, M_DEVBUF,
+		txr->br = buf_ring_alloc(igb_buf_ring_size, M_DEVBUF,
 		    M_WAITOK, &txr->tx_mtx);
 #endif
 	}