Have sis_encap use m_defrag if:

1.  The chain passed in is > 31 fragments long
or
2.  The chain will not fit in the remaining descriptors without
    defragmentation.

This is slightly less clear than other network drivers because the sis
chips share one descriptor list for all packets, it seems.

Before this change, a > 127 fragment chain would get stuck in the IFQUEUE
permanently, bringing all network traffic to a halt.

MFC after:	2 weeks

1 files changed, 25 insertions, 1 deletions

diff --git a/sys/pci/if_sis.c b/sys/pci/if_sis.c
index 7f72c39..fa4006e 100644
--- a/sys/pci/if_sis.c
+++ b/sys/pci/if_sis.c
@@ -1878,9 +1878,33 @@ sis_encap(sc, m_head, txidx)
 {
 	struct sis_desc		*f = NULL;
 	struct mbuf		*m;
-	int			frag, cur, cnt = 0;
+	int			frag, cur, cnt = 0, chainlen = 0;
 
 	/*
+	 * If there's no way we can send any packets, return now.
+	 */
+	if (SIS_TX_LIST_CNT - sc->sis_cdata.sis_tx_cnt < 2)
+		return (ENOBUFS);
+
+	/*
+	 * Count the number of frags in this chain to see if
+	 * we need to m_defrag.  Since the descriptor list is shared
+	 * by all packets, we'll m_defrag long chains so that they
+	 * do not use up the entire list, even if they would fit.
+	 */
+
+	for (m = m_head; m != NULL; m = m->m_next)
+		chainlen++;
+
+	if ((chainlen > SIS_TX_LIST_CNT / 4) ||
+	    ((SIS_TX_LIST_CNT - (chainlen + sc->sis_cdata.sis_tx_cnt)) < 2)) {
+		m = m_defrag(m_head, M_DONTWAIT);
+		if (m == NULL)
+			return (ENOBUFS);
+		m_head = m;
+	}
+	
+	/*
  	 * Start packing the mbufs in this chain into
 	 * the fragment pointers. Stop when we run out
  	 * of fragments or hit the end of the mbuf chain.