svn commit: r192869 - head/sys/dev/xen/netfront
Adrian Chadd
adrian at FreeBSD.org
Wed May 27 01:54:27 UTC 2009
Author: adrian
Date: Wed May 27 01:54:26 2009
New Revision: 192869
URL: http://svn.freebsd.org/changeset/base/192869
Log:
Flesh out some inline documentation which hopefully reflect the intended
reality of these functions.
Modified:
head/sys/dev/xen/netfront/netfront.c
Modified: head/sys/dev/xen/netfront/netfront.c
==============================================================================
--- head/sys/dev/xen/netfront/netfront.c Wed May 27 01:45:23 2009 (r192868)
+++ head/sys/dev/xen/netfront/netfront.c Wed May 27 01:54:26 2009 (r192869)
@@ -685,6 +685,23 @@ xn_free_tx_ring(struct netfront_info *sc
#endif
}
+/*
+ * Do some brief math on the number of descriptors available to
+ * determine how many slots are available.
+ *
+ * Firstly - wouldn't something with RING_FREE_REQUESTS() be more applicable?
+ * Secondly - MAX_SKB_FRAGS is a Linux construct which may not apply here.
+ * Thirdly - it isn't used here anyway; the magic constant '24' is possibly
+ * wrong?
+ * The "2" is presumably to ensure there are also enough slots available for
+ * the ring entries used for "options" (eg, the TSO entry before a packet
+ * is queued); I'm not sure why its 2 and not 1. Perhaps to make sure there's
+ * a "free" node in the tx mbuf list (node 0) to represent the freelist?
+ *
+ * This only figures out whether any xenbus ring descriptors are available;
+ * it doesn't at all reflect how many tx mbuf ring descriptors are also
+ * available.
+ */
static inline int
netfront_tx_slot_available(struct netfront_info *np)
{
@@ -1389,6 +1406,12 @@ xn_start_locked(struct ifnet *ifp)
if (m_head == NULL)
break;
+ /*
+ * netfront_tx_slot_available() tries to do some math to
+ * ensure that there'll be enough xenbus ring slots available
+ * for the maximum number of packet fragments (and a couple more
+ * for what I guess are TSO and other ring entry items.)
+ */
if (!netfront_tx_slot_available(sc)) {
IF_PREPEND(&ifp->if_snd, m_head);
ifp->if_drv_flags |= IFF_DRV_OACTIVE;
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