Date: Wed, 27 May 2009 01:54:26 +0000 (UTC) From: Adrian Chadd <adrian@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r192869 - head/sys/dev/xen/netfront Message-ID: <200905270154.n4R1sQEb080315@svn.freebsd.org>
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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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