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Flesh out some inline documentation which hopefully reflect the intended

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reality of these functions.
This commit is contained in:
adrian 2009-05-27 01:54:26 +00:00
parent 712491866f
commit fc20aee93b
1 changed files with 23 additions and 0 deletions
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23
sys/dev/xen/netfront/netfront.c
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@ -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;