Date: Fri, 17 Dec 2010 07:48:13 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-net@freebsd.org Cc: abcde abcde <abcde0@yahoo.com> Subject: Re: nfe_defrag() routine in nividia ethernet driver Message-ID: <201012170748.13980.jhb@freebsd.org> In-Reply-To: <808782.86181.qm@web53807.mail.re2.yahoo.com> References: <808782.86181.qm@web53807.mail.re2.yahoo.com>
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On Thursday, December 16, 2010 10:53:16 pm abcde abcde wrote: > Hi, we ported the nvidia ethernet driver to our product. It's been OK until > recently we ran into an error condition where packets would get dropped quietly. > The root cause resides in the nfe_encap() routine, where we call nfe_defrag() to > try to reduce the length of the mbuf chain to 32, if it's longer than 32. In the > event the 32 mbufs need more than 32 segments, the subsequent call to > bus_dmamap_load_mbuf_sg() would cause it to return an error then the packet is > subsequently dropped. > > > My questions are, > > 1. there appears to be a generic m_defrag() routine available, which doesn't > stop at 32 and is used by a couple of other drivers (Intel, Broadcom, to name a > few). What was the need for a nvidia version of the defrag routine? The nfe_defrag() routine is a copy of a similar method from another driver. It is now called m_collapse() in newer versions of FreeBSD. m_collapse() is less expensive than m_defrag() in that it can avoid extra allocations if they aren't needed. You can use m_defrag() if you wish, but I would still try nfe_defrag() first and only fall back to m_defrag() if that fails. > 2. The NFE_MAX_SCATTER constant, which limits how many segments can be used, is > defined to be 32, while the corresponding constants for other drivers are 100 or > 64 (again Intel or Broadcom). How was the value 32 picked? Anybody knows the > reasoning behind them? It may very well be a chip-dependent limitation due to something in the hardware or firmware of the nfe(4) devices. -- John Baldwinhelp
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