Date: Mon, 06 Feb 2006 15:06:10 -0800 From: Nate Lawson <nate@root.org> To: Oleg Bulyzhin <oleg@FreeBSD.org> Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org, Alan Cox <alc@cs.rice.edu>, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/dev/bge if_bge.c Message-ID: <43E7D662.6000608@root.org> In-Reply-To: <20060206223436.GB57775@lath.rinet.ru> References: <200602020958.k129wWtc066930@repoman.freebsd.org> <20060202100637.GB24350@lath.rinet.ru> <20060205235817.GQ5499@cs.rice.edu> <20060206221141.GA57775@lath.rinet.ru> <43E7CBD5.5090203@root.org> <20060206223436.GB57775@lath.rinet.ru>
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Oleg Bulyzhin wrote: > On Mon, Feb 06, 2006 at 02:21:09PM -0800, Nate Lawson wrote: >>Oleg Bulyzhin wrote: >>> nq = q->m_nextpkt; >>> q->m_nextpkt = NULL; >>> m->m_pkthdr.csum_flags &= q->m_pkthdr.csum_flags; >>>- m->m_pkthdr.csum_data += q->m_pkthdr.csum_data; >>>+ sum = m->m_pkthdr.csum_data + q->m_pkthdr.csum_data; >>>+ m->m_pkthdr.csum_data = (sum & 0xffff) + (sum >> 16); >>> m_cat(m, q); >>> } >>>#ifdef MAC >> >>I'm not familiar with this code. So m->m_pkthdr.csum_data is 32 bits? >>Couldn't the same thing be achieved with making it 16 bits since the add >>will wrap normally? > > It will not work cause it's not just a trivial sum it's so called > "1's complement sum" (refer rfc1071 for details). You're right, but aren't you missing the NOT step? 1. 2's complement sum 2. Add in carry 3. 1's complement of result (not) -- Nate
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