Date: Tue, 25 Jul 2006 19:20:01 +0000 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: David Malone <dwmalone@maths.tcd.ie> Cc: src-committers@FreeBSD.org, cvs-src@FreeBSD.org, "Christian S.J. Peron" <csjp@FreeBSD.org>, cvs-all@FreeBSD.org, Sam Leffler <sam@errno.com>, Jung-uk Kim <jkim@FreeBSD.org> Subject: Re: cvs commit: src/sys/net bpf.c Message-ID: <11633.1153855201@critter.freebsd.dk> In-Reply-To: Your message of "Tue, 25 Jul 2006 20:01:10 %2B0100." <200607252001.aa18647@salmon.maths.tcd.ie>
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In message <200607252001.aa18647@salmon.maths.tcd.ie>, David Malone writes: >> > libpcap does not need to be modified; it works already for >> > wireless. The fact that the DLT is named DLT_IEEE802_11_RADIO is a >> > bit of a misnomer; it's not entirely 802.11-specific. > >> Ah, you mean we just exploit DLT_IEEE802_11_RADIO. Hmm... How about >> processing overhead? Can we synchronize the timestamping with system >> time? > >It sounds to me like a reasonable thing to do would be to pass up >a raw version of the timestamp (as returned by the hardware). You can only do that for a very limited time. To make it work for more than a fraction of a second you would need to grab the following data: timecounter reading timehands->th_counter->tc_gettimecount() timecounter width timehands->th_counter->tc_counter_mask reference count timehands->th_offset_count reference timestamp timehands->th_offset scaling factor timehands->th_scale UTC offset boottimebin In total we're talking 4+4+4+12+8+12 bytes = 44 bytes. At the expense of a subtraction and an AND, you can save 8 bytes by storing only the masked counter delta instead of the raw values. At the expense of a 96 bit addition, you can add the utc offset to the boottimebin, and save another 12 bytes. That would bring it down to 4+12+8 = 24 bytes. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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