Date: Tue, 25 Jul 2006 15:38:34 -0400 From: Jung-uk Kim <jkim@FreeBSD.org> To: "Poul-Henning Kamp" <phk@phk.freebsd.dk> Cc: src-committers@FreeBSD.org, cvs-src@FreeBSD.org, "Christian S.J. Peron" <csjp@FreeBSD.org>, cvs-all@FreeBSD.org, David Malone <dwmalone@maths.tcd.ie>, Sam Leffler <sam@errno.com> Subject: Re: cvs commit: src/sys/net bpf.c Message-ID: <200607251538.36097.jkim@FreeBSD.org> In-Reply-To: <11633.1153855201@critter.freebsd.dk> References: <11633.1153855201@critter.freebsd.dk>
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On Tuesday 25 July 2006 03:20 pm, Poul-Henning Kamp wrote: > 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. I hit send button before I read this. :-) Yes, that's exactly what I meant. BTW, what's your opinion on extending timecounter API to be able to register/deregister trivial timecounter? Jung-uk Kim
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