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Date:      Tue, 25 Jul 2006 15:25:10 -0400
From:      Jung-uk Kim <jkim@FreeBSD.org>
To:        David Malone <dwmalone@maths.tcd.ie>
Cc:        Sam Leffler <sam@errno.com>, src-committers@FreeBSD.org, "Christian S.J. Peron" <csjp@FreeBSD.org>, cvs-all@FreeBSD.org, cvs-src@FreeBSD.org
Subject:   Re: cvs commit: src/sys/net bpf.c
Message-ID:  <200607251525.11623.jkim@FreeBSD.org>
In-Reply-To: <200607252001.aa18647@salmon.maths.tcd.ie>
References:  <200607252001.aa18647@salmon.maths.tcd.ie>

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On Tuesday 25 July 2006 03:01 pm, David Malone wrote:
> > > 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). We'd
> also pass up the regular microtime() timestamp. You can then do any
> postprocessing to syncronise timestamps later in userland?

Nope.  In that case, you actually need to export few more things, 
i.e., current hardware timecounter value, clock frequency, size of 
the timecounter, etc.  Even then, it's going to be hard to get 
correct timeval without exposing few kernel internals.

> > Okay.  But I am worried about timecounter <-> timeval conversion
> > because I want to know timeval delta from system time, not just
> > some timer value.
>
> If we do it in the kernel, then we probably need to look into
> expanding the timecounter infrastructure quite a lot, particularly
> if you want to be able to apply NTP adjustments to multiple
> counters at the same time.

Yes, I know the complexity because I had experimented on few ideas in 
the past.  Maybe phk can give us something magical again. ;-)

Jung-uk Kim



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