From: Richard Cochran <richardcochran@gmail.com>
To: Jeff Epler <jepler@unpythonic.net>
Cc: linux-man <linux-man@vger.kernel.org>,
Laurent Georget <laurent.georget@supelec.fr>,
linux-kernel@vger.kernel.org,
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Subject: Re: [PATCH] adjtimex: PPM scaling is by 2^-16
Date: Fri, 2 Jan 2015 07:29:20 +0100 [thread overview]
Message-ID: <20150102062920.GB4221@localhost.localdomain> (raw)
In-Reply-To: <20150102024118.GA8293@unpythonic.net>
On Thu, Jan 01, 2015 at 08:41:20PM -0600, Jeff Epler wrote:
> Like Laurent Georget, I found by reading ntpd source
> the scale is 2^16 (i.e., 1 ~= 1.5e-5 ppm, 65536 = 1ppm)
Yep, you wont find this documented in prose anywhere, not even at
ntp.org.
> - long freq; /* Frequency offset, as scaled PPM
> + long freq; /* Frequency offset, in units of 2^-16 PPM
> (parts per million) */
This wording is correct.
IMHO, it is more understandable when you explain that this field is a
fixed point number, with a 16 bit binary fraction. For example, see
the comment in the function, ppb_to_scaled_ppm(), in
Documentation/ptp/testptp.c.
Thanks,
Richard
next prev parent reply other threads:[~2015-01-02 6:29 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-30 17:19 [PATCH] adjtimex.2: Add details about ADJ_FREQUENCY Laurent Georget
[not found] ` <54A2DEAD.2050708-vbcOdlJ0SulGWvitb5QawA@public.gmane.org>
2014-12-30 18:05 ` Richard Cochran
[not found] ` <20141230180549.GB7403-bi+AKbBUZKY6gyzm1THtWbp2dZbC/Bob@public.gmane.org>
2014-12-30 18:47 ` Laurent Georget
2015-01-02 2:41 ` [PATCH] adjtimex: PPM scaling is by 2^-16 Jeff Epler
2015-01-02 6:29 ` Richard Cochran [this message]
[not found] ` <20150102062920.GB4221-bi+AKbBUZKY6gyzm1THtWbp2dZbC/Bob@public.gmane.org>
2015-01-02 16:39 ` Laurent Georget
[not found] ` <54A6C9B5.4050904-vbcOdlJ0SulGWvitb5QawA@public.gmane.org>
2015-01-02 18:51 ` Richard Cochran
2015-01-02 21:15 ` Jeff Epler
[not found] ` <20150102211519.GA55815-ixP+gI44yfQ4d9/VWYMlNA@public.gmane.org>
2015-01-02 22:48 ` Laurent Georget
2015-01-02 23:01 ` [PATCH 1/2] adjtimex.2: Clarify the 'ppm scaling' used in struct timex Laurent Georget
2015-01-02 23:07 ` [PATCH 2/2] adjtimex.2: Change 'PPM' (parts per million) to 'ppm' Laurent Georget
[not found] ` <54A724BE.8050409-vbcOdlJ0SulGWvitb5QawA@public.gmane.org>
2015-01-06 15:20 ` Michael Kerrisk (man-pages)
[not found] ` <54ABFD54.9000705-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-01-06 15:42 ` Laurent Georget
[not found] ` <54AC0250.5080005-vbcOdlJ0SulGWvitb5QawA@public.gmane.org>
2015-01-07 5:59 ` Michael Kerrisk (man-pages)
[not found] ` <54ACCB28.50100-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-01-08 16:29 ` Laurent Georget
[not found] ` <54A72360.3070807-vbcOdlJ0SulGWvitb5QawA@public.gmane.org>
2015-01-06 15:19 ` [PATCH 1/2] adjtimex.2: Clarify the 'ppm scaling' used in struct timex Michael Kerrisk (man-pages)
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