linux-kernel - Re: [PATCH] adjtimex: PPM scaling is by 2^-16

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Date:	Fri, 2 Jan 2015 07:29:20 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Jeff Epler <jepler@...ythonic.net>
Cc:	linux-man <linux-man@...r.kernel.org>,
	Laurent Georget <laurent.georget@...elec.fr>,
	linux-kernel@...r.kernel.org,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH] adjtimex: PPM scaling is by 2^-16

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
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