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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150102062920.GB4221@localhost.localdomain>
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/