linux-kernel - Re: [PATCH 1/2] adjtimex.2: Clarify the 'ppm scaling' used in struct timex

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Date:	Tue, 06 Jan 2015 16:19:00 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Laurent Georget <laurent.georget@...elec.fr>,
	linux-man <linux-man@...r.kernel.org>
CC:	mtk.manpages@...il.com, linux-kernel@...r.kernel.org,
	Jeff Epler <jepler@...ythonic.net>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH 1/2] adjtimex.2: Clarify the 'ppm scaling' used in struct
 timex

Hi Laurent,

On 01/03/2015 12:01 AM, Laurent Georget wrote:
> Hi,
> 
> this is the last version of a new series of patchs for adjtimex.2.
> Please ignore all previous versions of this series.
> 
> This patch makes explicit and clarifies the unit used for fields 
> "freq", "ppsfreq" and "stabil" in struct timex.
> It closes a FIXME in the man page.

Thanks -- nice work pulling together the input of Richard and Jeff.
Patch applied.

> Reviewed-By: Jeff Epler <jepler@...ythonic.net>

And for good measure I threw in a "Reviewed-by" from Richard as well.

Thanks,

Michael


> 
> ---
>  man2/adjtimex.2 | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/man2/adjtimex.2 b/man2/adjtimex.2
> index 18823c8..ff4b23e 100644
> --- a/man2/adjtimex.2
> +++ b/man2/adjtimex.2
> @@ -54,9 +54,8 @@ struct timex {
>      long offset;      /* Time offset; nanoseconds, if STA_NANO
>                           status flag is set, otherwise
>                           microseconds */
> -    long freq;        /* Frequency offset, as scaled PPM
> -                         (parts per million) */
> -.\" FIXME What is the scaling unit of timex.freq?  2^16 ?
> +    long freq;        /* Frequency offset, in units of 2^-16 PPM
> +                         (parts per million) (see NOTES below) */
>      long maxerror;    /* Maximum error (microseconds) */
>      long esterror;    /* Estimated error (microseconds) */
>      int  status;      /* Clock command/status */
> @@ -72,13 +71,13 @@ struct timex {
>                           flag is set, otherwise microseconds */
>      long tick;        /* Microseconds between clock ticks */
>      long ppsfreq;     /* PPS (pulse per second) frequency
> -                         (scaled PPM, read-only) */
> +                         (2^-16 PPM (see NOTES), read-only) */
>      long jitter;      /* PPS jitter (read-only); nanoseconds, if
>                           STA_NANO status flag is set, otherwise
>                           microseconds */
>      int  shift;       /* PPS interval duration
>                           (seconds, read-only) */
> -    long stabil;      /* PPS stability (scaled PPM, read-only) */
> +    long stabil;      /* PPS stability (2^-16 PPM (see NOTES), read-only) */
>      long jitcnt;      /* PPS jitter limit exceeded (read-only) */
>      long calcnt;      /* PPS calibration intervals (read-only) */
>      long errcnt;      /* PPS calibration errors (read-only) */
> @@ -343,6 +342,18 @@ and the caller does not have sufficient privilege.
>  Under Linux, the
>  .B CAP_SYS_TIME
>  capability is required.
> +.SH NOTES
> +In struct
> +.IR timex ,
> +.IR freq ,
> +.IR ppsfreq ,
> +and 
> +.I stabil
> +are PPM (parts per million) with a 16-bits fractional part, which means that a
> +value of 1 in one of those fields actually means 2^-16 PPM, and 2^16=65535 is 
> +1 PPM. This is the case for both input values (in the case of
> +.IR freq )
> +and output values.
>  .SH CONFORMING TO
>  .BR adjtimex ()
>  is Linux-specific and should not be used in programs
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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