Hi,
Le 02/01/2015 07:29, Richard Cochran a écrit :
> 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.
>
Ah, ok, I finally understood what you meant by "fractional part".
Perhaps we could explicit it in the man page like in the following patch?
---
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
--
2.0.4
Laurent
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