> + ts += offset;
I think there is a sign error here. This line of code is attempting to
reconstruct the 'tp' variable in sysoff_estimate(). In other words, ts
should become the ethernet device's PHC timestamp at the time of the
best sample.
sysoff_measure writes the approximate CLOCK_REALTIME timestamp ((t1 +
t2) / 2) into the 'ts' variable. However, as written, the line code
quoted above will adjust that timestamp in the wrong direction.
An example:
Inside sysoff_estimate() with SYSOFF_BASIC:
t1=1576539849164524663
t2=1576539849164526165
tp=1576539849919759249
sysoff_measure() writes:
offset=-755233835
delay=1502
ts=1576539849164525414 (halfway between t1 and t2)
so ts += offset is meant to recover tp, 1576539849919759249.
Instead, the code is doing 1576539849164525414 + -755233835 =
1576539848409291579.
Flipping the sign of the offset produces the correct result:
1576539849164525414 + 755233835 = 1576539849919759249
I discovered this because the clock servo would sometimes get
unrecoverably out-of-sync if the offset suddenly dramatically changed,
and I think this is the root cause.
Could you please check my work?
Thanks,
-cliff
On Mon, Nov 12, 2018 at 8:30 AM Miroslav Lichvar <mlich...@redhat.com> wrote:
>
> If synchronizing a PHC to the system clock, use one of the
> PTP_SYS_OFFSET ioctls (if supported) to measure the offset between the
> two clocks. Negate the offset and switch the timestamp before passing
> them to the servo.
>
> This makes the synchronization between PHC and system clock symmetric.
>
> Signed-off-by: Miroslav Lichvar <mlich...@redhat.com>
> ---
> phc2sys.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/phc2sys.c b/phc2sys.c
> index 2cd477a..b8f1ea0 100644
> --- a/phc2sys.c
> +++ b/phc2sys.c
> @@ -790,6 +790,16 @@ static int do_loop(struct node *node, int subscriptions)
> node->phc_readings,
> &offset, &ts, &delay) < 0)
> return -1;
> + } else if (node->master->clkid == CLOCK_REALTIME &&
> + clock->sysoff_method >= 0) {
> + /* use reversed sysoff */
> + if
> (sysoff_measure(CLOCKID_TO_FD(clock->clkid),
> + clock->sysoff_method,
> + node->phc_readings,
> + &offset, &ts, &delay) < 0)
> + return -1;
> + ts += offset;
> + offset = -offset;
> } else {
> /* use phc */
> if (!read_phc(node->master->clkid,
> clock->clkid,
> --
> 2.17.2
>
>
>
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> Linuxptp-devel@lists.sourceforge.net
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