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phc2sys: Fix frequency estimation when synchronizing a PHC to the system clock. 

When synchronizing a PHC to the Linux system clock (CLOCK_REALTIME),
the phc2sys uses the sysoff method, reversing the master and slave
roles.

The offset between a master clock and a slave clock is given by

    offset = slave_ts - master_ts,

and the call to sysoff_measure() provides the 'offset' and 'slave_ts'
values.  The needed local time stamp on the 'master' is given by

    master_ts = slave_ts - offset,

but the code calcuates

    master_ts = slave_ts + offset.

When passed to the servo, the local time stamp is used to estimate the
frequency offset between the two clocks before starting the main
synchronization loop.  The effect of the bug may be seen with a simple
test.  Here is a sample output with the existing code.

    $ sudo testptp -d /dev/ptp1 -f 62400000
    frequency adjustment okay
    $ sudo ./phc2sys -m -q -c eth6 -s CLOCK_REALTIME -O0
    phc2sys[90221.239]: eth6 sys offset 191001318 s0 freq -62400000 delay   5547
    phc2sys[90222.239]: eth6 sys offset 253380897 s1 freq  +8265884 delay   5507
    phc2sys[90223.239]: eth6 sys offset  -8301685 s2 freq    -35801 delay   5487
    phc2sys[90224.239]: eth6 sys offset  -8297136 s2 freq  -2521757 delay   5531
    phc2sys[90225.239]: eth6 sys offset  -5806117 s2 freq  -2519879 delay   5542
    phc2sys[90226.239]: eth6 sys offset  -3317009 s2 freq  -1772606 delay   5495
    phc2sys[90227.240]: eth6 sys offset  -1575231 s2 freq  -1025931 delay   5505
    phc2sys[90228.240]: eth6 sys offset   -580249 s2 freq   -503518 delay   5524
    phc2sys[90229.240]: eth6 sys offset   -107770 s2 freq   -205114 delay   5519
    phc2sys[90230.240]: eth6 sys offset     66298 s2 freq    -63377 delay   5490
    phc2sys[90230.881]: eth6 sys offset     86942 s2 freq    -22844 delay   5495

And this is the output with the bug fix in place.

    $ sudo testptp -d /dev/ptp1 -f 62400000
    frequency adjustment okay
    $ sudo ./phc2sys -m -q -c eth6 -s CLOCK_REALTIME -O0
    phc2sys[90365.624]: eth6 sys offset 311912675 s0 freq -62400000 delay   5490
    phc2sys[90366.624]: eth6 sys offset 374292766 s1 freq  -31098 delay   5642
    phc2sys[90367.624]: eth6 sys offset     -3825 s2 freq  -34923 delay   5617
    phc2sys[90368.625]: eth6 sys offset         6 s2 freq  -32240 delay   5564
    phc2sys[90369.625]: eth6 sys offset      1241 s2 freq  -31003 delay   5605
    phc2sys[90370.625]: eth6 sys offset      1131 s2 freq  -30741 delay   5600
    phc2sys[90371.625]: eth6 sys offset       801 s2 freq  -30732 delay   5621
    phc2sys[90372.625]: eth6 sys offset       458 s2 freq  -30834 delay   5640
    phc2sys[90373.626]: eth6 sys offset       186 s2 freq  -30969 delay   5598
    phc2sys[90374.626]: eth6 sys offset       134 s2 freq  -30965 delay   5599
    phc2sys[90375.626]: eth6 sys offset        43 s2 freq  -31016 delay   5595
    phc2sys[90375.681]: eth6 sys offset       -32 s2 freq  -31078 delay   5541

This patch fixes the issue by correcting the calculation of the local
time stamp value.

Fixes: 8142da41b6 ("phc2sys: Use reversed sysoff when synchronizing to system clock.")
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reported-by: Cliff Spradlin <cspradlin@google.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
master
Richard Cochran 2019-12-24 11:09:34 -08:00
parent a920c13413
commit e0580929f4
1 changed files with 1 additions and 1 deletions
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2
phc2sys.c
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@ -770,8 +770,8 @@ static int do_loop(struct node *node, int subscriptions)
node->phc_readings,
&offset, &ts, &delay) < 0)
return -1;
ts += offset;
offset = -offset;
ts += offset;
} else {
/* use phc */
if (!read_phc(node->master->clkid, clock->clkid,