commit abb09418b16993ecd8289dd459dff91701f4f971
Author: Miroslav Lichvar <mlichvar@redhat.com>
Date:   Wed Apr 19 12:20:14 2017 +0200

    sys_linux: don't drop PHC samples with zero delay
    
    When processing data from the PTP_SYS_OFFSET ioctl, the sample is
    dropped when an interval between two consecutive readings of the system
    clock is negative or zero, assuming the clock has been stepped between
    the two readings.
    
    With a real PHC the interval is normally expected to be at least a
    microsecond, but with a virtual PHC and a low-resolution system clock
    it's possible to get two readings with the same system time. Modify the
    check to drop only samples with a negative delay.

diff --git a/sys_linux.c b/sys_linux.c
index c06112a..649afb0 100644
--- a/sys_linux.c
+++ b/sys_linux.c
@@ -705,9 +705,11 @@ get_phc_sample(int phc_fd, double precision, struct timespec *phc_ts,
     phc_tss[i] = ts2;
     delays[i] = UTI_DiffTimespecsToDouble(&ts3, &ts1);
 
-    if (delays[i] <= 0.0)
+    if (delays[i] < 0.0) {
       /* Step in the middle of a PHC reading? */
+      DEBUG_LOG(LOGF_SysLinux, "Bad PTP_SYS_OFFSET sample delay=%e", delays[i]);
       return 0;
+    }
 
     if (!i || delays[i] < min_delay)
       min_delay = delays[i];