1 files changed, 82 insertions, 18 deletions

diff --git a/networking/ntpd.c b/networking/ntpd.c
index 17e5c7da6..8205ab271 100644
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -165,8 +165,12 @@
  * Using exact power of 2 (1/8) results in smaller code
  */
 #define SLEW_THRESHOLD 0.125
+//^^^^^^^^^^^^^^^^^^^^^^^^^^ TODO: man adjtimex about tmx.offset:
+// "Since Linux 2.6.26, the supplied value is clamped to the range (-0.5s, +0.5s)"
+// - can use this larger value instead?
+
 /* Stepout threshold (sec). std ntpd uses 900 (11 mins (!)) */
-#define WATCH_THRESHOLD  128
+//UNUSED: #define WATCH_THRESHOLD  128
 /* NB: set WATCH_THRESHOLD to ~60 when debugging to save time) */
 //UNUSED: #define PANIC_THRESHOLD 1000    /* panic threshold (sec) */
 
@@ -419,6 +423,7 @@ struct globals {
         uint8_t  discipline_state;      // doc calls it c.state
         uint8_t  poll_exp;              // s.poll
         int      polladj_count;         // c.count
+        int      FREQHOLD_cnt;
         long     kernel_freq_drift;
         peer_t   *last_update_peer;
         double   last_update_offset;    // c.last
@@ -1034,6 +1039,7 @@ step_time(double offset)
         tval = tvn.tv_sec;
         strftime_YYYYMMDDHHMMSS(buf, sizeof(buf), &tval);
         bb_error_msg("setting time to %s.%06u (offset %+fs)", buf, (unsigned)tvn.tv_usec, offset);
+        //maybe? G.FREQHOLD_cnt = 0;
 
         /* Correct various fields which contain time-relative values: */
 
@@ -1709,39 +1715,96 @@ update_local_clock(peer_t *p)
         tmx.freq = G.discipline_freq_drift * 65536e6;
 #endif
         tmx.modes = ADJ_OFFSET | ADJ_STATUS | ADJ_TIMECONST;// | ADJ_MAXERROR | ADJ_ESTERROR;
-        tmx.constant = (int)G.poll_exp - 4;
-        /* EXPERIMENTAL.
-         * The below if statement should be unnecessary, but...
-         * It looks like Linux kernel's PLL is far too gentle in changing
-         * tmx.freq in response to clock offset. Offset keeps growing
-         * and eventually we fall back to smaller poll intervals.
-         * We can make correction more aggressive (about x2) by supplying
-         * PLL time constant which is one less than the real one.
-         * To be on a safe side, let's do it only if offset is significantly
-         * larger than jitter.
-         */
-        if (G.offset_to_jitter_ratio >= TIMECONST_HACK_GATE)
-                tmx.constant--;
+
         tmx.offset = (long)(offset * 1000000); /* usec */
         if (SLEW_THRESHOLD < STEP_THRESHOLD) {
                 if (tmx.offset > (long)(SLEW_THRESHOLD * 1000000)) {
                         tmx.offset = (long)(SLEW_THRESHOLD * 1000000);
-                        tmx.constant--;
                 }
                 if (tmx.offset < -(long)(SLEW_THRESHOLD * 1000000)) {
                         tmx.offset = -(long)(SLEW_THRESHOLD * 1000000);
-                        tmx.constant--;
                 }
         }
-        if (tmx.constant < 0)
-                tmx.constant = 0;
 
         tmx.status = STA_PLL;
+        if (G.FREQHOLD_cnt != 0) {
+                /* man adjtimex on STA_FREQHOLD:
+                 * "Normally adjustments made via ADJ_OFFSET result in dampened
+                 * frequency adjustments also being made.
+                 * This flag prevents the small frequency adjustment from being
+                 * made when correcting for an ADJ_OFFSET value."
+                 *
+                 * Use this flag for a few first adjustments at the beginning
+                 * of ntpd execution, otherwise even relatively small initial
+                 * offset tend to cause largish changes to in-kernel tmx.freq.
+                 * If ntpd was restarted due to e.g. switch to another network,
+                 * this destroys already well-established tmx.freq value.
+                 */
+                if (G.FREQHOLD_cnt < 0) {
+                        /* Initialize it */
+// Example: a laptop whose clock runs slower when hibernated,
+// after wake up it still has good tmx.freq, but accumulated ~0.5 sec offset:
+// Run with code where initial G.FREQHOLD_cnt was always 8:
+//15:17:52.947 no valid datapoints, no peer selected
+//15:17:56.515 update from:<IP> offset:+0.485133 delay:0.157762 jitter:0.209310 clock drift:-1.393ppm tc:4
+//15:17:57.719 update from:<IP> offset:+0.483825 delay:0.158070 jitter:0.181159 clock drift:-1.393ppm tc:4
+//15:17:59.925 update from:<IP> offset:+0.479504 delay:0.158147 jitter:0.156657 clock drift:-1.393ppm tc:4
+//15:18:33.322 update from:<IP> offset:+0.428119 delay:0.158317 jitter:0.138071 clock drift:-1.393ppm tc:4
+//15:19:06.718 update from:<IP> offset:+0.376932 delay:0.158276 jitter:0.122075 clock drift:-1.393ppm tc:4
+//15:19:39.114 update from:<IP> offset:+0.327022 delay:0.158384 jitter:0.108538 clock drift:-1.393ppm tc:4
+//15:20:12.715 update from:<IP> offset:+0.275596 delay:0.158297 jitter:0.097292 clock drift:-1.393ppm tc:4
+//15:20:45.111 update from:<IP> offset:+0.225715 delay:0.158271 jitter:0.087841 clock drift:-1.393ppm tc:4
+// If allwed to continue, it would start increasing tmx.freq now.
+// Instead, it was ^Ced, and started anew:
+//15:21:15.043 no valid datapoints, no peer selected
+//15:21:17.408 update from:<IP> offset:+0.175910 delay:0.158314 jitter:0.076683 clock drift:-1.393ppm tc:4
+//15:21:19.774 update from:<IP> offset:+0.171784 delay:0.158401 jitter:0.066436 clock drift:-1.393ppm tc:4
+//15:21:22.140 update from:<IP> offset:+0.171660 delay:0.158592 jitter:0.057536 clock drift:-1.393ppm tc:4
+//15:21:22.140 update from:<IP> offset:+0.167126 delay:0.158507 jitter:0.049792 clock drift:-1.393ppm tc:4
+//15:21:55.696 update from:<IP> offset:+0.115223 delay:0.158277 jitter:0.050240 clock drift:-1.393ppm tc:4
+//15:22:29.093 update from:<IP> offset:+0.068051 delay:0.158243 jitter:0.049405 clock drift:-1.393ppm tc:5
+//15:23:02.490 update from:<IP> offset:+0.051632 delay:0.158215 jitter:0.043545 clock drift:-1.393ppm tc:5
+//15:23:34.726 update from:<IP> offset:+0.039984 delay:0.158157 jitter:0.038106 clock drift:-1.393ppm tc:5
+// STA_FREQHOLD no longer set, started increasing tmx.freq now:
+//15:24:06.961 update from:<IP> offset:+0.030968 delay:0.158190 jitter:0.033306 clock drift:+2.387ppm tc:5
+//15:24:40.357 update from:<IP> offset:+0.023648 delay:0.158211 jitter:0.029072 clock drift:+5.454ppm tc:5
+//15:25:13.774 update from:<IP> offset:+0.018068 delay:0.157660 jitter:0.025288 clock drift:+7.728ppm tc:5
+//15:26:19.173 update from:<IP> offset:+0.010057 delay:0.157969 jitter:0.022255 clock drift:+8.361ppm tc:6
+//15:27:26.602 update from:<IP> offset:+0.006737 delay:0.158103 jitter:0.019316 clock drift:+8.792ppm tc:6
+//15:28:33.030 update from:<IP> offset:+0.004513 delay:0.158294 jitter:0.016765 clock drift:+9.080ppm tc:6
+//15:29:40.617 update from:<IP> offset:+0.002787 delay:0.157745 jitter:0.014543 clock drift:+9.258ppm tc:6
+//15:30:47.045 update from:<IP> offset:+0.001324 delay:0.157709 jitter:0.012594 clock drift:+9.342ppm tc:6
+//15:31:53.473 update from:<IP> offset:+0.000007 delay:0.158142 jitter:0.010922 clock drift:+9.343ppm tc:6
+//15:32:58.902 update from:<IP> offset:-0.000728 delay:0.158222 jitter:0.009454 clock drift:+9.298ppm tc:6
+                        /*
+                         * This expression would choose 15 in the above example.
+                         */
+                        G.FREQHOLD_cnt = 8 + ((unsigned)(abs(tmx.offset)) >> 16);
+                }
+                G.FREQHOLD_cnt--;
+                tmx.status |= STA_FREQHOLD;
+        }
         if (G.ntp_status & LI_PLUSSEC)
                 tmx.status |= STA_INS;
         if (G.ntp_status & LI_MINUSSEC)
                 tmx.status |= STA_DEL;
 
+        tmx.constant = (int)G.poll_exp - 4;
+        /* EXPERIMENTAL.
+         * The below if statement should be unnecessary, but...
+         * It looks like Linux kernel's PLL is far too gentle in changing
+         * tmx.freq in response to clock offset. Offset keeps growing
+         * and eventually we fall back to smaller poll intervals.
+         * We can make correction more aggressive (about x2) by supplying
+         * PLL time constant which is one less than the real one.
+         * To be on a safe side, let's do it only if offset is significantly
+         * larger than jitter.
+         */
+        if (G.offset_to_jitter_ratio >= TIMECONST_HACK_GATE)
+                tmx.constant--;
+        if (tmx.constant < 0)
+                tmx.constant = 0;
+
         //tmx.esterror = (uint32_t)(clock_jitter * 1e6);
         //tmx.maxerror = (uint32_t)((sys_rootdelay / 2 + sys_rootdisp) * 1e6);
         rc = adjtimex(&tmx);
@@ -2227,6 +2290,7 @@ static NOINLINE void ntp_init(char **argv)
         if (BURSTPOLL != 0)
                 G.poll_exp = BURSTPOLL; /* speeds up initial sync */
         G.last_script_run = G.reftime = G.last_update_recv_time = gettime1900d(); /* sets G.cur_time too */
+        G.FREQHOLD_cnt = -1;
 
         /* Parse options */
         peers = NULL;