ts2phc: Add holdover support.

If the external PPS signal is generated by a clock with better long-term
stability than the clock synchronizing to it, it can work as a good time
source for some period of time after losing its own time source (e.g.
GPS receiver losing its signal).

Add an option to specify a holdover interval where ts2phc can continue
synchronizing the clock without any ToD information. Allow that only in
the LOCKED_STABLE servo state, which needs to be enabled by the
servo_num_offset_values option.

This is supported only in the non-automatic mode and when the pulse
polarity detection is disabled.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>

config.c

@@ -370,6 +370,7 @@ struct config_item config_tab[] = {
 	PORT_ITEM_INT("ts2phc.channel", 0, 0, INT_MAX),
 	PORT_ITEM_INT("ts2phc.extts_correction", 0, INT_MIN, INT_MAX),
 	PORT_ITEM_ENU("ts2phc.extts_polarity", PTP_RISING_EDGE, extts_polarity_enu),
+	PORT_ITEM_INT("ts2phc.holdover", 0, 0, INT_MAX),
 	PORT_ITEM_INT("ts2phc.master", 0, 0, 1),
 	PORT_ITEM_INT("ts2phc.nmea_baudrate", 9600, 300, INT_MAX),
 	GLOB_ITEM_STR("ts2phc.nmea_remote_host", ""),

ts2phc.8

@@ -170,6 +170,16 @@ with the log level of the message as a number.  The default is an empty string
 (which cannot be set in the configuration file as the option requires an
 argument).
 
+.TP
+.B ts2phc.holdover
+The holdover interval, specified in seconds. When the ToD information stops
+working (e.g. GNSS receiver lost its fix), ts2phc is allowed for the specified
+interval to continue synchronizing the target clock as long as the servo is in
+the SERVO_LOCKED_STABLE state. The servo state needs be enabled by the
+\fBservo_num_offset_values\fP option. The holdover is not supported with the
+\fB-a\fP option and when \fBts2phc.extts_polarity\fP is set to \fIboth\fP.
+The default is 0 (disabled).
+
 .TP
 .B sa_file
 Specifies the location of the file containing Security Associations used

ts2phc.c

@@ -435,9 +435,10 @@ static int ts2phc_pps_source_implicit_tstamp(struct ts2phc_private *priv,
 
 static void ts2phc_synchronize_clocks(struct ts2phc_private *priv, int autocfg)
 {
+	struct timespec source_ts, now;
 	tmv_t source_tmv;
 	struct ts2phc_clock *c;
-	int valid, err;
+	int holdover, valid;
 
 	if (autocfg) {
 		if (!priv->ref_clock) {
@@ -451,9 +452,20 @@ static void ts2phc_synchronize_clocks(struct ts2phc_private *priv, int autocfg)
 			return;
 		}
 	} else {
-		err = ts2phc_pps_source_implicit_tstamp(priv, &source_tmv);
-		if (err < 0)
+		valid = !ts2phc_pps_source_implicit_tstamp(priv, &source_tmv);
+	}
+
+	if (valid) {
+		priv->holdover_start = 0;
+		holdover = 0;
+	} else {
+		clock_gettime(CLOCK_MONOTONIC, &now);
+
+		if (!priv->holdover_start)
+			priv->holdover_start = now.tv_sec;
+		if (now.tv_sec >= priv->holdover_start + priv->holdover_length)
 			return;
+		holdover = 1;
 	}
 
 	LIST_FOREACH(c, &priv->clocks, list) {
@@ -470,6 +482,16 @@ static void ts2phc_synchronize_clocks(struct ts2phc_private *priv, int autocfg)
 			continue;
 		}
 
+		if (holdover) {
+			if (c->servo_state != SERVO_LOCKED_STABLE)
+				continue;
+			source_ts = tmv_to_timespec(ts);
+			if (source_ts.tv_nsec > NS_PER_SEC / 2)
+				source_ts.tv_sec++;
+			source_ts.tv_nsec = 0;
+			source_tmv = timespec_to_tmv(source_ts);
+		}
+
 		offset = tmv_to_nanoseconds(tmv_sub(ts, source_tmv));
 
 		if (c->no_adj) {
@@ -481,8 +503,15 @@ static void ts2phc_synchronize_clocks(struct ts2phc_private *priv, int autocfg)
 		adj = servo_sample(c->servo, offset, tmv_to_nanoseconds(ts),
 				   SAMPLE_WEIGHT, &c->servo_state);
 
-		pr_info("%s offset %10" PRId64 " s%d freq %+7.0f",
-			c->name, offset, c->servo_state, adj);
+		if (holdover && c->servo_state != SERVO_LOCKED_STABLE) {
+			pr_info("%s lost holdover lock (offset %10" PRId64 ")",
+				c->name, offset);
+			continue;
+		}
+
+		pr_info("%s offset %10" PRId64 " s%d freq %+7.0f%s",
+			c->name, offset, c->servo_state, adj,
+			holdover ? " holdover" : "");
 
 		switch (c->servo_state) {
 		case SERVO_UNLOCKED:
@@ -751,6 +780,9 @@ int main(int argc, char *argv[])
 		return -1;
 	}
 
+	priv.holdover_length = config_get_int(cfg, NULL, "ts2phc.holdover");
+	priv.holdover_start = 0;
+
 	while (is_running()) {
 		struct ts2phc_clock *clk;
 

ts2phc.h

@@ -55,6 +55,8 @@ struct ts2phc_private {
 	bool state_changed;
 	LIST_HEAD(port_head, ts2phc_port) ports;
 	LIST_HEAD(clock_head, ts2phc_clock) clocks;
+	int holdover_length;
+	time_t holdover_start;
 };
 
 struct ts2phc_clock *ts2phc_clock_add(struct ts2phc_private *priv,