conf: add precision option to hwtimestamp directive

conf.c

@@ -1259,13 +1259,17 @@ parse_hwtimestamp(char *line)
 
   iface = ARR_GetNewElement(hwts_interfaces);
   iface->name = Strdup(p);
+  iface->precision = 100.0e-9;
   iface->tx_comp = 0.0;
   iface->rx_comp = 0.0;
 
   for (p = line; *p; line += n, p = line) {
     line = CPS_SplitWord(line);
 
-    if (!strcasecmp(p, "rxcomp")) {
+    if (!strcasecmp(p, "precision")) {
+      if (sscanf(line, "%lf%n", &iface->precision, &n) != 1)
+        break;
+    } else if (!strcasecmp(p, "rxcomp")) {
       if (sscanf(line, "%lf%n", &iface->rx_comp, &n) != 1)
         break;
     } else if (!strcasecmp(p, "txcomp")) {

conf.h

@@ -121,6 +121,7 @@ extern double CNF_GetInitStepThreshold(void);
 
 typedef struct {
   char *name;
+  double precision;
   double tx_comp;
   double rx_comp;
 } CNF_HwTsInterface;

doc/chrony.conf.adoc

@@ -1818,6 +1818,9 @@ on all available interfaces.
 +
 The *hwtimestamp* directive has the following options:
 +
+*precision* _precision_:::
+This option specifies the assumed precision of reading of the NIC clock. The
+default value is 100e-9 (100 nanoseconds).
 *txcomp* _compensation_:::
 This option specifies the difference in seconds between the actual transmission
 time at the physical layer and the reported transmit timestamp. This value will

ntp_io_linux.c

@@ -175,7 +175,7 @@ add_interface(CNF_HwTsInterface *conf_iface)
   iface->l2_udp4_ntp_start = 42;
   iface->l2_udp6_ntp_start = 62;
 
-  iface->precision = 100.0e-9;
+  iface->precision = conf_iface->precision;
   iface->tx_comp = conf_iface->tx_comp;
   iface->rx_comp = conf_iface->rx_comp;