conf: add minpoll option to hwtimestamp directive

conf.c

@@ -1259,6 +1259,7 @@ parse_hwtimestamp(char *line)
 
   iface = ARR_GetNewElement(hwts_interfaces);
   iface->name = Strdup(p);
+  iface->minpoll = 0;
   iface->precision = 100.0e-9;
   iface->tx_comp = 0.0;
   iface->rx_comp = 0.0;
@@ -1266,7 +1267,10 @@ parse_hwtimestamp(char *line)
   for (p = line; *p; line += n, p = line) {
     line = CPS_SplitWord(line);
 
-    if (!strcasecmp(p, "precision")) {
+    if (!strcasecmp(p, "minpoll")) {
+      if (sscanf(line, "%d%n", &iface->minpoll, &n) != 1)
+        break;
+    } else if (!strcasecmp(p, "precision")) {
       if (sscanf(line, "%lf%n", &iface->precision, &n) != 1)
         break;
     } else if (!strcasecmp(p, "rxcomp")) {

conf.h

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

doc/chrony.conf.adoc

@@ -1818,6 +1818,12 @@ on all available interfaces.
 +
 The *hwtimestamp* directive has the following options:
 +
+*minpoll* _poll_:::
+This option specifies the minimum interval between readings of the NIC clock.
+It's defined as a power of two. It should correspond to the minimum polling
+interval of all NTP sources and the minimum expected polling interval of NTP
+clients. The default value is 0 (1 second) and the minimum value is -6 (1/64th
+of a second).
 *precision* _precision_:::
 This option specifies the assumed precision of reading of the NIC clock. The
 default value is 100e-9 (100 nanoseconds).

ntp_io_linux.c

@@ -77,6 +77,9 @@ struct Interface {
 /* Number of PHC readings per HW clock sample */
 #define PHC_READINGS 10
 
+/* Minimum interval between PHC readings */
+#define MIN_PHC_POLL -6
+
 /* Maximum acceptable offset between HW and daemon/kernel timestamp */
 #define MAX_TS_DELAY 1.0
 
@@ -179,7 +182,7 @@ add_interface(CNF_HwTsInterface *conf_iface)
   iface->tx_comp = conf_iface->tx_comp;
   iface->rx_comp = conf_iface->rx_comp;
 
-  iface->clock = HCL_CreateInstance(1.0);
+  iface->clock = HCL_CreateInstance(UTI_Log2ToDouble(MAX(conf_iface->minpoll, MIN_PHC_POLL)));
 
   DEBUG_LOG(LOGF_NtpIOLinux, "Enabled HW timestamping on %s", iface->name);