local: make maximum frequency offset configurable

Add maxdrift directive to set the maximum assumed drift of the clock,
which sets the maximum frequency offset chronyd is allowed to use to
to correct the drift.

diff --git a/conf.c b/conf.c
index 682b97174cbaab3a46a935fa54829860cb19b98f..b7c3ddc7f418364e150636da76e03b952aa875a9 100644 (file)
--- a/conf.c
+++ b/conf.c
@@ -85,6 +85,7 @@ static char *rtc_file = NULL;
 static double max_update_skew = 1000.0;
 static double correction_time_ratio = 3.0;
 static double max_clock_error = 1.0; /* in ppm */
+static double max_drift = 500000.0; /* in ppm */
 static double max_slew_rate = 1e6 / 12.0; /* in ppm */
 
 static double max_distance = 3.0;
@@ -489,6 +490,8 @@ CNF_ParseLine(const char *filename, int number, char *line)
     parse_double(p, &max_clock_error);
   } else if (!strcasecmp(command, "maxdistance")) {
     parse_double(p, &max_distance);
+  } else if (!strcasecmp(command, "maxdrift")) {
+    parse_double(p, &max_drift);
   } else if (!strcasecmp(command, "maxsamples")) {
     parse_int(p, &max_samples);
   } else if (!strcasecmp(command, "maxslewrate")) {
@@ -1486,6 +1489,14 @@ CNF_GetMaxUpdateSkew(void)
 
 /* ================================================== */
 
+double
+CNF_GetMaxDrift(void)
+{
+  return max_drift;
+}
+
+/* ================================================== */
+
 double
 CNF_GetMaxClockError(void)
 {

diff --git a/conf.h b/conf.h
index b5ac5cfaec01bfeed0c33b273e7cf1482b38be6e..34c7e41e63ef3a955edf3fc195ffa35293c21dcf 100644 (file)
--- a/conf.h
+++ b/conf.h
@@ -83,6 +83,7 @@ extern char *CNF_GetLeapSecTimezone(void);
 /* Value returned in ppm, as read from file */
 extern double CNF_GetMaxUpdateSkew(void);
 extern double CNF_GetMaxClockError(void);
+extern double CNF_GetMaxDrift(void);
 extern double CNF_GetCorrectionTimeRatio(void);
 extern double CNF_GetMaxSlewRate(void);
 

diff --git a/doc/chrony.conf.adoc b/doc/chrony.conf.adoc
index 55d20caf1072b2842e7af42a096c17ed99753127..bd44b9027b74dbe7e591a52f9289bca635c8d784 100644 (file)
--- a/doc/chrony.conf.adoc
+++ b/doc/chrony.conf.adoc
@@ -805,6 +805,16 @@ By default, the maximum error is 1 ppm.
 Typical values for _error-in-ppm_ might be 10 for a low quality clock and 0.1
 for a high quality clock using a temperature compensated crystal oscillator.
 
+[[maxdrift]]*maxdrift* _drift-in-ppm_::
+This directive specifies the maximum assumed drift (frequency error) of the
+system clock. It limits the frequency adjustment that *chronyd* is allowed to
+use to correct the measured drift. It is an additional limit to the maximum
+adjustment that can be set by the system driver (100000 ppm on Linux, 500 ppm
+on FreeBSD and NetBSD, 32500 ppm on Solaris).
++
+By default, the maximum assumed drift is 500000 ppm, i.e. the adjustment is
+limited by the system driver rather than this directive.
+
 [[maxupdateskew]]*maxupdateskew* _skew-in-ppm_::
 One of *chronyd*'s tasks is to work out how fast or slow the computer's clock
 runs relative to its reference sources. In addition, it computes an estimate of

diff --git a/local.c b/local.c
index 848cc3edf1e8feeb822e57780c14dadba386683f..2ec173eb4243fef13023098e4bd28a946c6c4dbc 100644 (file)
--- a/local.c
+++ b/local.c
@@ -42,13 +42,12 @@
 
 /* ================================================== */
 
-/* Maximum allowed frequency offset in ppm, the time must not stop
-   or run backwards */
-#define MAX_FREQ 500000.0
-
 /* Variable to store the current frequency, in ppm */
 static double current_freq_ppm;
 
+/* Maximum allowed frequency, in ppm */
+static double max_freq_ppm;
+
 /* Temperature compensation, in ppm */
 static double temp_comp_ppm;
 
@@ -166,6 +165,11 @@ LCL_Initialise(void)
 
   calculate_sys_precision();
 
+  /* This is the maximum allowed frequency offset in ppm, the time must
+     never stop or run backwards */
+  max_freq_ppm = CNF_GetMaxDrift();
+  max_freq_ppm = CLAMP(0.0, max_freq_ppm, 500000.0);
+
   max_clock_error = CNF_GetMaxClockError() * 1e-6;
 }
 
@@ -406,12 +410,12 @@ LCL_ReadAbsoluteFrequency(void)
 static double
 clamp_freq(double freq)
 {
-  if (freq <= MAX_FREQ && freq >= -MAX_FREQ)
+  if (freq <= max_freq_ppm && freq >= -max_freq_ppm)
     return freq;
 
   LOG(LOGS_WARN, LOGF_Local, "Frequency %.1f ppm exceeds allowed maximum", freq);
 
-  return freq >= MAX_FREQ ? MAX_FREQ : -MAX_FREQ;
+  return CLAMP(-max_freq_ppm, freq, max_freq_ppm);
 }
 
 /* ================================================== */