[Linuxptp-devel] [PATCH v3 4/4] Extend clockcheck to check for changes in frequency.

[Miroslav L. <mli...@re...>]

Before setting the new frequency offset on a clock update, compare the
current frequency returned by the kernel with the value saved from the
previous update. Print a warning message if the difference is larger
than 1 ppb, allowing for rounding errors in conversion to and from
double. The kernel caches the value set by clock_adjtime() in shifted
ppm, it doesn't request it from the driver, which can have a lower
resulution.

This should detect misconfigurations where multiple processes are trying
to control the clock (e.g. another ptp4l/phc2sys instance or an NTP
client), even when they don't step the clock.

Signed-off-by: Miroslav Lichvar <mli...@re...>
---
 clock.c      |  3 +++
 clockcheck.c | 10 ++++++++++
 clockcheck.h |  8 ++++++++
 phc2sys.c    |  3 +++
 ptp4l.8      |  6 ++++--
 5 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/clock.c b/clock.c
index 46ac9c2..8177e77 100644
--- a/clock.c
+++ b/clock.c
@@ -1776,6 +1776,9 @@ static void clock_step_window(struct clock *c)
 
 static void clock_synchronize_locked(struct clock *c, double adj)
 {
+	if (c->sanity_check) {
+		clockcheck_freq(c->sanity_check, clockadj_get_freq(c->clkid));
+	}
 	clockadj_set_freq(c->clkid, -adj);
 	if (c->clkid == CLOCK_REALTIME) {
 		sysclk_set_sync();
diff --git a/clockcheck.c b/clockcheck.c
index f0141be..b5a69cc 100644
--- a/clockcheck.c
+++ b/clockcheck.c
@@ -123,6 +123,16 @@ void clockcheck_set_freq(struct clockcheck *cc, int freq)
 	cc->freq_known = 1;
 }
 
+int clockcheck_freq(struct clockcheck *cc, int freq)
+{
+	/* Allow difference of 1 ppb due to conversion to/from double */
+	if (cc->freq_known && abs(cc->current_freq - freq) > 1) {
+		pr_warning("clockcheck: clock frequency changed unexpectedly!");
+		return 1;
+	}
+	return 0;
+}
+
 void clockcheck_step(struct clockcheck *cc, int64_t step)
 {
 	if (cc->last_ts)
diff --git a/clockcheck.h b/clockcheck.h
index 1ff86eb..4b09b98 100644
--- a/clockcheck.h
+++ b/clockcheck.h
@@ -54,6 +54,14 @@ int clockcheck_sample(struct clockcheck *cc, uint64_t ts);
  */
 void clockcheck_set_freq(struct clockcheck *cc, int freq);
 
+/**
+ * Check whether the frequency correction did not change unexpectedly.
+ * @param cc   Pointer to a clock check obtained via @ref clockcheck_create().
+ * @param freq Current reading of the frequency correction in ppb.
+ * @return Zero if the frequency did not change, non-zero otherwise.
+ */
+int clockcheck_freq(struct clockcheck *cc, int freq);
+
 /**
  * Inform clock check that the clock was stepped.
  * @param cc   Pointer to a clock check obtained via @ref clockcheck_create().
diff --git a/phc2sys.c b/phc2sys.c
index ebc43e5..88ed00c 100644
--- a/phc2sys.c
+++ b/phc2sys.c
@@ -561,6 +561,9 @@ static void update_clock(struct phc2sys_private *priv, struct clock *clock,
 		/* Fall through. */
 	case SERVO_LOCKED:
 	case SERVO_LOCKED_STABLE:
+		if (clock->sanity_check)
+			clockcheck_freq(clock->sanity_check,
+					clockadj_get_freq(clock->clkid));
 		clockadj_set_freq(clock->clkid, -ppb);
 		if (clock->clkid == CLOCK_REALTIME)
 			sysclk_set_sync();
diff --git a/ptp4l.8 b/ptp4l.8
index 1268802..cd6299f 100644
--- a/ptp4l.8
+++ b/ptp4l.8
@@ -619,8 +619,10 @@ This option used to be called
 The maximum allowed frequency offset between uncorrected clock and the system
 monotonic clock in parts per billion (ppb). This is used as a sanity check of
 the synchronized clock. When a larger offset is measured, a warning message
-will be printed and the servo will be reset. When set to 0, the sanity check is
-disabled. The default is 200000000 (20%).
+will be printed and the servo will be reset. If the frequency correction set by
+ptp4l changes unexpectedly between updates of the clock (e.g. due to another
+process trying to control the clock), a warning message will be printed. When
+set to 0, the sanity check is disabled. The default is 200000000 (20%).
 .TP
 .B initial_delay
 The initial path delay of the clock in nanoseconds used for synchronization of
-- 
2.37.3