X-Git-Url: http://git.lttng.org/?a=blobdiff_plain;f=lttng-events.c;h=1e988ebef2c5cf708e55e74399ad40823ba4b310;hb=5283350bf46f4cec3de30e4fc9b215d88d020d98;hp=f0f5d0f80dbe340bf1e4663681ff621acd25f504;hpb=9c1f4643eb4a11d451a979d81389f0c2ff666af2;p=lttng-modules.git

diff --git a/lttng-events.c b/lttng-events.c
index f0f5d0f8..1e988ebe 100644
--- a/lttng-events.c
+++ b/lttng-events.c
@@ -2023,12 +2023,15 @@ int _lttng_event_header_declare(struct lttng_session *session)
  * taken at start of trace.
  * Yes, this is only an approximation. Yes, we can (and will) do better
  * in future versions.
+ * This function may return a negative offset. It may happen if the
+ * system sets the REALTIME clock to 0 after boot.
  */
 static
-uint64_t measure_clock_offset(void)
+int64_t measure_clock_offset(void)
 {
-	uint64_t offset, monotonic[2], realtime;
+	uint64_t monotonic_avg, monotonic[2], realtime;
 	uint64_t tcf = trace_clock_freq();
+	int64_t offset;
 	struct timespec rts = { 0, 0 };
 	unsigned long flags;
 
@@ -2039,7 +2042,7 @@ uint64_t measure_clock_offset(void)
 	monotonic[1] = trace_clock_read64();
 	local_irq_restore(flags);
 
-	offset = (monotonic[0] + monotonic[1]) >> 1;
+	monotonic_avg = (monotonic[0] + monotonic[1]) >> 1;
 	realtime = (uint64_t) rts.tv_sec * tcf;
 	if (tcf == NSEC_PER_SEC) {
 		realtime += rts.tv_nsec;
@@ -2049,7 +2052,7 @@ uint64_t measure_clock_offset(void)
 		do_div(n, NSEC_PER_SEC);
 		realtime += n;
 	}
-	offset = realtime - offset;
+	offset = (int64_t) realtime - monotonic_avg;
 	return offset;
 }
 
@@ -2161,11 +2164,11 @@ int _lttng_session_metadata_statedump(struct lttng_session *session)
 		"	description = \"%s\";\n"
 		"	freq = %llu; /* Frequency, in Hz */\n"
 		"	/* clock value offset from Epoch is: offset * (1/freq) */\n"
-		"	offset = %llu;\n"
+		"	offset = %lld;\n"
 		"};\n\n",
 		trace_clock_description(),
 		(unsigned long long) trace_clock_freq(),
-		(unsigned long long) measure_clock_offset()
+		(long long) measure_clock_offset()
 		);
 	if (ret)
 		goto end;