1 files changed, 81 insertions, 0 deletions
diff --git a/usr.sbin/xntpd/util/precision.c b/usr.sbin/xntpd/util/precision.c
new file mode 100644
index 0000000..69af19f
--- /dev/null
+++ b/usr.sbin/xntpd/util/precision.c
@@ -0,0 +1,81 @@
+#include <sys/types.h>
+#include <sys/time.h>
+
+#define	DEFAULT_SYS_PRECISION	-99
+
+int default_get_precision();
+
+int
+main() {
+	printf("log2(precision) = %d\n", default_get_precision());
+	return 0;
+}
+
+/* Find the precision of the system clock by watching how the current time
+ * changes as we read it repeatedly.
+ *
+ * struct timeval is only good to 1us, which may cause problems as machines
+ * get faster, but until then the logic goes:
+ *
+ * If a machine has precision (i.e. accurate timing info) > 1us, then it will
+ * probably use the "unused" low order bits as a counter (to force time to be
+ * a strictly increaing variable), incrementing it each time any process
+ * requests the time [[ or maybe time will stand still ? ]].
+ *
+ * SO: the logic goes:
+ *
+ *      IF      the difference from the last time is "small" (< MINSTEP)
+ *      THEN    this machine is "counting" with the low order bits
+ *      ELIF    this is not the first time round the loop
+ *      THEN    this machine *WAS* counting, and has now stepped
+ *      ELSE    this machine has precision < time to read clock
+ *
+ * SO: if it exits on the first loop, assume "full accuracy" (1us)
+ *     otherwise, take the log2(observered difference, rounded UP)
+ *
+ * MINLOOPS > 1 ensures that even if there is a STEP between the initial call
+ * and the first loop, it doesn't stop too early.
+ * Making it even greater allows MINSTEP to be reduced, assuming that the
+ * chance of MINSTEP-1 other processes getting in and calling gettimeofday
+ * between this processes's calls.
+ * Reducing MINSTEP may be necessary as this sets an upper bound for the time
+ * to actually call gettimeofday.
+ */
+
+#define	DUSECS	1000000
+#define	HUSECS	(1024 * 1024)
+#define	MINSTEP	5	/* some systems increment uS on each call */
+			/* Don't use "1" as some *other* process may read too*/
+			/*We assume no system actually *ANSWERS* in this time*/
+#define	MAXLOOPS HUSECS	/* Assume precision < .1s ! */
+
+int default_get_precision()
+{
+	struct timeval tp;
+	struct timezone tzp;
+	long last;
+	int i;
+	long diff;
+	long val;
+	int minsteps = 2;	/* need at least this many steps */
+
+	gettimeofday(&tp, &tzp);
+	last = tp.tv_usec;
+	for (i = - --minsteps; i< MAXLOOPS; i++) {
+		gettimeofday(&tp, &tzp);
+		diff = tp.tv_usec - last;
+		if (diff < 0) diff += DUSECS;
+		if (diff > MINSTEP) if (minsteps-- <= 0) break;
+		last = tp.tv_usec;
+	}
+
+	printf("precision calculation given %dus after %d loop%s\n",
+		diff, i, (i==1) ? "" : "s");
+
+	diff = (diff *3)/2;
+        if (i >= MAXLOOPS)      diff = 1; /* No STEP, so FAST machine */
+	if (i == 0)             diff = 1; /* time to read clock >= precision */
+	for (i=0, val=HUSECS; val>0; i--, val >>= 1) if (diff >= val) return i;
+	return DEFAULT_SYS_PRECISION /* Something's BUST, so lie ! */;
+}
+