Guard against divide by zero.

If a buggy driver or hardware delivers bogus time stamps, then we might
crash with a divide by zero exception.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>

clock.c

@@ -209,6 +209,10 @@ static enum servo_state clock_no_adjust(struct clock *c)
 	if (f->count < f->max_count) {
 		return state;
 	}
+	if (tmv_eq(c->t2, f->ingress1)) {
+		pr_warning("bad timestamps in rate ratio calculation");
+		return state;
+	}
 	/*
 	 * origin2 = c->t1 (+c->path_delay) + c->c1 + c->c2;
 	 */

port.c

@@ -407,6 +407,10 @@ static void port_nrate_calculate(struct port *p, tmv_t t3, tmv_t t4, tmv_t c)
 		return;
 	}
 	origin2 = tmv_add(t3, c);
+	if (tmv_eq(t4, n->ingress1)) {
+		pr_warning("bad timestamps in nrate calculation");
+		return;
+	}
 	n->ratio =
 		tmv_dbl(tmv_sub(origin2, n->origin1)) /
 		tmv_dbl(tmv_sub(t4, n->ingress1));

tmv.h

@@ -51,6 +51,11 @@ static inline tmv_t tmv_div(tmv_t a, int divisor)
 	return a / divisor;
 }
 
+static inline int tmv_eq(tmv_t a, tmv_t b)
+{
+	return a == b ? 1 : 0;
+}
+
 static inline int tmv_is_zero(tmv_t x)
 {
 	return x == ((tmv_t) 0) ? 1 : 0;