bugfix: servo: don't throw out precision

When calculating the running average (and the average between two as well), we must first add all items and then divide. Everyone does like this, so we should as well, to avoid loosing the low bits Example: 3 and 5, average is 4: (3+5) / 2 = 4, but 3/2 + 5/2 = 3. Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

bugfix: servo: don't throw out precision
When calculating the running average (and the average between two as well), we must first add all items and then divide. Everyone does like this, so we should as well, to avoid loosing the low bits Example: 3 and 5, average is 4: (3+5) / 2 = 4, but 3/2 + 5/2 = 3. Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
d7b813da · Alessandro Rubini · 9bf96bd8 · d7b813da
Commit d7b813da authored Mar 18, 2013 by Alessandro Rubini
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servo.c proto-standard/servo.c +7 -6

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--- a/proto-standard/servo.c
+++ b/proto-standard/servo.c
@@ -86,13 +86,14 @@ void pp_update_delay(struct pp_instance *ppi, TimeInternal *correction_field)
 	else if (owd_fltr->s_exp > 1 << s)
 		owd_fltr->s_exp = 1 << s;

-	/* filter 'meanPathDelay' */
-	owd_fltr->y = (owd_fltr->s_exp - 1) *
-		owd_fltr->y / owd_fltr->s_exp +
-		(mpd->nanoseconds / 2 +
-		 owd_fltr->nsec_prev / 2) / owd_fltr->s_exp;
-
+	/* Use the average between current value and previous one */
+	mpd->nanoseconds = (mpd->nanoseconds + owd_fltr->nsec_prev) / 2;
 	owd_fltr->nsec_prev = mpd->nanoseconds;
+
+	/* filter 'meanPathDelay' (running average) */
+	owd_fltr->y = (owd_fltr->y * (owd_fltr->s_exp - 1) + mpd->nanoseconds)
+		/ owd_fltr->s_exp;
+
 	mpd->nanoseconds = owd_fltr->y;

 	pp_diag(ppi, servo, 1, "delay filter %d, %d\n",