samus: Only update fan speeds every N seconds

This adds CONFIG_FAN_UPDATE_PERIOD to limit the frequency at which
the fan speeds are updated. Short version: the CPU core temp
fluctuates rapidly, causing the fans turn off and on annoyingly
often (assuming you have good hearing and are in a quiet room).

With this CL, we limit the speed changes to only once every N
seconds. N should be long enough to be less annoying, yet short
enough that the CPU doesn't overheat while we're not looking.

BUG=chrome-os-partner:34789
BRANCH=ToT,samus
TEST=manual

Let it sit quietly, then visit a busy webpage, then let it sit a
while. The fan speed should only change every 10 seconds or so,
not every second.

Change-Id: Id985350394f24d56dc4a1e51af09487ac643285b
Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/250501
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

board/samus/board.h

@@ -53,6 +53,7 @@
 #define CONFIG_CHARGER_DISCHARGE_ON_AC
 #define CONFIG_CHARGER_MIN_BAT_PCT_FOR_POWER_ON 1
 #define CONFIG_FANS 2
+#define CONFIG_FAN_UPDATE_PERIOD 10
 #define CONFIG_GESTURE_DETECTION
 #define CONFIG_GESTURE_SAMPLING_INTERVAL_MS 5
 #undef  CONFIG_HIBERNATE_DELAY_SEC

common/fan.c

@@ -19,6 +19,11 @@
  * things manually. */
 static int thermal_control_enabled[CONFIG_FANS];
 
+#ifdef CONFIG_FAN_UPDATE_PERIOD
+/* Should we ignore the fans for a while? */
+static int fan_update_counter[CONFIG_FANS];
+#endif
+
 #ifndef CONFIG_FAN_RPM_CUSTOM
 /* This is the default implementation. It's only called over [0,100].
  * Convert the percentage to a target RPM. We can't simply scale all
@@ -49,6 +54,13 @@ test_mockable void fan_set_percent_needed(int fan, int pct)
 	if (!thermal_control_enabled[fan])
 		return;
 
+#ifdef CONFIG_FAN_UPDATE_PERIOD
+	/* Only set each fan every so often, to avoid rapid changes. */
+	fan_update_counter[fan] %= CONFIG_FAN_UPDATE_PERIOD;
+	if (fan_update_counter[fan]++)
+		return;
+#endif
+
 	new_rpm = fan_percent_to_rpm(fan, pct);
 	actual_rpm = fan_get_rpm_actual(fans[fan].ch);
 

include/config.h

@@ -551,6 +551,13 @@
  */
 #undef CONFIG_FAN_RPM_CUSTOM
 
+/*
+ * We normally check and update the fans once per second (HOOK_SECOND). If this
+ * is #defined to a postive integer N, we will only update the fans every N
+ * seconds instead.
+ */
+#undef CONFIG_FAN_UPDATE_PERIOD
+
 /*****************************************************************************/
 /* Flash configuration */