fan: npcx: fine tune the fan control

The fan used on the Vell platform requires a longer response time than
others. It needs about 1 second to output the TACH signal to EC after EC
starts to toggle the PWM signal. During this period, the fan driver
adjusts the PWM duty per 200 milli-seconds and has the chance to let the
duty reach 100% with the original fan control mechanism. It makes the
user feel the fan spins unsmoothly. To bypass it, this CL steps the PWM
duty not too aggressively when two consecutive RPM reading is zero.

BUG=b:225208265
BRANCH=none
TEST=pass "make buildall"
TEST=set RPM=3000 on VELL, the fan duty doesn't reach 100% afer 1
second.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>
Change-Id: Idc14c859576247759e9119abee5fdeb1f483331d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3539776
Tested-by: Devin Lu <Devin.Lu@quantatw.com>
Reviewed-by: caveh jalali <caveh@chromium.org>
Commit-Queue: CH Lin <chlin56@nuvoton.com>
Tested-by: CH Lin <chlin56@nuvoton.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/chip/npcx/fan.c b/chip/npcx/fan.c
index f0b3215ce0..5b56f33edf 100644
--- a/chip/npcx/fan.c
+++ b/chip/npcx/fan.c
@@ -293,11 +293,21 @@ enum fan_status fan_smart_control(int ch, int rpm_actual, int rpm_target)
 		return FAN_STATUS_CHANGING;
 	}
 
+	/*
+	 * A specific type of fan needs a longer time to output the TACH
+	 * signal to EC after EC outputs the PWM signal to the fan.
+	 * During this period, the driver will read two consecutive RPM = 0.
+	 * In this case, don't step the PWM duty too aggressively.
+	 * See b:225208265 for more detail.
+	 */
+	if (rpm_pre[ch] == 0 && rpm_actual == 0)  {
+		rpm_diff = RPM_MARGIN(rpm_target) + 1;
+	} else {
+		rpm_diff = rpm_target - rpm_actual;
+	}
+
 	/* Record previous rpm */
 	rpm_pre[ch] = rpm_actual;
-
-	/* Adjust PWM duty */
-	rpm_diff = rpm_target - rpm_actual;
 	duty = fan_get_duty(ch);
 	if (duty == 0 && rpm_target == 0)
 		return FAN_STATUS_STOPPED;