Sync fans based on temp instead of duty

Fans may be different sizes, placed in asymmetrical positions, or have
different amounts of venting through the chassis. These characteristics
affect the ability for each fan, separately, to dissipate heat and
generate noise.

Replace syncing fans to the highest duty calculated for each fan, based
on separate thermal sensors, to using the highest reported temperature
across all sensors to calculate each fan's duty for that highest
temperature.

In other words: The old behavior synced fans based on the *output* value
(duty), while this new behavior syncs fans based on the *input* value
(temperature).

This allows tuning fans separately to better manage total system
thermals and mitigate noise.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/board/system76/common/fan.c

@@ -220,18 +220,15 @@ static uint8_t fan_get_duty(const struct Fan *const fan, int16_t temp) {
 }
 
 void fan_update_duty(void) {
-#if defined(FAN2_PWM) && !CONFIG_HAVE_DGPU
-    int16_t dgpu_temp = peci_temp;
+#if CONFIG_HAVE_DGPU
+    int16_t sys_temp = MAX(peci_temp, dgpu_temp);
+#else
+    int16_t sys_temp = peci_temp;
 #endif
 
-    uint8_t fan1_duty = fan_get_duty(&FAN1, peci_temp);
+    uint8_t fan1_duty = fan_get_duty(&FAN1, sys_temp);
 #ifdef FAN2_PWM
-    uint8_t fan2_duty = fan_get_duty(&FAN2, dgpu_temp);
-
-#if SYNC_FANS != 0
-    fan1_duty = MAX(fan1_duty, fan2_duty);
-    fan2_duty = MAX(fan1_duty, fan2_duty);
-#endif // SYNC_FANS
+    uint8_t fan2_duty = fan_get_duty(&FAN2, sys_temp);
 #endif // FAN2_PWM
 
     // set FAN1 duty

src/board/system76/common/include/board/fan.h

@@ -14,10 +14,6 @@
 #define SMOOTH_FANS 1 // default to fan smoothing
 #endif
 
-#ifndef SYNC_FANS
-#define SYNC_FANS 1 // default to syncing fan speeds
-#endif
-
 #if SMOOTH_FANS != 0
 #ifndef SMOOTH_FANS_UP
 #define SMOOTH_FANS_UP 45 // default to ~11 seconds for full ramp-up