mt76: mt7915e: Fix degraded performance after temporary overheat

mt7915e registers a cooling_device with wrong semantics:

1. cooling_device expect that higher states values should cool more, but
   mt7915e did the opposite...  with the exception of state == 0, which
   should "disable thermal management", but does not seem to have any
   effect since the previous state is kept.

The result is that when the thermal zone heats up a bit and bumps the
cooling_device state from 0 to 1 to cool a bit, the performance is
destroyed, and when going back from 1 to 0, the performance stays bad.

2. Reading the cooling_device state does not always return the last
   written state, but can return the actual hardware throttle state,
   which is different.

This is a problem because the mt7915 firmware actually implement the
equivalent of a thermal zone with trip points.  Setting the cooling
device state actually changes the throttles at each trip point, so the
following could occur if the first issue is fixed:

- thermal subsystem set state to 100% power (state=0)
- mt7915e driver set trip throttles to [100%, 50%, 25%, 12%]
- hardware heats up and decides to switch to 50% power
- thermal subsystem see that power is 50% (state=50), decide to increase
  it to 60% (state=40) because the rest of the system is cool.
- mt7915e driver set trip throttle to [60%, 30%, 15%, 7%]
- hardware thus switches to 30% power
[race to the bottom continues...]

This patch corrects the semantics of the cooling_device to the one that
the thermal subsystem expect it.
Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/init.c

@@ -97,7 +97,7 @@ static int
 mt7915_thermal_get_max_throttle_state(struct thermal_cooling_device *cdev,
 				      unsigned long *state)
 {
-	*state = MT7915_THERMAL_THROTTLE_MAX;
+	*state = MT7915_CDEV_THROTTLE_MAX;
 
 	return 0;
 }
@@ -108,7 +108,7 @@ mt7915_thermal_get_cur_throttle_state(struct thermal_cooling_device *cdev,
 {
 	struct mt7915_phy *phy = cdev->devdata;
 
-	*state = phy->throttle_state;
+	*state = phy->cdev_state;
 
 	return 0;
 }
@@ -118,22 +118,27 @@ mt7915_thermal_set_cur_throttle_state(struct thermal_cooling_device *cdev,
 				      unsigned long state)
 {
 	struct mt7915_phy *phy = cdev->devdata;
+	u8 throttling = MT7915_THERMAL_THROTTLE_MAX - state;
 	int ret;
 
-	if (state > MT7915_THERMAL_THROTTLE_MAX)
+	if (state > MT7915_CDEV_THROTTLE_MAX)
 		return -EINVAL;
 
 	if (phy->throttle_temp[0] > phy->throttle_temp[1])
 		return 0;
 
-	if (state == phy->throttle_state)
+	if (state == phy->cdev_state)
 		return 0;
 
-	ret = mt7915_mcu_set_thermal_throttling(phy, state);
+	/*
+	 * cooling_device convention: 0 = no cooling, more = more cooling
+	 * mcu convention: 1 = max cooling, more = less cooling
+	 */
+	ret = mt7915_mcu_set_thermal_throttling(phy, throttling);
 	if (ret)
 		return ret;
 
-	phy->throttle_state = state;
+	phy->cdev_state = state;
 
 	return 0;
 }

drivers/net/wireless/mediatek/mt76/mt7915/mt7915.h

@@ -60,6 +60,7 @@
 #define MT7915_CFEND_RATE_11B		0x03	/* 11B LP, 11M */
 
 #define MT7915_THERMAL_THROTTLE_MAX	100
+#define MT7915_CDEV_THROTTLE_MAX	99
 
 #define MT7915_SKU_RATE_NUM		161
 
@@ -229,6 +230,7 @@ struct mt7915_phy {
 	struct ieee80211_vif *monitor_vif;
 
 	struct thermal_cooling_device *cdev;
+	u8 cdev_state;
 	u8 throttle_state;
 	u32 throttle_temp[2]; /* 0: critical high, 1: maximum */