thermal/drivers/step_wise: Fix temperature regulation misbehavior

There is a particular situation when the cooling device is cpufreq and the heat
dissipation is not efficient enough where the temperature increases little by
little until reaching the critical threshold and leading to a SoC reset.

The behavior is reproducible on a hikey6220 with bad heat dissipation (eg.
stacked with other boards).

Running a simple C program doing while(1); for each CPU of the SoC makes the
temperature to reach the passive regulation trip point and ends up to the
maximum allowed temperature followed by a reset.

This issue has been also reported by running the libhugetlbfs test suite.

What is observed is a ping pong between two cpu frequencies, 1.2GHz and 900MHz
while the temperature continues to grow.

It appears the step wise governor calls get_target_state() the first time with
the throttle set to true and the trend to 'raising'. The code selects logically
the next state, so the cpu frequency decreases from 1.2GHz to 900MHz, so far so
good. The temperature decreases immediately but still stays greater than the
trip point, then get_target_state() is called again, this time with the
throttle set to true *and* the trend to 'dropping'. From there the algorithm
assumes we have to step down the state and the cpu frequency jumps back to
1.2GHz. But the temperature is still higher than the trip point, so
get_target_state() is called with throttle=1 and trend='raising' again, we jump
to 900MHz, then get_target_state() is called with throttle=1 and
trend='dropping', we jump to 1.2GHz, etc ... but the temperature does not
stabilizes and continues to increase.

[  237.922654] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
[  237.922678] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
[  237.922690] thermal cooling_device0: cur_state=0
[  237.922701] thermal cooling_device0: old_target=0, target=1
[  238.026656] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
[  238.026680] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=1
[  238.026694] thermal cooling_device0: cur_state=1
[  238.026707] thermal cooling_device0: old_target=1, target=0
[  238.134647] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
[  238.134667] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
[  238.134679] thermal cooling_device0: cur_state=0
[  238.134690] thermal cooling_device0: old_target=0, target=1

In this situation the temperature continues to increase while the trend is
oscillating between 'dropping' and 'raising'. We need to keep the current state
untouched if the throttle is set, so the temperature can decrease or a higher
state could be selected, thus preventing this oscillation.

Keeping the next_target untouched when 'throttle' is true at 'dropping' time
fixes the issue.

The following traces show the governor does not change the next state if
trend==2 (dropping) and throttle==1.

[ 2306.127987] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
[ 2306.128009] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
[ 2306.128021] thermal cooling_device0: cur_state=0
[ 2306.128031] thermal cooling_device0: old_target=0, target=1
[ 2306.231991] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
[ 2306.232016] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=1
[ 2306.232030] thermal cooling_device0: cur_state=1
[ 2306.232042] thermal cooling_device0: old_target=1, target=1
[ 2306.335982] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=0,throttle=1
[ 2306.336006] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=0,throttle=1
[ 2306.336021] thermal cooling_device0: cur_state=1
[ 2306.336034] thermal cooling_device0: old_target=1, target=1
[ 2306.439984] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
[ 2306.440008] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=0
[ 2306.440022] thermal cooling_device0: cur_state=1
[ 2306.440034] thermal cooling_device0: old_target=1, target=0

[ ... ]

After a while, if the temperature continues to increase, the next state becomes
2 which is 720MHz on the hikey. That results in the temperature stabilizing
around the trip point.

[ 2455.831982] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
[ 2455.832006] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=0
[ 2455.832019] thermal cooling_device0: cur_state=1
[ 2455.832032] thermal cooling_device0: old_target=1, target=1
[ 2455.935985] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=0,throttle=1
[ 2455.936013] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=0,throttle=0
[ 2455.936027] thermal cooling_device0: cur_state=1
[ 2455.936040] thermal cooling_device0: old_target=1, target=1
[ 2456.043984] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=0,throttle=1
[ 2456.044009] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=0,throttle=0
[ 2456.044023] thermal cooling_device0: cur_state=1
[ 2456.044036] thermal cooling_device0: old_target=1, target=1
[ 2456.148001] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
[ 2456.148028] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
[ 2456.148042] thermal cooling_device0: cur_state=1
[ 2456.148055] thermal cooling_device0: old_target=1, target=2
[ 2456.252009] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
[ 2456.252041] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=0
[ 2456.252058] thermal cooling_device0: cur_state=2
[ 2456.252075] thermal cooling_device0: old_target=2, target=1

IOW, this change is needed to keep the state for a cooling device if the
temperature trend is oscillating while the temperature increases slightly.

Without this change, the situation above leads to a catastrophic crash by a
hardware reset on hikey. This issue has been reported to happen on an OMAP
dra7xx also.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Keerthy <j-keerthy@ti.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Leo Yan <leo.yan@linaro.org>
Tested-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

drivers/thermal/step_wise.c

@@ -31,8 +31,7 @@
  * If the temperature is higher than a trip point,
  *    a. if the trend is THERMAL_TREND_RAISING, use higher cooling
  *       state for this trip point
- *    b. if the trend is THERMAL_TREND_DROPPING, use lower cooling
- *       state for this trip point
+ *    b. if the trend is THERMAL_TREND_DROPPING, do nothing
  *    c. if the trend is THERMAL_TREND_RAISE_FULL, use upper limit
  *       for this trip point
  *    d. if the trend is THERMAL_TREND_DROP_FULL, use lower limit
@@ -94,9 +93,11 @@ static unsigned long get_target_state(struct thermal_instance *instance,
 			if (!throttle)
 				next_target = THERMAL_NO_TARGET;
 		} else {
-			next_target = cur_state - 1;
-			if (next_target > instance->upper)
-				next_target = instance->upper;
+			if (!throttle) {
+				next_target = cur_state - 1;
+				if (next_target > instance->upper)
+					next_target = instance->upper;
+			}
 		}
 		break;
 	case THERMAL_TREND_DROP_FULL: