thermal: gov_step_wise: Restore passive polling management

Consider a thermal zone with one passive trip point, a cooling device
with 3 states (0, 1, 2) bound to it, passive polling enabled (nonzero
passive_delay_jiffies) and no regular polling (polling_delay_jiffies
equal to 0) that is managed by the Step-Wise governor.  Suppose that
the initial state of the cooling device is 0 and the zone temperature
is below the trip point to start with.

When the trip point is crossed, tz->passive is incremented by the
thermal core and the governor's .manage() callback is invoked.  It
sets 'throttle' to 'true' for the trip in question and
get_target_state() returns 1 for the instance corresponding to the
cooling device (say that 'upper' and 'lower' are set to 2 and 0 for
it, respectively), so its state changes to 1.

Passive polling is still active for the zone, so next time the
temperature is updated, the governor's .manage() callback will be
invoked again.  If the temperature is still rising, it will change
the state of the cooling device to 2.

Now suppose that next time the zone temperature is updated, it falls
below the trip point, so tz->passive is decremented for the zone (say
it becomes 0 then) and the governor's .manage() callbacks runs.

It finds that the temperature trend for the zone is 'falling' and
'throttle' will be set to 'false' for the trip in question, so the
cooling device's state will be changed to 1.  However, because
tz->polling is 0 for the zone, the governor's .manage() callback
may not be invoked again for a long time and the cooling device's
state will not be reset back to 0.

This can happen because commit 042a3d80 ("thermal: core: Move
passive polling management to the core") removed passive polling
management from the Step-Wise governor.

Before that change, thermal_zone_trip_update() would bump up
tz->passive when changing the target state for a thermal instance
from "no target" to a specific value and it would drop tz->passive
when changing it back to "no target" which would cause passive
polling to be active for the zone until the governor has reset the
states of all cooling devices.  In particular, in the example above
tz->passive would be incremented when changing the state of the
cooling device from 0 to 1 and then it would be still nonzero when
the state of the cooling device was changed from 2 to 1.

To prevent this problem from occurring, restore the passive polling
management in the Step-Wise governor by partially reverting the
commit in question and update the comment in the restored code
to explain its role more clearly.

Fixes: 042a3d80 ("thermal: core: Move passive polling management to the core")
Closes: https://lore.kernel.org/linux-pm/ZmVfcEOxmjUHZTSX@hovoldconsulting.comReported-by: Johan Hovold <johan+linaro@kernel.org>
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/thermal/gov_step_wise.c

@@ -93,6 +93,23 @@ static void thermal_zone_trip_update(struct thermal_zone_device *tz,
 		if (instance->initialized && old_target == instance->target)
 			continue;
 
+		if (trip->type == THERMAL_TRIP_PASSIVE) {
+			/*
+			 * If the target state for this thermal instance
+			 * changes from THERMAL_NO_TARGET to something else,
+			 * ensure that the zone temperature will be updated
+			 * (assuming enabled passive cooling) until it becomes
+			 * THERMAL_NO_TARGET again, or the cooling device may
+			 * not be reset to its initial state.
+			 */
+			if (old_target == THERMAL_NO_TARGET &&
+			    instance->target != THERMAL_NO_TARGET)
+				tz->passive++;
+			else if (old_target != THERMAL_NO_TARGET &&
+				 instance->target == THERMAL_NO_TARGET)
+				tz->passive--;
+		}
+
 		instance->initialized = true;
 
 		mutex_lock(&instance->cdev->lock);