Re: [PATCH] drivers: thermal: step_wise: add support for hysteresis

[Thara Gopinath <thara.gopinath@linaro.org>]

On 11/21/2019 12:50 AM, Amit Kucheria wrote:
> From: Ram Chandrasekar <rkumbako@codeaurora.org>
> 
> Currently, step wise governor increases the mitigation when the
> temperature goes above a threshold and decreases the mitigation when the
> temperature goes below the threshold. If there is a case where the
> temperature is wavering around the threshold, the mitigation will be
> applied and removed every iteration, which is not very efficient.
> 
> The use of hysteresis temperature could avoid this ping-pong of
> mitigation by relaxing the mitigation to happen only when the
> temperature goes below this lower hysteresis value.
Hi Amit,

Can this not lead to ping-pong around the hysteresis temperature?
If the idea is to minimize ping-pong isn't average a better method?

Warm Regards
Thara

> 
> Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>
> Signed-off-by: Lina Iyer <ilina@codeaurora.org>
> [Rebased patch from downstream]
> Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
> ---
>  drivers/thermal/step_wise.c | 35 ++++++++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/thermal/step_wise.c b/drivers/thermal/step_wise.c
> index 6e051cbd824ff..2c8a34a7cf959 100644
> --- a/drivers/thermal/step_wise.c
> +++ b/drivers/thermal/step_wise.c
> @@ -24,7 +24,7 @@
>   *       for this trip point
>   *    d. if the trend is THERMAL_TREND_DROP_FULL, use lower limit
>   *       for this trip point
> - * If the temperature is lower than a trip point,
> + * If the temperature is lower than a hysteresis temperature,
>   *    a. if the trend is THERMAL_TREND_RAISING, do nothing
>   *    b. if the trend is THERMAL_TREND_DROPPING, use lower cooling
>   *       state for this trip point, if the cooling state already
> @@ -115,30 +115,31 @@ static void update_passive_instance(struct thermal_zone_device *tz,
>  
>  static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
>  {
> -	int trip_temp;
> +	int trip_temp, hyst_temp;
>  	enum thermal_trip_type trip_type;
>  	enum thermal_trend trend;
>  	struct thermal_instance *instance;
> -	bool throttle = false;
> +	bool throttle;
>  	int old_target;
>  
>  	if (trip == THERMAL_TRIPS_NONE) {
> -		trip_temp = tz->forced_passive;
> +		hyst_temp = trip_temp = tz->forced_passive;
>  		trip_type = THERMAL_TRIPS_NONE;
>  	} else {
>  		tz->ops->get_trip_temp(tz, trip, &trip_temp);
> +		hyst_temp = trip_temp;
> +		if (tz->ops->get_trip_hyst) {
> +			tz->ops->get_trip_hyst(tz, trip, &hyst_temp);
> +			hyst_temp = trip_temp - hyst_temp;
> +		}
>  		tz->ops->get_trip_type(tz, trip, &trip_type);
>  	}
>  
>  	trend = get_tz_trend(tz, trip);
>  
> -	if (tz->temperature >= trip_temp) {
> -		throttle = true;
> -		trace_thermal_zone_trip(tz, trip, trip_type);
> -	}
> -
> -	dev_dbg(&tz->device, "Trip%d[type=%d,temp=%d]:trend=%d,throttle=%d\n",
> -				trip, trip_type, trip_temp, trend, throttle);
> +	dev_dbg(&tz->device,
> +		"Trip%d[type=%d,temp=%d,hyst=%d]:trend=%d,throttle=%d\n",
> +		trip, trip_type, trip_temp, hyst_temp, trend, throttle);
>  
>  	mutex_lock(&tz->lock);
>  
> @@ -147,6 +148,18 @@ static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
>  			continue;
>  
>  		old_target = instance->target;
> +		throttle = false;
> +		/*
> +		 * Lower the mitigation only if the temperature
> +		 * goes below the hysteresis temperature.
> +		 */
> +		if (tz->temperature >= trip_temp ||
> +		    (tz->temperature >= hyst_temp &&
> +		     old_target != THERMAL_NO_TARGET)) {
> +			throttle = true;
> +			trace_thermal_zone_trip(tz, trip, trip_type);
> +		}
> +
>  		instance->target = get_target_state(instance, trend, throttle);
>  		dev_dbg(&instance->cdev->device, "old_target=%d, target=%d\n",
>  					old_target, (int)instance->target);
>