Date:   Fri, 08 Jul 2022 11:56:45 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>, rafael@...nel.org
Cc:     quic_manafm@...cinc.com, Amit Kucheria <amitk@...nel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] thermal/core: Fix thermal trip cross point

On Thu, 2022-07-07 at 23:45 +0200, Daniel Lezcano wrote:
> The routine doing trip point crossing the way up or down is actually
> wrong.
> 
> A trip point is composed with a trip temperature and a hysteresis.
> 
> The trip temperature is used to detect when the trip point is crossed
> the way up.
> 
> The trip temperature minus the hysteresis is used to detect when the
> trip point is crossed the way down.
> 
> > -----------low--------high------------|
> 
>              |<--------->|
>              |    hyst   |
>              |           |
>              |          -|--> crossed the way up
>              |
>          <---|-- crossed the way down
> 
> For that, there is a two point comparison: the current temperature
> and
> the previous temperature.
> 
> The actual code assumes if the current temperature is greater than
> the
> trip temperature and the previous temperature was lesser, then the
> trip point is crossed the way up. That is true only if we crossed the
> way down the low temperature boundary from the previous temperature
> or
> if the hysteresis is zero. The temperature can decrease between the
> low and high, so the trip point is not crossed the way down and then
> increase again and cross the high temperature raising a new trip
> point
> crossed detection which is incorrect. The same scenario happens when
> crossing the way down.
> 
> The trip point crossing the way up and down must act as parenthesis,
> a
> trip point down must close a trip point up. Today we have multiple
> trip point up without the corresponding trip point down.
> 
> In order to fix that, we store the previous trip point which gives
> the
> information about the previous trip.
> 
> As a sidenote, the thermal_zone_device structure has already the
> prev_trip_low and prev_trip_high information which are used by the
> thermal_zone_set_trips() function. This one can be changed to be
> triggered by the trip temperature crossing function, which makes more
> sense, and the two fields will disappear.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> ---
>  drivers/thermal/thermal_core.c | 32 ++++++++++++++++++++++----------
>  include/linux/thermal.h        |  2 ++
>  2 files changed, 24 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/thermal/thermal_core.c
> b/drivers/thermal/thermal_core.c
> index f66036b3daae..92bc9ddb6904 100644
> --- a/drivers/thermal/thermal_core.c
> +++ b/drivers/thermal/thermal_core.c
> @@ -357,19 +357,30 @@ static void handle_critical_trips(struct
> thermal_zone_device *tz,
>  static void handle_thermal_trip_crossed(struct thermal_zone_device
> *tz, int trip,
>  					int trip_temp, int trip_hyst,
> enum thermal_trip_type trip_type)
>  {
> +	int trip_low_temp = trip_temp - trip_hyst;
> +
>  	if (tz->last_temperature == THERMAL_TEMP_INVALID)
>  		return;
>  
> -	if (tz->last_temperature < trip_temp &&
> -	    tz->temperature >= trip_temp) {
> -		thermal_notify_tz_trip_up(tz->id, trip,
> -					  tz->temperature);
> -	}
> -
> -	if (tz->last_temperature >= trip_temp &&
> -	    tz->temperature < (trip_temp - trip_hyst)) {
> -		thermal_notify_tz_trip_down(tz->id, trip,
> -					    tz->temperature);
> +	/*
> +	 * Due to the hysteresis, a third information is needed to
> +	 * detect when the temperature is wavering between the
> +	 * trip_low_temp and the trip_temp. A trip point is crossed
> +	 * the way up only if the temperature is above it while the
> +	 * previous temperature was below *and* we crossed the
> +	 * trip_temp_low before. The previous trip point give us the
> +	 * previous trip point transition. The similar problem exists
> +	 * when crossing the way down.
> +	 */
> +	if (tz->last_temperature < trip_temp && tz->temperature >=
> trip_temp &&
> +	    trip != tz->prev_trip) {
> +		thermal_notify_tz_trip_up(tz->id, trip, tz-
> >temperature);
> +		tz->prev_trip = trip;
> +		
> +	} else if (tz->last_temperature >= trip_low_temp && tz-
> >temperature < trip_low_temp &&
> +	    trip == tz->prev_trip) {
> +		thermal_notify_tz_trip_down(tz->id, trip, tz-
> >temperature);
> +		tz->prev_trip = trip - 1;

Say, let's assume hysteresis is Zero,
When the temperature increases and we do thermal_notify_tz_trip_up()
for trip 0 and trip 1, tz->prev_trip is set to 1 in this case.
And then the temperature drops below trip 0, we don't have chance to do
thermal_notify_tz_trip_down() for trip 0, because we always handle the
trips in ascending order, and tz->prev_trip is 1 when we do
handle_thermal_trip(0).

thanks,
rui

>  	}
>  }
>  
> @@ -427,6 +438,7 @@ static void thermal_zone_device_init(struct
> thermal_zone_device *tz)
>  {
>  	struct thermal_instance *pos;
>  	tz->temperature = THERMAL_TEMP_INVALID;
> +	tz->prev_trip = -1;
>  	tz->prev_low_trip = -INT_MAX;
>  	tz->prev_high_trip = INT_MAX;
>  	list_for_each_entry(pos, &tz->thermal_instances, tz_node)
> diff --git a/include/linux/thermal.h b/include/linux/thermal.h
> index 231bac2768fb..5b3bfb902d10 100644
> --- a/include/linux/thermal.h
> +++ b/include/linux/thermal.h
> @@ -124,6 +124,7 @@ struct thermal_cooling_device {
>   * @last_temperature:	previous temperature read
>   * @emul_temperature:	emulated temperature when using
> CONFIG_THERMAL_EMULATION
>   * @passive:		1 if you've crossed a passive trip point, 0
> otherwise.
> + * @prev_trip:		previous trip point the thermal zone
> was, -1 if below all of them
>   * @prev_low_trip:	the low current temperature if you've crossed a
> passive
>  			trip point.
>   * @prev_high_trip:	the above current temperature if you've crossed
> a
> @@ -159,6 +160,7 @@ struct thermal_zone_device {
>  	int last_temperature;
>  	int emul_temperature;
>  	int passive;
> +	int prev_trip;
>  	int prev_low_trip;
>  	int prev_high_trip;
>  	atomic_t need_update;

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