linux-kernel - Re: [PATCH v1] thermal: trip: Avoid skipping trips in thermal_zone_set

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Message-ID: <b0bd35b2-25f0-4968-853e-6369cabae307@linaro.org>
Date: Wed, 31 Jul 2024 12:08:28 +0200
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>,
 Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH v1] thermal: trip: Avoid skipping trips in
 thermal_zone_set_trips()

On 30/07/2024 18:46, Rafael J. Wysocki wrote:
> On Tue, Jul 30, 2024 at 4:57 PM Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
>>
>> On 30/07/2024 16:41, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>
>>> Say there are 3 trip points A, B, C sorted in ascending temperature
>>> order with no hysteresis.  If the zone temerature is exactly equal to
>>> B, thermal_zone_set_trips() will set the boundaries to A and C and the
>>> hardware will not catch any crossing of B (either way) until either A
>>> or C is crossed and the boundaries are changed.
>>
>> When the temperature is B, an interrupt fired which led to the
>> thermal_zone_update() and in turn it calls thermal_zone_set_trips()
>>
>> As the current temperature is equal to trip B, it makes sense to set A
>> and C, as B has been processes when handling the thermal trips right
>> before calling thermal_zone_set_trips()
> 
> So say that A, B and C are active trips and the thermal zone uses the
> Bang-bang governor.  Say that each trip point has a fan associated
> with it, so that every time it is crossed on the way up, the fan
> should be turned on, and every time it is crossed on the way down, the
> fan should be turned off.  Denote these fans as f_A, f_B, and f_C,
> respectively.
> 
> Say the initial thermal zone temperature is below A, so the initial
> thermal_zone_set_trips() interval is {-INT_MAX, A}.  The zone
> temperature starts to rise and A is reached, so an interrupt triggers.
> __thermal_zone_device_update() runs and it sees that the zone
> temperature is equal to A, so thermal_zone_set_trips() sets the new
> interval to {-INT_MAX, B} and f_A is turned on.
> 
> Say the zone temperature is still rising, despite f_A being on, and B
> is reached.   __thermal_zone_device_update() runs and it sees that the
> zone temperature is equal to B, so thermal_zone_set_trips() sets the
> new interval to {A, C} and f_B is turned on.
> 
> Say the temperature rises somewhat above B, but does not reach C and
> starts to fall down.  B is crossed on the way down, so f_B should be
> turned off, but nothing happens, because an interrupt will only
> trigger when A is reached.
> 
>> I'm failing to understand the issue you describe
> 
> I hope the above helps.

Yes, I understand now. This is really specific when it is with active 
trip points, so passive delay is not involved.

Thanks for the clarification


>> Were you able to reproduce the issue with emul_temp ?
> 
> I haven't tried, but I'm sure I'd be able to reproduce it.

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