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

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Message-ID: <CAJZ5v0iAd18d3_o8i74ao9tg=JPXhBZ29wMiYBd8xS64B7wGsA@mail.gmail.com>
Date: Tue, 30 Jul 2024 18:46:49 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Daniel Lezcano <daniel.lezcano@...aro.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 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.

> 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.