linux-kernel - Re: [PATCH 1/2] thermal: core: add initial support for cold and critical

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Message-ID: <CAJZ5v0iN6quW7c3B+e7eDcXFA7wjWkw3nJpxr6fV6SoVH6U_KA@mail.gmail.com>
Date:   Wed, 13 Dec 2023 15:51:07 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Christian Marangi <ansuelsmth@...il.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Lukasz Luba <lukasz.luba@....com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] thermal: core: add initial support for cold and
 critical_cold trip point

On Wed, Dec 13, 2023 at 3:43 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Dec 13, 2023 at 3:20 PM Christian Marangi <ansuelsmth@...il.com> wrote:
> >
> > On Wed, Dec 13, 2023 at 03:12:41PM +0100, Daniel Lezcano wrote:
> > > On 12/12/2023 23:13, Christian Marangi wrote:
> > > > Add initial support for cold and critical_cold trip point. Many if not
> > > > all hwmon and thermal device have normally trip point for hot
> > > > temperature and for cold temperature.
> > > >
> > > > Till now only hot temperature were supported. Add support for also cold
> > > > temperature to permit complete definition of cold trip point in DT.
> > > >
> > > > Thermal driver may use these additional trip point to correctly set
> > > > interrupt for cold temperature values and react based on that with
> > > > various measure like enabling attached heater, forcing higher voltage
> > > > and other specialaized peripherals.
> > > >
> > > > For hwmon drivers this is needed as currently there is a problem with
> > > > setting the full operating range of the device for thermal devices
> > > > defined with hwmon. To better describe the problem, the following
> > > > example is needed:
> > > >
> > > > In the scenario of a simple hwmon with an active trip point declared
> > > > and a cooling device attached, the hwmon subsystem currently set the
> > > > min and max trip point based on the single active trip point.
> > > > Thermal subsystem parse all the trip points and calculate the lowest and
> > > > the highest trip point and calls the .set_trip of hwmon to setup the
> > > > trip points.
> > > >
> > > > The fact that we currently don't have a way to declare the cold/min
> > > > temperature values, makes the thermal subsystem to set the low value as
> > > > -INT_MAX.
> > > > For hwmon drivers that doesn't use clamp_value and actually reject
> > > > invalid values for the trip point, this results in the hwmon settings to
> > > > be rejected.
> > > >
> > > > To permit to pass the correct range of trip point, permit to set in DT
> > > > also cold and critical_cold trip point.
> > > >
> > > > Thermal driver may also define .cold and .critical_cold to act on these
> > > > trip point tripped and apply the required measure.
> > >
> > > Agree with the feature but we need to clarify the semantic of the trip
> > > points first. What actions do we expect for them in order to have like a
> > > mirror reflection of the existing hot trip points.
> > >
> > > What action do you expect with:
> > >
> > >  - 'cold' ?
> > >
> > >  - 'critical_cold' ?
> > >
> > >
> >
> > This is more of a sensible topic but I think it's the thermal driver
> > that needs to implement these. As said in the commit description,
> > examples are setting higher voltage from the attached regulator,
> > enabling some hardware heater.
>
> So how is it different from an active trip point?  There are heating
> rather than cooling devices associated with it, but other than this??

And, of course, the mitigation would trigger when crossed on the way
down and the hysteresis value would be added to the temperature.

So it all looks like a "reverse" active trip point.

> > Maybe with critical cold bigger measure can be applied. Currently for
> > critical trip point we shutdown the system (if the critical ops is not
> > declared) but with critical_cold condition I think it won't work... I
> > expect a system in -40°C would just lock up/glitch so rebooting in that
> > condition won't change a thing...
> >
> > Anyway yes we can define a shutdown by default for that but IMHO it
> > wouldn't make much sense.
>
> So why do you want to add it at all?