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

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Message-ID: <CAJZ5v0iyagOX-bL4XhfGd5H-ubK=HjaniDvYB_ZJhKSfqaS09g@mail.gmail.com>
Date:   Wed, 13 Dec 2023 16:03:40 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Christian Marangi <ansuelsmth@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.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 3:56 PM Christian Marangi <ansuelsmth@...il.com> wrote:
>
> On Wed, Dec 13, 2023 at 03:43:54PM +0100, Rafael J. Wysocki 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??
> >
>
> From what I read from documentation, active trip point are used to
> trigger cooling-device. Cold (and crit_cold) are to setup trip point to
> the device. The device will normally trigger an interrupt

Well, that's how thermal sensors work in general IIUC.

> (or even internally with the correct register set autonomously apply some measure
> to handle the problem)
>
> In theory it's possible to have passive trip point for cold condition
> but still we lack any definition of the lower spectrum of the trip
> point.

Such that it will increase power of some devices in order to warm the
system up?  Fair enough.

> > > 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?
>
> Again it's really to fill a hole we have from a long time... One example
> is the qcom tsens driver that have trip point for cold and crit_cold.
> Those in theory can be set in DT with the trip point but we lack any
> definition for them. (using passive trip point would be confusing IMHO)
>
> Another example is an Aquantia PHY that have register for the cold and
> critical_cold trip point.

My point is about the crit_cold trips in particular.  If there is no
common action to trigger when they are crossed, what are they actually
good for?

> Currently defining a critical trip point for the negative temp results
> in the system shutdown.

Sure.