linux-kernel - Re: [PATCH v2 2/2] thermal: core: Add sanity check for polling_delay and passive

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Message-ID: <CAJZ5v0jgTN+6WC9nPGCidMnMjSsYMcMe+m=8Ge7Hr--utefM2Q@mail.gmail.com>
Date: Mon, 8 Jul 2024 15:38:07 +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 v2 2/2] thermal: core: Add sanity check for polling_delay
 and passive_delay

On Mon, Jul 8, 2024 at 2:12 PM Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>
> On 05/07/2024 21:46, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > If polling_delay is nonzero and passive_delay is 0, the thermal zone
> > will use polling except when tz->passive is nonzero, which does not make
> > sense.
> >
> > Also if polling_delay is nonzero and passive_delay is greater than
> > polling_delay, the thermal zone temperature will be updated less often
> > when tz->passive is nonzero.  This does not make sense either.
> >
> > Ensure that none of the above will happen.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >
> > v1 -> v2: The patch actually matches the changelog
> >
> > ---
> >   drivers/thermal/thermal_core.c |    3 +++
> >   1 file changed, 3 insertions(+)
> >
> > Index: linux-pm/drivers/thermal/thermal_core.c
> > ===================================================================
> > --- linux-pm.orig/drivers/thermal/thermal_core.c
> > +++ linux-pm/drivers/thermal/thermal_core.c
> > @@ -1440,6 +1440,9 @@ thermal_zone_device_register_with_trips(
> >               td->threshold = INT_MAX;
> >       }
> >
> > +     if (polling_delay && (passive_delay > polling_delay || !passive_delay))
> > +             passive_delay = polling_delay;
>
> Given this is a system misconfiguration, it would make more sense to
> bail out with -EINVAL. Assigning a default value in the back of the
> caller will never raise its attention and can make a bad configuration
> staying for a long time.

This works except for the case mentioned below.

I think that passive_delay > polling_delay can trigger a -EINVAL, but
(polling_delay && !passive_delay) cannot do it because it is regarded
as a valid case as per the below.

> That said, there are configurations with a passive delay set to zero but
> with a non zero polling delay. For instance, a thermal zone mitigated
> with a fan, so active trip points are set. Another example is when there
> is only critical trip points for a thermal zone.
>
> Actually there are multiple combinations with delays value which may
> look invalid but which are actually valid.
>
> For example, a setup with polling_delay > 0, passive_delay = 0, active
> trip points, cooling map to this active trips, passive trip points
> without cooling map.
>
> IMHO, it is better to do the configuration the system is asking for,
> even if it sounds weird

Except that it doesn't work as expected because if passive_delay = 0,
polling is paused when tz->passive is set.

Thanks!