linux-kernel - Re: [PATCH v2 2/2] thermal/core: Make deprecated cooling device state change from userspace

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Date:   Tue, 19 Oct 2021 17:43:28 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Lukasz Luba <lukasz.luba@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Amit Kucheria <amitk@...nel.org>
Subject: Re: [PATCH v2 2/2] thermal/core: Make deprecated cooling device state
 change from userspace

On Tue, Oct 19, 2021 at 5:40 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Tue, Oct 19, 2021 at 5:02 PM Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
> >
> > The cooling devices have their cooling device set_cur_state
> > read-writable all the time in the sysfs directory, thus allowing the
> > userspace to act on it.
> >
> > The thermal framework is wrongly used by userspace as a power capping
> > framework by acting on the cooling device opaque state. This one then
> > competes with the in-kernel governor decision.
> >
> > We have seen in out-of-tree kernels, a big number of devices which are
> > abusely declaring themselves as cooling device just to act on their
> > power.
> >
> > The role of the thermal framework is to protect the junction
> > temperature of the silicon. Letting the userspace to play with a
> > cooling device is invalid and potentially dangerous.
> >
> > The powercap framework is the right framework to do power capping and
> > moreover it deals with the aggregation via the dev pm qos.
> >
> > As the userspace governor is marked deprecated and about to be
> > removed, there is no point to keep this file writable also in the
> > future.
> >
> > Emit a warning and deprecate the interface.
> >
> > Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> > ---
> >  drivers/thermal/thermal_sysfs.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sysfs.c
> > index 1c4aac8464a7..730f1361dbef 100644
> > --- a/drivers/thermal/thermal_sysfs.c
> > +++ b/drivers/thermal/thermal_sysfs.c
> > @@ -610,6 +610,8 @@ cur_state_store(struct device *dev, struct device_attribute *attr,
> >         unsigned long state;
> >         int result;
> >
> > +       pr_warn_once("Setting cooling device state is deprecated\n");
>
> Maybe dev_warn_once() even?  I guess it won\t hurt to print it once
> per the affected device?
>
> > +
> >         if (sscanf(buf, "%ld\n", &state) != 1)
> >                 return -EINVAL;
> >
> > --

Also I think that changing the subject to something like "Deprecate
changing cooling device state from userspace" would help.