linux-kernel - Re: [PATCH 2/5] thermal/core: Reset cooling state during cooling device unregistration

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Date:   Tue, 28 Mar 2023 02:46:41 +0000
From:   "Zhang, Rui" <rui.zhang@...el.com>
To:     "rafael@...nel.org" <rafael@...nel.org>
CC:     "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] thermal/core: Reset cooling state during cooling
 device unregistration

On Mon, 2023-03-27 at 17:13 +0200, Rafael J. Wysocki wrote:
> On Mon, Mar 27, 2023 at 4:50 PM Zhang, Rui <rui.zhang@...el.com>
> wrote:
> > On Fri, 2023-03-24 at 14:19 +0100, Rafael J. Wysocki wrote:
> > > On Fri, Mar 24, 2023 at 8:08 AM Zhang Rui <rui.zhang@...el.com>
> > > wrote:
> > > > When unregistering a cooling device, it is possible that the
> > > > cooling
> > > > device has been activated. And once the cooling device is
> > > > unregistered,
> > > > no one will deactivate it anymore.
> > > > 
> > > > Reset cooling state during cooling device unregistration.
> > > > 
> > > > Signed-off-by: Zhang Rui <rui.zhang@...el.com>
> > > > ---
> > > > In theory, this problem that this patch fixes can be triggered
> > > > on a
> > > > platform with ACPI Active cooling, by
> > > > 1. overheat the system to trigger ACPI active cooling
> > > > 2. unload ACPI fan driver
> > > > 3. check if the fan is still spinning
> > > > But I don't have such a system so I didn't trigger then problem
> > > > and
> > > > I
> > > > only did build & boot test.
> > > 
> > > So I'm not sure if this change is actually safe.
> > > 
> > > In the example above, the system will still need the fan to spin
> > > after
> > > the ACPI fan driver is unloaded in order to cool down, won't it?
> > 
> > Then we can argue that the ACPI fan driver should not be unloaded
> > in
> > this case.
> 
> I don't think that whether or not the driver is expected to be
> unloaded at a given time has any bearing on how it should behave when
> actually unloaded.
> 
> Leaving the cooling device in its current state is "safe" from the
> thermal control perspective, but it may affect the general user
> experience (which may include performance too) going forward, so
> there
> is a tradeoff.

Right.
If we don't have a third choice, then the question is simple.
"thermal safety" vs. "user experience"?

I'd vote for "thermal safety" and drop this patch series.
> 
> What do the other cooling device drivers do in general when they get
> removed?

No cooling device driver has extra handling after cdev unregistration.

thanks,
rui