linux-kernel - Re: [PATCH 2/3] thermal: core: fix: Initialize the max

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Date:	Fri, 03 Oct 2014 10:26:28 +0200
From:	Lukasz Majewski <l.majewski@...sung.com>
To:	Eduardo Valentin <edubezval@...il.com>
Cc:	Zhang Rui <rui.zhang@...el.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Lukasz Majewski <l.majewski@...ess.pl>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] thermal: core: fix: Initialize the max_state variable
 to 0

Hi Eduardo,

> Hello Lukasz,
> 
> On Wed, Sep 24, 2014 at 10:27:11AM +0200, Lukasz Majewski wrote:
> > Pointer to the uninitialized max_state variable is passed to get the
> > maximal cooling state.
> > For CPU cooling device (cpu_cooling.c) the cpufreq_get_max_state()
> > is called, which even when error occurs will return the max_state
> > variable unchanged.
> > Since error for ->get_max_state() is not checked, the automatically
> 
> 
> Good that you added a fix in your series for this.
> 
> 
> > allocated value of max_state is used for (upper > max_state)
> > comparison. For any possible max_state value it is very unlikely
> > that it will be less than upper.
> > As a consequence, the cooling device is bind even without the backed
> > cpufreq table initialized.
> > 
> > This initialization will prevent from accidental binding trip
> > points to cpu freq cooling frequencies when cpufreq driver itself
> > is not yet fully initialized.
> 
> Although I agree with the fix, as long as we also include a check for
> the .get_max_state return value,  I believe the problem you are
> describing is about initialization sequence.

As you pointed out - the problem here is with initialization sequence.
Thermal and cpufreq cores are initialized very early.

However, the get_max_state() for cpu_cooling.c device (as it is the
pervasive way of cooling things) accesses cpufreq policy to get the
freq_table and count available states.

The issue here is with late initialization of cpufreq policy.

Up till now the cpu_cooling device was bind even when the
get_max_state() returned -EINVAL and everything worked after late
cpufreq policy initialization. However, during this time window the
thermal driver is not configured.

> 
> In general, I believe we need a better
> sequencing between thermal and cpufreq subsystems. One way out is to
> include a check for cpufreq driver in the thermal driver, and return
> -EPROBE_DEFER  when cpufreq is not ready.

I think that we could return -EPROBE_DEFER when cpufreq's policy is not
yet available and subscribe to cpufreq notifier to call
bind_cooling_device then.

Let's wait for Zhang opinion since he looks after the thermal core code.

> 
> > 
> > Signed-off-by: Lukasz Majewski <l.majewski@...sung.com>
> > ---
> >  drivers/thermal/thermal_core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/thermal/thermal_core.c
> > b/drivers/thermal/thermal_core.c index 454884a..747618a 100644
> > --- a/drivers/thermal/thermal_core.c
> > +++ b/drivers/thermal/thermal_core.c
> > @@ -927,7 +927,7 @@ int thermal_zone_bind_cooling_device(struct
> > thermal_zone_device *tz, struct thermal_instance *pos;
> >  	struct thermal_zone_device *pos1;
> >  	struct thermal_cooling_device *pos2;
> > -	unsigned long max_state;
> > +	unsigned long max_state = 0;
> >  	int result;
> >  
> >  	if (trip >= tz->trips || (trip < 0 && trip !=
> > THERMAL_TRIPS_NONE)) -- 
> > 2.0.0.rc2
> > 
> 



-- 
Best regards,

Lukasz Majewski

Samsung R&D Institute Poland (SRPOL) | Linux Platform Group
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