linux-kernel - Re: [PATCH v3 3/6] thermal: Added Bang-bang thermal governor

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Date:	Fri, 18 Jul 2014 18:24:29 +0200
From:	Peter Feuerer <peter@...e.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zhang Rui <rui.zhang@...el.com>, Andreas Mohr <andi@...as.de>,
	Borislav Petkov <bp@...e.de>, Javi Merino <javi.merino@....com>
Subject: Re: [PATCH v3 3/6] thermal: Added Bang-bang thermal governor

Borislav Petkov writes:

> On Sat, May 03, 2014 at 07:59:23PM +0200, Peter Feuerer wrote:
>> The bang-bang thermal governor uses a hysteresis to switch abruptly on
>> or off a cooling device.  It is intended to control fans, which can
>> not be throttled but just switched on or off.
>> Bang-bang cannot be set as default governor as it is intended for
>> special devices only.  For those special devices the driver needs to
>> explicitely request it.
>> 
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Zhang Rui <rui.zhang@...el.com>
>> Cc: Andreas Mohr <andi@...as.de>
>> Cc: Borislav Petkov <bp@...e.de>
>> Cc: Javi Merino <javi.merino@....com>
>> Signed-off-by: Peter Feuerer <peter@...e.net>
> 
> ...
> 
>> +static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
>> +{
>> +	long trip_temp;
>> +	unsigned long trip_hyst;
>> +	struct thermal_instance *instance;
>> +
>> +	tz->ops->get_trip_temp(tz, trip, &trip_temp);
>> +	tz->ops->get_trip_hyst(tz, trip, &trip_hyst);
>> +
>> +	dev_dbg(&tz->device, "Trip%d[temp=%ld]:temp=%d:hyst=%ld\n",
>> +				trip, trip_temp, tz->temperature,
>> +				trip_hyst);
>> +
>> +	mutex_lock(&tz->lock);
>> +
>> +	list_for_each_entry(instance, &tz->thermal_instances, tz_node) {
>> +		if (instance->trip != trip)
>> +			continue;
>> +
>> +		/* in case fan is in initial state, switch the fan off */
>> +		if (instance->target == THERMAL_NO_TARGET)
>> +			instance->target = 0;
>> +
>> +		/* in case fan is neither on nor off set the fan to active */
>> +		if (instance->target != 0 && instance->target != 1) {
> 
> ->target is unsigned long so
> 
> 		if (instance->target > 1)

Right, but I wanted to clearly point out the two possible states.


> (and btw, a whole unsigned long for a cooling state... 2^64-1 cooling
> states, nice).

Hehe, true, maybe u8 makes more sense, so the compiler can take the action to align it accordingly to prevent bit shifting.

> 
>> +			pr_warn("Thermal instance %s controlled by bang-bang has unexpected state: %ld\n",
>> +					instance->name, instance->target);
>> +			instance->target = 1;
>> +		}
> 
> Can that even happen?

It should not, but if so, we have a warning and I have the feeling we should 
keep it there.

-- 
--peter;
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