linux-kernel - Re: [PATCH v2 3/4] thermal: Added Bang-bang thermal governor

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Date:	Tue, 29 Apr 2014 18:37:28 +0200
From:	Peter Feuerer <peter@...e.net>
To:	Javi Merino <javi.merino@....com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Zhang Rui <rui.zhang@...el.com>, Andreas Mohr <andi@...as.de>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH v2 3/4] thermal: Added Bang-bang thermal governor

Javi Merino writes:

> On Tue, Apr 29, 2014 at 10:17:56AM +0100, 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.
> 
> I don't really understand why step-wise doesn't work for you (AIUI,
> this governor should be a subset of it.  I'll let others comment on
> that, just a minor comment below.

That's what we thought in the first place too and we've been doing a lot of 
fiddling in the past to somehow fit acerhdf into stepwise. But it never 
really fitted and it broke all the time. Furthermore stepwise has a lot of 
features like trend analysis, which is just overkill for the two step 
regulation of acerhdf.

If I recall correctly, even Rui claimed in some email, that inventing a 
separate governor would be a good long term solution.


> [...]
>> diff --git a/drivers/thermal/gov_bang_bang.c b/drivers/thermal/gov_bang_bang.c
>> new file mode 100644
>> index 0000000..328dde0
>> --- /dev/null
>> +++ b/drivers/thermal/gov_bang_bang.c
>> @@ -0,0 +1,124 @@
>> +/*
>> + *  gov_bang_bang.c - A simple thermal throttling governor using hysteresis
>> + *
>> + *  Copyright (C) 2014 Peter Feuerer <peter@...e.net>
>> + *
>> + *  Based on step_wise.c with following Copyrights:
>> + *  Copyright (C) 2012 Intel Corp
>> + *  Copyright (C) 2012 Durgadoss R <durgadoss.r@...el.com>
>> + *
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License as published by
>> + * the Free Software Foundation, version 2.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
>> + * the GNU General Public License for more details.
>> + *
>> + */
>> +
>> +#include <linux/thermal.h>
>> +
>> +#include "thermal_core.h"
>> +
>> +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 neither on nor off set the fan to active */
>> +		if (instance->target != 0 && instance->target != 1)
>> +			instance->target = 1;
> 
> I think you should add a pr_warn() here to warn the user that the
> governor is being used with a cooling device that seems to support
> more than one cooling state.

Strange thing is, that the first time it is actually called with acerhdf 
attached, it comes in with instance→target = -1 … I did not yet find out, 
why.

I'll further investigate on this and add some warning to it.

thanks for your comments.

-- 
kind regards,
--peter;
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