linux-kernel - Re: [PATCH 11/15] thermal: thermal: Add support for hardware-tracked trip points

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Date:	Mon, 18 May 2015 14:09:44 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Mikko Perttunen <mikko.perttunen@...si.fi>
Cc:	linux-pm@...r.kernel.org, Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>, kernel@...gutronix.de,
	linux-mediatek@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	Brian Norris <computersforpeace@...il.com>
Subject: Re: [PATCH 11/15] thermal: thermal: Add support for hardware-tracked
 trip points

Hi Mikko,

On Mon, May 18, 2015 at 12:06:50PM +0300, Mikko Perttunen wrote:
> > +	for (i = 0; i < tz->trips; i++) {
> > +		int trip_low;
> > +
> > +		tz->ops->get_trip_temp(tz, i, &trip_temp);
> > +		tz->ops->get_trip_hyst(tz, i, &hysteresis);
> > +
> > +		trip_low = trip_temp - hysteresis;
> > +
> > +		if (trip_low < temp && trip_low > low)
> > +			low = trip_low;
> > +
> > +		if (trip_temp > temp && trip_temp < high)
> > +			high = trip_temp;
> > +	}
> > +
> > +	tz->prev_low_trip = low;
> > +	tz->prev_high_trip = high;
> > +
> > +	dev_dbg(&tz->device, "new temperature boundaries: %d < x < %d\n",
> > +			low, high);
> > +
> > +	tz->ops->set_trips(tz, low, high);
> 
> This should probably do something if set_trips returns an error
> code; at least an error message, perhaps enable polling? I'm not
> exactly sure what safety features the thermal framework has in
> general if errors happen..

Currently a thermal zone has the passive_delay and polling_delay
variables. If these are nonzero the thermal core will always poll. A
purely interrupt driven thermal zone would set these values to zero.
In this case the thermal core has no basis for polling, so we would
have to make up polling intervals when set_trips fails. Another
possibility would be to interpret the *_delay variables as 'when
set_trips is available, do not poll. When something goes wrong, use
*_delay as polling intervals'

> 
> One interesting thing I noticed was that at least the bang-bang
> governor only acts if the temperature is properly smaller than (trip
> temp - hysteresis). So perhaps we should specify the non-tripping
> range as [low, high)? Or we could change bang-bang.

I wonder how we can protect against such off-by-one errors anyway.
Generally a hardware might operate on raw values rather than directly
in temperature values in °C. This means a driver for this must have
celsius_to_raw and raw_to_celsius conversion functions. Now it can
happen that due to rounding errors celsius_to_raw(Tcrit) returns a raw
value that when converted back to celsius is different from the
original value in °C. This would mean the hardware triggers an interrupt
for a trip point and the thermal core does not react because get_temp
actually returns a different temperature than previously programmed as
interrupt trigger. This way we would lose hot (or cold) events.

> 
> > +}
> > +
> >  void thermal_zone_device_update(struct thermal_zone_device *tz)
> >  {
> >  	int temp, ret, count;
> > @@ -479,6 +518,8 @@ void thermal_zone_device_update(struct
> thermal_zone_device *tz)
> >  	dev_dbg(&tz->device, "last_temperature=%d, current_temperature=%d\n",
> >  				tz->last_temperature, tz->temperature);
> >
> > +	thermal_zone_set_trips(tz);
> > +
> >  	for (count = 0; count < tz->trips; count++)
> >  		handle_thermal_trip(tz, count);
> >  }
> 
> set_trips should also be called from temp_store and other places
> that modify values that affect the trip points.

Good point.

Sascha

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