Re: Using a temperature sensor with 1-bit output for CPU throttling

[Mason <slash.tmp@free.fr>]

On 29/04/2015 15:47, Mason wrote:

> On 28/04/2015 13:27, Mason wrote:
> 
>> The SoC I'm working on provides a temperature sensor (NXP) in the CPU block.
>> The sensor seems to be very primitive, so I wanted to ask experienced people
>> what would be the best way to use it from Linux.
>>
>> General Description
>> "The sensor generates an output signal that indicates if the die temperature
>> exceeds a programmable threshold. This makes it particularly suitable for
>> detecting overheating."
>>
>> So it seems that the original purpose of this sensor was to periodically
>> check that the temperature has not exceeded a given threshold.
>>
>> - Is the CPU temp higher than 100°C ?
>> - No.
>> - OK. Business as usual.
>>
>> (1 second later)
>> - Is the CPU temp higher than 100°C ?
>> - Yes.
>> - Uh-oh! I need to do something about it.
>>
>>
>> Basic Functions
>> "The temp sensor uses a bandgap type of circuit to compare a voltage which
>> has a negative temperature coefficient with a voltage that is proportional
>> to absolute temperature. A resistor bank allows 40 different temperature
>> thresholds to be selected and the logic output 'out_temperature' will then
>> indicate whether the actual die temperature lies above or below the selected
>> threshold."
>>
>> The available thresholds seem to be chosen somewhat arbitrarily:
>>
>>   -45.1, -39.7, -33.7, -29.4, -24.4, -20.4, -15.4, -10.1,
>>   -6.4, -1.4, 3.6, 7.6, 12.9, 16.6, 20.6, 25.6, 30.9,
>>   34.9, 38.6, 43.9, 48.9, 52.9, 57.9, 61.9, 66.9, 70.9,
>>   76.3, 81.3, 85.3, 90.3, 95.3, 98.9, 102.9, 108.3, 111.9,
>>   117.3, 122.3, 126.3, 131.3, 135.3, 139.3
>>
>> The spacing between values seems arbitrary also.
>> (Is there an underlying physical explanation?)
>>
>> I'm not sure that there is much point in testing for temperatures lower
>> than 50°C ? (I'm told that the SoC can reliably function up to 125°C.)
>>
>> Do higher temperatures shorten the lifespan of a component?
>> In other words, would a CPU running 24/7 at 100°C "break" sooner
>> than one running 24/7 at 50°C ?
>>
>>
>> Characteristics
>>
>> Symbol      Parameter             Min  Typ  Max  Unit
>>
>> (Operating conditions)
>> Tjunc      Junction temperature   -40   25   125  °C
>> Vdd        Supply voltage         1.0  1.1  1.26   V
>>
>> (Normal operating mode)
>> Idd         Supply current              50    60  μA
>> Vbandgapref Ref output voltage   0.72  0.8  0.88   V
>> ∆outtemp    Absolute Temp               ±2   ±10  °C
>>             threshold error
>> T_res       Temp resolution        3    4.5    7  °C
>>
>>
>> Given the semantics of the temperature sensor hardware block, I was
>> tempted to implement something along these lines:
>>
>> Create a kernel thread that runs periodically (e.g. every second)
>> to check if the temperature is above 100°C.
>> - If not, do nothing
>> - If yes, somehow prevent the CPU from using the highest frequencies
>> defined in cpufreq's freq table
>> (They are 1000, 500, 333, 200, 100 MHz)
>>
>> Is that a sensible approach?
>> Is there a way to implement this using the thermal framework?
>>
>> Or am I looking at this wrong, and things should be done a
>> different way? (I'm using 3.14 by the way.)
>>
>> I suppose I could perform some kind of binary search to zoom in
>> on the current threshold (although it might change during the
>> measurements, so I'd rather not go there.)
> 
> I'm aware that I posted many questions. I'd be grateful if someone
> would answer even a tiny subset. That would get the ball rolling.
> 
> If I understand correctly, if I want to use the CPU throttling
> framework, I need to define a "thermal zone device" and a
> "cooling device". AFAIU, the cooling device is taken care of
> by cpu_cooling.c
> 
>   cpufreq_cooling_register(cpu_present_mask);
> 
> My temperature sensor would be the thermal zone device?
> How do I tie the two devices together?
> Is that where a thermal governor comes in play?
> 
> I took a look at the dove_thermal driver, because it seems simple
> enough to understand (by me).
> 
> Looking at ti-soc-thermal/omap?-thermal-data.c
> the lookup table looks familiar. Are they using the same kind
> of technology as my primitive sensor? (bandgap)
> I do note that the precision is much higher though.

Hello everyone,

Is there, perhaps, a better place to discuss these issues?
(IRC, web forum, other mailing list, Stack Overflow, ...)

Regards.