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

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

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.

Regards.