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. -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
