hwmon : raw reading -> temperature conversion
Hi,
I have a question related to hwmon driver and need suggestions.
I am working on a temperature sensor driver that is hwmon driver.
- The temperature is calculated from raw sensor reading and certain initialization parameters.
- Raw reading obtained from 2 different sensor instances under same conditions can differ. Further, initialization parameters are specific to each hardware instance.
- Expressions with floating point operands are used to compute the temperature value.
In our platform there are multiple kernel level clients to the temperature sensor driver.
Hence I am planning to present temperature to these clients from kernel driver itself.
But looking at the hwmon linux documentation, seems the sensor kernel drivers should report only raw readings.
The raw readings can be converted into required output,
e.g. temperature in this case, by respective user space implementation.
However because of my driver clients being in kernel space, I am thinking of doing fixed point calculations in the sensor driver and get the temperature corresponding to raw sensor readings.
Please let me know if this a correct approach?
If there is a better way, please suggest.
-regards,
Bitan
Quoted 18 lines · Bitan Biswas
Hi,
I have a question related to hwmon driver and need suggestions.
I am working on a temperature sensor driver that is hwmon driver.
- The temperature is calculated from raw sensor reading and certain initialization parameters.
- Raw reading obtained from 2 different sensor instances under same conditions can differ. Further, initialization parameters are specific to each hardware instance.
- Expressions with floating point operands are used to compute the temperature value.In our platform there are multiple kernel level clients to the temperature sensor driver.
Hence I am planning to present temperature to these clients from kernel driver itself.
Quoted 5 lines
But looking at the hwmon linux documentation, seems the sensor kernel drivers should report only raw readings.
The raw readings can be converted into required output,
e.g. temperature in this case, by respective user space implementation.
"raw" means the value as reported to the sensor. For example, for an ADC, the raw value means the voltage in mV as seen on the sensor's input pins. This voltage is the voltage to be reported. Converting it to a "real" voltage as, typically, determined by a set of voltage divider resistors should be done in user space.
For temperature sensors this is a bit more tricky. Presumably you get readings from a thermistor or similar. The hwmon subsystem includes a driver for NTC thermistors; maybe you can get some ideas from it. Maybe you can even use it and/or extend it to support your hardware.
Quoted 6 lines
However because of my driver clients being in kernel space, I am thinking of doing fixed point calculations in the sensor driver and get the temperature corresponding to raw sensor readings.
Please let me know if this a correct approach?
Question is really what chip you are using, and how exactly your hardware looks like. Do you use a generic ADC ? If so, is it already supported in the kernel ? How are the thermistor readings converted and reported to SW ?
Thanks,
Guenter
Hi Guenter,
Thank you for the guidance.
Quoted 12 lines
Quoted 6 lines
But looking at the hwmon linux documentation, seems the sensor kernel drivers should report only raw readings.
The raw readings can be converted into required output,
e.g. temperature in this case, by respective user space implementation."raw" means the value as reported to the sensor. For example, for an ADC, the raw value means the voltage in mV as seen on the sensor's input pins. This voltage is the voltage to be reported. Converting it to a "real" voltage as, typically, determined by a set of voltage divider resistors should be done in user space.
Quoted 4 lines
For temperature sensors this is a bit more tricky. Presumably you get readings from a thermistor or similar. The hwmon subsystem includes a driver for NTC thermistors; maybe you can get some ideas from it. Maybe you can even use it and/or extend it to support your hardware.
The sensor is Tegra3 integrated thermal sensor. It is not a thermistor, but gives a counter reading. The expressions to convert the counter to temperature use characterization data and certain initialization parameters. I looked at the NTC thermistor driver. The expressions I need to use to convert counter value to temperature would need floating point arguments.
Quoted 10 lines
Quoted 7 lines
However because of my driver clients being in kernel space, I am thinking of doing fixed point calculations in the sensor driver and get the temperature corresponding to raw sensor readings.
Please let me know if this a correct approach?
Question is really what chip you are using, and how exactly your hardware looks like. Do you use a generic ADC ? If so, is it already supported in the kernel ? How are the thermistor readings converted and reported to SW ?
I think this is somewhat like an ADC but I need to implement the expressions to convert the counter readings to temperature. Using fixed point approximations instead of the floating point operands, the expressions can be implemented in kernel. The expressions to be used for calculation are specific to chip and I could not find a existing driver that could support this sensor.
There are expressions that can convert the counter readings to
temperature and vice-versa.
1) SW could call the driver callbacks and get the temperature.
2) SW could also request temperature thresholds to be configured for thermal monitoring.
The software that may need to use the temperature are some other kernel drivers.
Is it not allowed to implement the counter raw readings to temperature in the hwmon sensor driver?
Quoted 4 lines
Thanks,
Guenter
Quoted 52 lines · Bitan Biswas
Hi Guenter,
Thank you for the guidance.
Quoted 12 lines
Quoted 6 lines
But looking at the hwmon linux documentation, seems the sensor kernel drivers should report only raw readings.
The raw readings can be converted into required output,
e.g. temperature in this case, by respective user space implementation."raw" means the value as reported to the sensor. For example, for an ADC, the raw value means the voltage in mV as seen on the sensor's input pins. This voltage is the voltage to be reported. Converting it to a "real" voltage as, typically, determined by a set of voltage divider resistors should be done in user space.
Quoted 4 lines
For temperature sensors this is a bit more tricky. Presumably you get readings from a thermistor or similar. The hwmon subsystem includes a driver for NTC thermistors; maybe you can get some ideas from it. Maybe you can even use it and/or extend it to support your hardware.
The sensor is Tegra3 integrated thermal sensor. It is not a thermistor, but gives a counter reading. The expressions to convert the counter to temperature use characterization data and certain initialization parameters. I looked at the NTC thermistor driver. The expressions I need to use to convert counter value to temperature would need floating point arguments.
Quoted 10 lines
Quoted 7 lines
However because of my driver clients being in kernel space, I am thinking of doing fixed point calculations in the sensor driver and get the temperature corresponding to raw sensor readings.
Please let me know if this a correct approach?
Question is really what chip you are using, and how exactly your hardware looks like. Do you use a generic ADC ? If so, is it already supported in the kernel ? How are the thermistor readings converted and reported to SW ?
I think this is somewhat like an ADC but I need to implement the expressions to convert the counter readings to temperature. Using fixed point approximations instead of the floating point operands, the expressions can be implemented in kernel. The expressions to be used for calculation are specific to chip and I could not find a existing driver that could support this sensor.
There are expressions that can convert the counter readings to
temperature and vice-versa.
1) SW could call the driver callbacks and get the temperature.
2) SW could also request temperature thresholds to be configured for thermal monitoring.
Quoted 6 lines
The software that may need to use the temperature are some other kernel drivers.
Is it not allowed to implement the counter raw readings to temperature in the hwmon sensor driver?
If you look at the NTC driver, that is pretty much exactly what it does. It accepts Ohm or Volt as input, and calculates a temperature from it. One can think of a counter reading as to be similar to a resistance reading; if you do that, you have a good starting point for your driver. You can use either fixed point approximations, or a table approach as in the NTC driver.
Thanks,
Guenter
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