Re: [PATCH v2 5/5] arm64: dts: qcom: sm7225-fairphone-fp4: Add battery temperature node

["Luca Weiss" <luca.weiss@fairphone.com>]

On Fri Feb 20, 2026 at 11:51 AM CET, Konrad Dybcio wrote:
> On 2/20/26 11:40 AM, Luca Weiss wrote:
>> On Fri Feb 20, 2026 at 11:00 AM CET, Konrad Dybcio wrote:
>>> On 2/20/26 10:19 AM, Luca Weiss wrote:
>>>> Add a generic-adc-thermal node to convert the voltage read by the
>>>> battery temperature ADC into degree Celsius using the provided lookup
>>>> table.
>>>>
>>>> This will later be used as input for the fuel gauge node (QGauge on the
>>>> PM7250B).
>>>>
>>>> Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
>>>> ---
>>>>  arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts | 83 +++++++++++++++++++++++
>>>>  1 file changed, 83 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts b/arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts
>>>> index b697051a0aaa..7857003099a6 100644
>>>> --- a/arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts
>>>> +++ b/arch/arm64/boot/dts/qcom/sm7225-fairphone-fp4.dts
>>>> @@ -108,6 +108,89 @@ rear_cam_sensor: thermal-sensor-rear-cam {
>>>>  		io-channel-names = "sensor-channel";
>>>>  	};
>>>>  
>>>> +	bat_therm_sensor: thermal-sensor-bat-therm {
>>>
>>> nit: this should be a little higher
>> 
>> meh, it's surprisingly easy to miss this sorting stuff. Will fix in v3.
>> 
>>>
>>>> +		compatible = "generic-adc-thermal";
>>>> +		#thermal-sensor-cells = <0>;
>>>> +		#io-channel-cells = <0>;
>>>> +		io-channels = <&pm7250b_adc ADC5_BAT_THERM_30K_PU>;
>>>> +		io-channel-names = "sensor-channel";
>>>> +		/*
>>>> +		 * Voltage to temperature table for 10kΩ (B=3435K) NTC with a
>>>> +		 * 1.875V reference and 30kΩ pull-up.
>>>> +		 */
>>>
>>> I think this looks good. Is this data going to be correct for all/most
>>> devices (i.e. is there a single battery sku)?
>> 
>> Yes, from my info there's just a single battery SKU, so that makes it
>> easy here.
>> 
>> For Fairphone 3 there's two battery SKUs:
>> 
>> * (Fuji) F3AC with NTC 100kOhm B=4100, ID resistor 10kOhm
>> * (Kayo) F3AC1 with NTC 100kOhm B=4050, ID resistor 49.9kOhm
>> 
>> In reality, one can probably ignore the difference between the LUT for
>> either B value since it only differs by a marginal amount, but
>> conceptually I'm not sure how this should really be resolved.
>> 
>> We could have both battery definitions in the dtb, and then the charging
>> driver could determine the battery that's actually present in the
>> system (based on the BATT_ID measurement), but given the design here
>> now, I'm not sure how this temperature lookup table would be propagated
>> to the rest of the system...
>
> The path of least resistance (pun intended) would probably be to make
> generic-adc-thermal consume an ID channel and accept a number of LUTs..

Not the worst idea ;)

>
> That sounds sensible since most battery ID mechanisms are probably also
> ADC-based and one would hope (tm) that the values output by these ADC channels
> would then be distinct enough for the driver to have an easy time confidently
> selecting one of the options (or a fallback)

Charger / fuel guage and everything else battery-related would also need
to get the correct battery properties for the actual one present, not
just this generic-adc-thermal driver.

But I feel like soon DT maintainers will say that Linux shouldn't
dynamically detect hardware that's present and the DT should be the
absolute source of truth. That works fine in simple cases, but in case
of interchangeable batteries, display panels, camera sensors, this won't
work. *Something* needs to determine what's actually there.

And for most of the ways to detect which of those are present in the
device that is booting, you need half a kernel to power up the various
hardware and do some basic communication to figure out what's there. Of
course you could say that's U-Boot's job for example but not sure you
want to add a CCI (I2C), ADC driver and much more...

Regards
Luca

> That said, this is just my guesstimates and perhaps the IIO folks could comment
> on that
>
> Konrad