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 {
+		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.
+		 */
+		temperature-lookup-table = <
+			(-40000) 1673
+			(-38000) 1649
+			(-36000) 1623
+			(-34000) 1596
+			(-32000) 1566
+			(-30000) 1535
+			(-28000) 1502
+			(-26000) 1467
+			(-24000) 1430
+			(-22000) 1392
+			(-20000) 1352
+			(-18000) 1311
+			(-16000) 1269
+			(-14000) 1226
+			(-12000) 1182
+			(-10000) 1138
+			 (-8000) 1093
+			 (-6000) 1049
+			 (-4000) 1004
+			 (-2000) 960
+			       0 917
+			    2000 874
+			    4000 832
+			    6000 791
+			    8000 752
+			   10000 713
+			   12000 676
+			   14000 640
+			   16000 606
+			   18000 573
+			   20000 541
+			   22000 511
+			   24000 483
+			   26000 455
+			   28000 430
+			   30000 405
+			   32000 382
+			   34000 360
+			   36000 340
+			   38000 320
+			   40000 302
+			   42000 285
+			   44000 269
+			   46000 253
+			   48000 239
+			   50000 225
+			   52000 213
+			   54000 201
+			   56000 190
+			   58000 179
+			   60000 169
+			   62000 160
+			   64000 152
+			   66000 143
+			   68000 136
+			   70000 128
+			   72000 122
+			   74000 115
+			   76000 109
+			   78000 104
+			   80000 98
+			   82000 93
+			   84000 89
+			   86000 84
+			   88000 80
+			   90000 76
+			   92000 73
+			   94000 69
+			   96000 66
+			   98000 63>;
+	};
+
 	thermal-zones {
 		chg-skin-thermal {
 			thermal-sensors = <&pm7250b_adc_tm 0>;

-- 
2.53.0

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

> +		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)?

Konrad

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...

Regards
Luca

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..

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)

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

Konrad

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

On Fri, Feb 20, 2026 at 12:26:48PM +0100, Luca Weiss wrote:
> 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.

How is it handled for the Android boots? I assume there are (at least)
two DTBOs and the correct one is being selected somehow (via the msm-id
/ board-id?). Or does ABL pass some kind of battery identifier to the
kernel?

> 
> 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...

-- 
With best wishes
Dmitry

On Sat Feb 21, 2026 at 3:49 AM CET, Dmitry Baryshkov wrote:
> On Fri, Feb 20, 2026 at 12:26:48PM +0100, Luca Weiss wrote:
>> 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.
>
> How is it handled for the Android boots? I assume there are (at least)
> two DTBOs and the correct one is being selected somehow (via the msm-id
> / board-id?). Or does ABL pass some kind of battery identifier to the
> kernel?

On downstream the Linux driver will do the selection, there you have two
batterydata nodes in the dtb with each their qcom,batt-id-kohm property
and the driver will choose the correct one at runtime.

Similar with multiple display panels, but I think there usually the
'detection' happens via what's passed on cmdline from the bootloader.
But not with two dtbs, the driver is selecting the correct panel from
one dtb.

For cameras, the camera stack is 95% in user space, so it's not quite
comparable but also there usually I think there it's trying probe camera
#1, if it fails try probing camera #2.

Regards
Luca

>
>> 
>> 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...

On Mon, Feb 23, 2026 at 08:50:50AM +0100, Luca Weiss wrote:
> On Sat Feb 21, 2026 at 3:49 AM CET, Dmitry Baryshkov wrote:
> > On Fri, Feb 20, 2026 at 12:26:48PM +0100, Luca Weiss wrote:
> >> 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.
> >
> > How is it handled for the Android boots? I assume there are (at least)
> > two DTBOs and the correct one is being selected somehow (via the msm-id
> > / board-id?). Or does ABL pass some kind of battery identifier to the
> > kernel?
> 
> On downstream the Linux driver will do the selection, there you have two
> batterydata nodes in the dtb with each their qcom,batt-id-kohm property
> and the driver will choose the correct one at runtime.
> 
> Similar with multiple display panels, but I think there usually the
> 'detection' happens via what's passed on cmdline from the bootloader.
> But not with two dtbs, the driver is selecting the correct panel from
> one dtb.

I remembered that all panels are a part of a single DTB, I didn't
remember about the batteries. If there is a bootparam, in theory we can
use it to identify the battery. Or we can find how it is being
identified in the first place and use the same logic in the kernel /
userspace.

> 
> For cameras, the camera stack is 95% in user space, so it's not quite
> comparable but also there usually I think there it's trying probe camera
> #1, if it fails try probing camera #2.
> 
> Regards
> Luca
> 
> >
> >> 
> >> 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...
> 

-- 
With best wishes
Dmitry