Re: [PATCH 2/2] dt-bindings: iio: adc: document TS voltage in AXP PMICs

[Evgeny Boger <boger@wirenboard.com>]

(added linux-pm@ list and maintainers)


Actually, on second though, I think it might be doable to add voltage to 
temperature conversion to this driver.

I think since the NTC thermistor belongs to the battery, not charger, 
the thermistor should be described in monitored battery node.
So I propose to extend battery node (power/supply/battery.yaml) by 
adding something like:

thermistor-resistance-temp-table = <25 10000>, <35 6530>, ...;

This driver will then interpolate between points to report temperature.

We can also adjust PMIC voltage thresholds based on this table and 
"alert-celsius" property already described in battery.yaml.

I think the driver should report raw TS voltage as well, because the TS 
pin can also be used as general-purpose ADC pin.




22.11.2021 14:35, Samuel Holland пишет:
> On 11/22/21 5:17 AM, Evgeny Boger wrote:
>> 22.11.2021 13:49, Maxime Ripard пишет:
>>> On Thu, Nov 18, 2021 at 05:12:33PM +0300, Evgeny Boger wrote:
>>>> Most AXPxxx-based reference designs place a 10k NTC thermistor on a
>>>> TS pin. axp20x IIO driver now report the voltage of this pin via
>>>> additional IIO channel. Add new "ts_v" channel to the channel
>>>> description.
>>>>
>>>> Signed-off-by: Evgeny Boger <boger@wirenboard.com>
>>> Would it make sense to put the resistance in the DT as well or is it
>>> made mandatory by Allwinner?
>>>
>>> Maxime
>> Well, I don't think so. Basically, by default AXP20x injects 80uA
>> current into the TS pin and measure the voltage. Then, there are
>> voltage thresholds to stop charging if the battery is too hot or too
>> cold. The default thresholds were calculated by the manufacturer for
>> default 10k resistance and 80uA current. Finally, if TS pin is
>> shorted to GND, the AXP2xx will detect it and won't shut down
>> charging. Note that AXP2xx doesn't convert the measured voltage to
>> temperature.
> Agreed, since the ADC driver only works with voltages, the resistance is
> not relevant to it, so a resistance property does not belong here.
>
>> So while it's possible to use AXP2xx with resistance other than 10k,
>> it will require us to override these protection thresholds.
>> Moreover, if one want to put the actual resistance in DT, then the
>> driver would need to calculate these protection thresholds based on
>> NTC parameters and injection current.
> That means we do need a resistance property for the battery charger
> driver, because it does need to calculate temperature.
>
> Regardless of the reference design, the resistance is variable in
> practice. At least some early v1.0 PinePhones shipped with batteries
> containing a 3 kOhm NTC. And the battery is removable, with an
> off-the-shelf form factor, so users could install aftermarket batteries
> with any NTC resistance.
>
> Right now, people with these batteries are disabling the TS; otherwise
> the PMIC refuses to charge them. It would be good to re-enable the TS by
> coming up with the proper voltages for the min/max thresholds. And there
> are power supply properties we can use to expose the current temperature
> and those thresholds to userspace (at least as read-only).
>
> Regards,
> Samuel