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@xxxxxxxxxxxxxx>
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.
Right now the charger driver just doesn't touch the default voltage
thresholds at all.
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.
I think we can easily expose *voltage* thresholds as DT properties.
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).
So my idea was to convert voltage to temperature elsewhere. Again, I
personally
think that it makes sense, because the hardware itself (including
charger) doesn't
deal with temperature at all, only with threshold voltages and injection
current.
For instance, in our board we now use hwmon NTC driver, like this:
/* Huge pullup voltage is here to emulate constant current */
bat-temp {
compatible = "murata,ncp15xh103";
pullup-uv = <1000000000>; // 1E9 uV
pullup-ohm = <12500000>; // pullup_uv/80
pulldown-ohm = <0>;
io-channels = <&axp_adc 4>;
};
As far as I know, there are IIO AFE patch set under review, which also add
voltage to temperature conversion.
If we indeed want the axp20x charger driver to convert voltage to
temperature
(and vice versa, for computing charging thresholds), then all that logic
will need to
be reimplemented in driver. Note that there wouldn't be a single
"resistance" property,
instead there are huge resistance vs voltage tables which are slightly
different for
different vendors.
In principle, having temperature reported by power supply driver would
be great. But
I don't see how it can be implemented without a massive effort.
Regards,
Samuel
--
С уважением,
Евгений Богер / Evgeny Boger
CTO, Wiren Board Team
https://wirenboard.com/ru
+7 495 150 66 19 (# 33)
