linux-kernel - Re: [RFC PATCH 03/13] dt-bindings: power: supply: BD72720 managed battery

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Message-ID: <CACRpkdbHBQQnnTUrUzOrYxzQKCzDyy8aNK7w8OEFz-ic8ic1FQ@mail.gmail.com>
Date: Mon, 13 Oct 2025 14:45:50 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Matti Vaittinen <mazziesaccount@...il.com>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>, Lee Jones <lee@...nel.org>, 
	Pavel Machek <pavel@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Sebastian Reichel <sre@...nel.org>, Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>, 
	Bartosz Golaszewski <brgl@...ev.pl>, Andreas Kemnade <andreas@...nade.info>, linux-leds@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-pm@...r.kernel.org, linux-gpio@...r.kernel.org
Subject: Re: [RFC PATCH 03/13] dt-bindings: power: supply: BD72720 managed battery

Hi Matti,

thanks for your patch!

On Tue, Oct 7, 2025 at 10:33 AM Matti Vaittinen
<mazziesaccount@...il.com> wrote:

> The BD72720 PMIC has a battery charger + coulomb counter block. These
> can be used to manage charging of a lithium-ion battery and to do fuel
> gauging.
>
> ROHM has developed a so called "zero-correction" -algotihm to improve

algorithm?

> the fuel-gauging accuracy close to the point where battery is depleted.
> This relies on battery specific "VDR" tables, which are measured from
> the battery, and which describe the voltage drop rate. More thorough
> explanation about the "zero correction" and "VDR" parameters is here:
> https://lore.kernel.org/all/676253b9-ff69-7891-1f26-a8b5bb5a421b@fi.rohmeurope.com/
>
> Document the VDR zero-correction specific battery properties used by the
> BD72720 and some other ROHM chargers.
>
> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>

> The parameters are describing the battery voltage drop rates - so they
> are properties of the battery, not the charger. Thus they do not belong
> in the charger node.

Right!

> The right place for them is the battery node, which is described by the
> generic "battery.yaml". I was not comfortable with adding these
> properties to the generic battery.yaml because they are:
>   - Meaningful only for those charger drivers which have the VDR
>     algorithm implemented. (And even though the algorithm is not charger
>     specific, AFAICS, it is currently only used by some ROHM PMIC
>     drivers).
>   - Technique of measuring the VDR tables for a battery is not widely
>     known. AFAICS, only folks at ROHM are measuring those for some
>     customer products. We do have those tables available for some of the
>     products though (Kobo?).

It would be sad if we later on have to convert it to a standard property
because it turns out to be wider used than we know.

But I buy your reasoning!

> +properties:
> +  rohm,voltage-vdr-thresh-microvolt:
> +    description: Threshold for starting the VDR correction
> +
> +  rohm,volt-drop-soc:
> +    description: Table of capacity values matching the values in VDR tables.
> +    $ref: /schemas/types.yaml#/definitions/uint32-array

Which unit is this? Seems to be capacity in % *10?

> +  rohm,volt-drop-high-temp-microvolt:
> +    description: VDR table for high temperature
> +
> +  rohm,volt-drop-normal-temp-microvolt:
> +    description: VDR table for normal temperature
> +
> +  rohm,volt-drop-low-temp-microvolt:
> +    description: VDR table for low temperature
> +
> +  rohm,volt-drop-very-low-temp-microvolt:
> +    description: VDR table for very low temperature

Doesn't the four last properties require to be defined as uint32-array?

> +        rohm,volt-drop-soc = <1000 1000 950 900 850 800 750 700 650 600 550 500
> +          450 400 350 300 250 200 150 100 50 00 (-50)>;

This one makes a lot of sense.

> +        rohm,volt-drop-high-temp-microvolt =  <100 100 102 104 106 109 114 124
> +          117 107 107 109 112 116 117 108 109 109 108 109 122 126 130>;
> +
> +        rohm,volt-drop-normal-temp-microvolt = <100 100 102 105 98 100 105 102
> +          101 99 98 100 103 105 109 117 111 109 110 114 128 141 154>;
> +
> +        rohm,volt-drop-low-temp-microvolt = <100 100 98 107 112 114 118 118 112
> +          108 108 110 111 113 117 123 131 144 157 181 220 283 399>;
> +
> +        rohm,volt-drop-very-low-temp-microvolt = <86 86 105 109 114 110 115 115
> +          110 108 110 112 114 118 124 134 136 160 177 201 241 322 403>;

I would have expected something like this, to avoid the a bit fuzzy definitions
of high, normal, low and very low temperature either:

Provide an array of temperatures in millicentigrades (I just guessed
these temperatures, you will know the real ones!):

rohm,vold-drop-temperatures-millicelsius = <500, 250, 100, (-50)>;
rohm,volt-drop-microvolt-0 = <...>;
rohm,volt-drop-microvolt-1 = <...>;
rohm,volt-drop-microvolt-2 = <...>;
rohm,volt-drop-microvolt-3 = <...>;

Where each array correspond to the temperature, or if you wanna
go all-out custom:

rohm,volt-drop-50-celsius-microvolt = <...>;
(...)

So we get the actual temperature in there one way or the other.

> +        rohm,voltage-vdr-thresh-microvolt = <4150000>;

This property seems to be missing from the bindings?

Yours,
Linus Walleij