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

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 13/10/2025 15:45, Linus Walleij wrote:
> Hi Matti,
> 
> thanks for your patch!
> 
> On Tue, Oct 7, 2025 at 10:33 AM Matti Vaittinen
> <mazziesaccount@gmail.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?

Indeed :)

> 
>> 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@gmail.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?

Ah, right. Should've documented this! Thanks.

>> +  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?

I have been under impression that the "-microvolt" ending suffices, but 
I may be wrong. At last the 'make dt_binding_check' didn't give me a shout.

> 
>> +        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.

I agree. This is a good idea. I'll try one of these :)

> 
>> +        rohm,voltage-vdr-thresh-microvolt = <4150000>;
> 
> This property seems to be missing from the bindings?

I think it is the first binding in the file :)

Yours,
	-- Matti