linux-kernel - Re: [PATCH 2/2] power: supply: add support for S2MU005 battery fuel gauge device

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Message-Id: <DG2QBROWWMLI.2EE3CBLLPAJHR@disroot.org>
Date: Sat, 31 Jan 2026 16:44:36 +0530
From: "Kaustabh Chakraborty" <kauschluss@...root.org>
To: "Sebastian Reichel" <sebastian.reichel@...labora.com>, "Kaustabh
 Chakraborty" <kauschluss@...root.org>
Cc: "Yassine Oudjana" <y.oudjana@...tonmail.com>, "Rob Herring"
 <robh@...nel.org>, "Krzysztof Kozlowski" <krzk+dt@...nel.org>, "Conor
 Dooley" <conor+dt@...nel.org>, <linux-pm@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] power: supply: add support for S2MU005 battery fuel
 gauge device

On 2026-01-29 02:59 +01:00, Sebastian Reichel wrote:
> Hi,
>
> On Mon, Jan 26, 2026 at 09:09:49PM +0530, Kaustabh Chakraborty wrote:
>> From: Yassine Oudjana <y.oudjana@...tonmail.com>
>> 
>> Samsung's S2MU005 PMIC, which contains battery charger functionality
>> also includes a battery fuel gauge device, which is separate from the
>> PMIC itself, and typically connected to an I2C bus. Add a generic driver
>> to support said device.
>> 
>> Signed-off-by: Yassine Oudjana <y.oudjana@...tonmail.com>
>> Co-developed-by: Kaustabh Chakraborty <kauschluss@...root.org>
>> Signed-off-by: Kaustabh Chakraborty <kauschluss@...root.org>
>> ---

[...]

>> +
>> +static const struct regmap_config s2mu005_fg_regmap_config = {
>> +	.reg_bits = 8,
>> +	.val_bits = 8,
>> +};
>
> Looks like all register addresses are 2 byte aligned and you are
> always using regmap_raw_read to get 16bit values. So just use
> .val_bits = 16 here?

Sure.

>> +static int s2mu005_fg_get_status(struct s2mu005_fg *priv, int *value)
>> +{
>> +	int current_now;
>> +	int capacity;
>> +	int ret;
>> +
>> +	ret = s2mu005_fg_get_current_now(priv, &current_now);
>> +	if (ret)
>> +		return ret;
>> +
>> +	if (current_now <= 0) {
>> +		*value = POWER_SUPPLY_STATUS_DISCHARGING;
>> +		return 0;
>> +	}
>> +
>> +	ret = s2mu005_fg_get_capacity(priv, &capacity);
>> +	if (ret)
>> +		return ret;
>> +
>> +	if (capacity < 90)
>> +		*value = POWER_SUPPLY_STATUS_CHARGING;
>> +	else
>> +		*value = POWER_SUPPLY_STATUS_FULL;
>
> Usually there is some kind of hysteresis that stops charging
> when the battery is full and then restarts charging once the
> battery drops under a certain capacity. As this code first
> checks the current to determine if the battery is discharging
> and only then checks if the battery is full - does your code
> toggle between FULL and DISCHARGING?

(Perhaps I misunderstood your query, let me know if I did)

It has been a while since I last tested this - but as far as I can
recall, after a certain threshold, the current would constantly bounce
between -ve and +ve. I believe it was somewhere around 90% to 95% and
above. If that's what 'hysteresis' is, then yes.

An older revision of this driver (I don't have it anymore) used to add
up the consecutive values of current in order to reduce the effect of
this inconsistency, but it was still unreliable.

Moreover, I do not possess any documentation for this device, so it's
not possible for me to know what or how.