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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aX8yPp2JVubxHNFJ@venus>
Date: Sun, 1 Feb 2026 12:14:58 +0100
From: Sebastian Reichel <sebastian.reichel@...labora.com>
To: 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

Hi,

On Sat, Jan 31, 2026 at 04:44:36PM +0530, Kaustabh Chakraborty wrote:
> On 2026-01-29 02:59 +01:00, Sebastian Reichel wrote:
> >> +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.

A typical charging setup is:

1. charge to 100%
2. stop charging
3. wait for charge to drop under 95% (or some other treshold)
4. restart charging
5. go to 1

> 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.

For the setup I described above, you consider everything above the
treshold as POWER_SUPPLY_STATUS_FULL independent of the current
direction. So you need to reorder:

if (capacity >= 90)
    return POWER_SUPPLY_STATUS_FULL;

if (current_now < 0)
    return POWER_SUPPLY_STATUS_DISCHARGING;
else if (current_now == 0)
    return POWER_SUPPLY_STATUS_NOT_CHARGING;
else
    return POWER_SUPPLY_STATUS_CHARGING;

Greetings,

-- Sebastian