Re: [REGRESSION] - Multiple userspace implementations of battery estimate broken after "ACPI: battery: negate current when discharging"

[Matthew Schwartz <matthew.schwartz@linux.dev>]

> On Jul 3, 2025, at 7:47 AM, Hans de Goede <hansg@kernel.org> wrote:
> 
> Hi Matthew,
> 
>> On 3-Jul-25 3:54 AM, Matthew Schwartz wrote:
>> Hello,
>> 
>> I installed kernel 6.15.4 to find that my battery estimate on my handheld gaming device was completely inaccurate, instead giving negative values and an unknown estimated battery life in multiple places.
>> 
>> After bisecting, I landed on "ACPI: battery: negate current when discharging” as the bad commit. This commit breaks not one but several userspace implementations of battery monitoring: Steam and MangoHud. Perhaps it breaks more, but those are the two I have noticed so far.
> 
> Thank you for reporting this.
> 
> As Rafael already indicated this patch will need to be reverted to
> unbreak userspace.
> 
> But, the patch was actually doing the right thing, according to:
> 
> Documentation/ABI/testing/sysfs-class-power
> 
> What:           /sys/class/power_supply/<supply_name>/current_avg
> Date:           May 2007
> Contact:        linux-pm@vger.kernel.org
> Description:
>                Battery:
> ...
>                Access: Read
> 
>                Valid values: Represented in microamps. Negative values are
>                used for discharging batteries, positive values for charging
>                batteries and for USB IBUS current.
> 
> (and the same for current_now)
> 
> and there are many power_supply fuel-gauge drivers (1) under
> drivers/power/supply/ which do adhere to this specification
> and report a negative current for discharging.
> 
> So if any of the userspace consumers of this API you mention
> were to run on hw with these drivers the same problem will
> be hit. Can you please file bugs against these userspace
> projects so that they can fix this?

Sure, I filed a bug with MangoHud last night about the kernel change breaking the current battery logic so I will mention this concern when updating that issue, and I will also file a new bug with Steam.

Thanks,
Matt

> 
> Regards,
> 
> Hans
> 
> 
> 
> 1) For directly accessing fuel-gauge chips on devices where these
> are directly accessible instead of being exposed through ACPI