Re: [PATCH] ACPI: SBS: report relative state of charge as CAPACITY

Sebastian Reichel <[email protected]> Sat, 5 Sep 2026 19:46:01 +0200

Newsgroups	org.kernel.vger.linux-acpi,org.kernel.vger.linux-kernel,org.kernel.vger.linux-pm
Message-ID	<apxVEvGoao0iVGlh@venus>

Hi,

On Thu, Sep 03, 2026 at 01:42:44PM +0300, Itay Shem-tov wrote:
> acpi_sbs reads SBS command 0x0e (AbsoluteStateOfCharge) into
> battery->state_of_charge, which is exported unmodified as
> POWER_SUPPLY_PROP_CAPACITY.
> 
> Per the Smart Battery Data Specification 1.1, 0x0e is expressed as a
> percentage of DesignCapacity and is explicitly permitted to exceed 100%.
> Documentation/ABI/testing/sysfs-class-power specifies the capacity
> attribute as "Valid values: 0 - 100 (percent)", so any pack whose
> FullChargeCapacity exceeds its DesignCapacity - the normal state of a
> new or recently replaced battery - makes the driver report out of range.
> 
> The correct source is 0x0d (RelativeStateOfCharge), a percentage of
> FullChargeCapacity, which the specification bounds to 0..100.
> 
> This is the same defect that was fixed in the i2c SBS driver by commit
> b1f092f6480e ("sbs-battery.c: Capacity attr = remaining relative
> capacity"), whose reasoning applies verbatim here; drivers/acpi/sbs.c was
> not updated at the time. drivers/power/supply/sbs-battery.c has used 0x0d
> since, so the two SBS drivers currently disagree about what CAPACITY
> means.
> 
> Observed on a MacBookPro11,1 with an SMP/bq20z451 pack
> (FullChargeCapacity 6775 mAh, DesignCapacity 6400 mAh). Both registers
> read back-to-back from the pack at a full charge:
> 
>   0x0d RelativeStateOfCharge = 100 %
>   0x0e AbsoluteStateOfCharge = 106 %
> 
> /sys/class/power_supply/BAT0/capacity reported 106 while upower, which
> computes charge_now/charge_full itself rather than trusting the driver,
> reported 100.
> 
> battery->state_of_charge has no other consumer, so no other property
> changes behaviour.
> 
> Signed-off-by: Itay Shem-tov <[email protected]>
> ---

That also matches the sbs-battery driver (drivers/power/supply/sbs-battery.c):

Reviewed-by: Sebastian Reichel <[email protected]>

Greetings,

-- Sebastian

>  drivers/acpi/sbs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/sbs.c b/drivers/acpi/sbs.c
> index 86b7c79..862cb94 100644
> --- a/drivers/acpi/sbs.c
> +++ b/drivers/acpi/sbs.c
> @@ -318,7 +318,7 @@ static struct acpi_battery_reader state_readers[] = {
>  	{0x0a, SMBUS_READ_WORD, offsetof(struct acpi_battery, rate_now)},
>  	{0x0b, SMBUS_READ_WORD, offsetof(struct acpi_battery, rate_avg)},
>  	{0x0f, SMBUS_READ_WORD, offsetof(struct acpi_battery, capacity_now)},
> -	{0x0e, SMBUS_READ_WORD, offsetof(struct acpi_battery, state_of_charge)},
> +	{0x0d, SMBUS_READ_WORD, offsetof(struct acpi_battery, state_of_charge)},
>  	{0x16, SMBUS_READ_WORD, offsetof(struct acpi_battery, state)},
>  };
> 
> --
> 2.51.0

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