power: supply: core: Add state_of_health power supply property

Add state_of_health power supply property to represent battery
health percentage.

Signed-off-by: Fenglin Wu <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

Documentation/ABI/testing/sysfs-class-power

···
 
 		Valid values: Represented in microohms
 
+What:		/sys/class/power_supply/<supply_name>/state_of_health
+Date:		August 2025
+Contact:	[e-mail]
+Description:
+		The state_of_health parameter quantifies the overall condition
+		of a battery as a percentage, reflecting its ability to deliver
+		rated performance relative to its original specifications. It is
+		dynamically computed using a combination of learned capacity
+		and impedance-based degradation indicators, both of which evolve
+		over the battery's lifecycle.
+		Note that the exact algorithms are kept secret by most battery
+		vendors and the value from different battery vendors cannot be
+		compared with each other as there is no vendor-agnostic definition
+		of "performance". Also this usually cannot be used for any
+		calculations (i.e. this is not the factor between charge_full and
+		charge_full_design).
+
+		Access: Read
+
+		Valid values: 0 - 100 (percent)
+
 **USB Properties**
 
 What:		/sys/class/power_supply/<supply_name>/input_current_limit

drivers/power/supply/power_supply_sysfs.c

···
 	POWER_SUPPLY_ATTR(MANUFACTURE_MONTH),
 	POWER_SUPPLY_ATTR(MANUFACTURE_DAY),
 	POWER_SUPPLY_ATTR(INTERNAL_RESISTANCE),
+	POWER_SUPPLY_ATTR(STATE_OF_HEALTH),
 	/* Properties of type `const char *' */
 	POWER_SUPPLY_ATTR(MODEL_NAME),
 	POWER_SUPPLY_ATTR(MANUFACTURER),

include/linux/power_supply.h

···
 	POWER_SUPPLY_PROP_MANUFACTURE_MONTH,
 	POWER_SUPPLY_PROP_MANUFACTURE_DAY,
 	POWER_SUPPLY_PROP_INTERNAL_RESISTANCE,
+	POWER_SUPPLY_PROP_STATE_OF_HEALTH,
 	/* Properties of type `const char *' */
 	POWER_SUPPLY_PROP_MODEL_NAME,
 	POWER_SUPPLY_PROP_MANUFACTURER,