power: supply: core: add capacity error margin property

Add a property for reporting the error margin expected
by fuel gauge chips.
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/ABI/testing/sysfs-class-power

@@ -74,6 +74,21 @@ Description:
 		Access: Read, Write
 		Valid values: 0 - 100 (percent)
 
+What:		/sys/class/power_supply/<supply_name>/capacity_error_margin
+Date:		April 2019
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Battery capacity measurement becomes unreliable without
+		recalibration. This values provides the maximum error
+		margin expected to exist by the fuel gauge in percent.
+		Values close to 0% will be returned after (re-)calibration
+		has happened. Over time the error margin will increase.
+		100% means, that the capacity related values are basically
+		completely useless.
+
+		Access: Read
+		Valid values: 0 - 100 (percent)
+
 What:		/sys/class/power_supply/<supply_name>/capacity_level
 Date:		June 2009
 Contact:	linux-pm@vger.kernel.org

drivers/power/supply/power_supply_sysfs.c

@@ -178,6 +178,7 @@ static struct power_supply_attr power_supply_attrs[] = {
 	POWER_SUPPLY_ATTR(CAPACITY),
 	POWER_SUPPLY_ATTR(CAPACITY_ALERT_MIN),
 	POWER_SUPPLY_ATTR(CAPACITY_ALERT_MAX),
+	POWER_SUPPLY_ATTR(CAPACITY_ERROR_MARGIN),
 	POWER_SUPPLY_ENUM_ATTR(CAPACITY_LEVEL),
 	POWER_SUPPLY_ATTR(TEMP),
 	POWER_SUPPLY_ATTR(TEMP_MAX),

include/linux/power_supply.h

@@ -139,6 +139,7 @@ enum power_supply_property {
 	POWER_SUPPLY_PROP_CAPACITY, /* in percents! */
 	POWER_SUPPLY_PROP_CAPACITY_ALERT_MIN, /* in percents! */
 	POWER_SUPPLY_PROP_CAPACITY_ALERT_MAX, /* in percents! */
+	POWER_SUPPLY_PROP_CAPACITY_ERROR_MARGIN, /* in percents! */
 	POWER_SUPPLY_PROP_CAPACITY_LEVEL,
 	POWER_SUPPLY_PROP_TEMP,
 	POWER_SUPPLY_PROP_TEMP_MAX,