power: supply: core: add UAPI to discover currently used extensions

Userspace wants to now about the used power supply extensions,
for example to handle a device extended by a certain extension
differently or to discover information about the extending device.

Add a sysfs directory to the power supply device.
This directory contains links which are named after the used extension
and point to the device implementing that extension.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20241211-power-supply-extensions-v6-4-9d9dc3f3d387@weissschuh.net
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/ABI/testing/sysfs-class-power

@@ -813,3 +813,12 @@ Description:
 
 		Access: Read
 		Valid values: 1-31
+
+What:		/sys/class/power_supply/<supply_name>/extensions/<extension_name>
+Date:		March 2025
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Reports the extensions registered to the power supply.
+		Each entry is a link to the device which registered the extension.
+
+		Access: Read

drivers/power/supply/cros_charge-control.c

@@ -31,6 +31,7 @@
  */
 
 struct cros_chctl_priv {
+	struct device *dev;
 	struct cros_ec_device *cros_ec;
 	struct acpi_battery_hook battery_hook;
 	struct power_supply *hooked_battery;
@@ -202,6 +203,7 @@ static int cros_chctl_psy_prop_is_writeable(struct power_supply *psy,
 	};									\
 										\
 	static const struct power_supply_ext _name = {				\
+		.name			= "cros-charge-control",		\
 		.properties		= _name ## _props,			\
 		.num_properties		= ARRAY_SIZE(_name ## _props),		\
 		.charge_behaviours	= EC_CHARGE_CONTROL_BEHAVIOURS,		\
@@ -233,7 +235,7 @@ static int cros_chctl_add_battery(struct power_supply *battery, struct acpi_batt
 		return 0;
 
 	priv->hooked_battery = battery;
-	return power_supply_register_extension(battery, priv->psy_ext, priv);
+	return power_supply_register_extension(battery, priv->psy_ext, priv->dev, priv);
 }
 
 static int cros_chctl_remove_battery(struct power_supply *battery, struct acpi_battery_hook *hook)
@@ -299,6 +301,7 @@ static int cros_chctl_probe(struct platform_device *pdev)
 
 	dev_dbg(dev, "Command version: %u\n", (unsigned int)priv->cmd_version);
 
+	priv->dev = dev;
 	priv->cros_ec = cros_ec;
 
 	if (priv->cmd_version == 1)

drivers/power/supply/power_supply.h

@@ -25,6 +25,7 @@ extern bool power_supply_ext_has_property(const struct power_supply_ext *ext,
 struct power_supply_ext_registration {
 	struct list_head list_head;
 	const struct power_supply_ext *ext;
+	struct device *dev;
 	void *data;
 };
 
@@ -39,6 +40,7 @@ struct power_supply_ext_registration {
 
 extern void __init power_supply_init_attrs(void);
 extern int power_supply_uevent(const struct device *dev, struct kobj_uevent_env *env);
+extern const struct attribute_group power_supply_extension_group;
 extern const struct attribute_group *power_supply_attr_groups[];
 
 #else

drivers/power/supply/power_supply_core.c

@@ -1358,17 +1358,21 @@ static int power_supply_update_sysfs_and_hwmon(struct power_supply *psy)
 }
 
 int power_supply_register_extension(struct power_supply *psy, const struct power_supply_ext *ext,
-				    void *data)
+				    struct device *dev, void *data)
 {
 	struct power_supply_ext_registration *reg;
 	size_t i;
 	int ret;
 
-	if (!psy || !ext || !ext->properties || !ext->num_properties)
+	if (!psy || !dev || !ext || !ext->name || !ext->properties || !ext->num_properties)
 		return -EINVAL;
 
 	guard(rwsem_write)(&psy->extensions_sem);
 
+	power_supply_for_each_extension(reg, psy)
+		if (strcmp(ext->name, reg->ext->name) == 0)
+			return -EEXIST;
+
 	for (i = 0; i < ext->num_properties; i++)
 		if (power_supply_has_property(psy, ext->properties[i]))
 			return -EEXIST;
@@ -1378,9 +1382,15 @@ int power_supply_register_extension(struct power_supply *psy, const struct power
 		return -ENOMEM;
 
 	reg->ext = ext;
+	reg->dev = dev;
 	reg->data = data;
 	list_add(&reg->list_head, &psy->extensions);
 
+	ret = sysfs_add_link_to_group(&psy->dev.kobj, power_supply_extension_group.name,
+				      &dev->kobj, ext->name);
+	if (ret)
+		goto sysfs_link_failed;
+
 	ret = power_supply_update_sysfs_and_hwmon(psy);
 	if (ret)
 		goto sysfs_hwmon_failed;
@@ -1388,6 +1398,8 @@ int power_supply_register_extension(struct power_supply *psy, const struct power
 	return 0;
 
 sysfs_hwmon_failed:
+	sysfs_remove_link_from_group(&psy->dev.kobj, power_supply_extension_group.name, ext->name);
+sysfs_link_failed:
 	list_del(&reg->list_head);
 	kfree(reg);
 	return ret;
@@ -1403,6 +1415,9 @@ void power_supply_unregister_extension(struct power_supply *psy, const struct po
 	power_supply_for_each_extension(reg, psy) {
 		if (reg->ext == ext) {
 			list_del(&reg->list_head);
+			sysfs_remove_link_from_group(&psy->dev.kobj,
+						     power_supply_extension_group.name,
+						     reg->ext->name);
 			kfree(reg);
 			power_supply_update_sysfs_and_hwmon(psy);
 			return;

drivers/power/supply/power_supply_sysfs.c

@@ -458,8 +458,18 @@ static const struct attribute_group power_supply_attr_group = {
 	.is_visible = power_supply_attr_is_visible,
 };
 
+static struct attribute *power_supply_extension_attrs[] = {
+	NULL
+};
+
+const struct attribute_group power_supply_extension_group = {
+	.name = "extensions",
+	.attrs = power_supply_extension_attrs,
+};
+
 const struct attribute_group *power_supply_attr_groups[] = {
 	&power_supply_attr_group,
+	&power_supply_extension_group,
 	NULL
 };
 

drivers/power/supply/test_power.c

@@ -293,6 +293,7 @@ static int test_power_battery_extproperty_is_writeable(struct power_supply *psy,
 }
 
 static const struct power_supply_ext test_power_battery_ext = {
+	.name			= "test_power",
 	.properties		= test_power_battery_extprops,
 	.num_properties		= ARRAY_SIZE(test_power_battery_extprops),
 	.get_property		= test_power_battery_extget_property,
@@ -307,7 +308,8 @@ static void test_power_configure_battery_extension(bool enable)
 	psy = test_power_supplies[TEST_BATTERY];
 
 	if (enable) {
-		if (power_supply_register_extension(psy, &test_power_battery_ext, NULL)) {
+		if (power_supply_register_extension(psy, &test_power_battery_ext, &psy->dev,
+						    NULL)) {
 			pr_err("registering battery extension failed\n");
 			return;
 		}

include/linux/power_supply.h

@@ -286,6 +286,7 @@ struct power_supply_desc {
 };
 
 struct power_supply_ext {
+	const char *const name;
 	u8 charge_behaviours;
 	const enum power_supply_property *properties;
 	size_t num_properties;
@@ -911,6 +912,7 @@ extern int power_supply_powers(struct power_supply *psy, struct device *dev);
 extern int __must_check
 power_supply_register_extension(struct power_supply *psy,
 				const struct power_supply_ext *ext,
+				struct device *dev,
 				void *data);
 extern void power_supply_unregister_extension(struct power_supply *psy,
 					      const struct power_supply_ext *ext);