power_supply: allow a power supply to explicitly point to powered device

If a power supply has a scope of "Device", then allow the power supply
to indicate what device it actually powers. This is represented in the
power supply's sysfs directory as a symlink named "powers", which points to
the sysfs directory of the powered device.

If the device has children, then the sub-devices are also powered by
the same power supply.
Signed-off-by: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Richard Hughes <richard@hughsie.com>

drivers/power/power_supply_core.c

@@ -147,6 +147,12 @@ struct power_supply *power_supply_get_by_name(char *name)
 }
 EXPORT_SYMBOL_GPL(power_supply_get_by_name);
 
+int power_supply_powers(struct power_supply *psy, struct device *dev)
+{
+	return sysfs_create_link_nowarn(&psy->dev->kobj, &dev->kobj, "powers");
+}
+EXPORT_SYMBOL_GPL(power_supply_powers);
+
 static void power_supply_dev_release(struct device *dev)
 {
 	pr_debug("device: '%s': %s\n", dev_name(dev), __func__);
@@ -202,6 +208,7 @@ EXPORT_SYMBOL_GPL(power_supply_register);
 void power_supply_unregister(struct power_supply *psy)
 {
 	cancel_work_sync(&psy->changed_work);
+	sysfs_remove_link(&psy->dev->kobj, "powers");
 	power_supply_remove_triggers(psy);
 	device_unregister(psy->dev);
 }

include/linux/power_supply.h

@@ -218,6 +218,7 @@ static inline int power_supply_is_system_supplied(void) { return -ENOSYS; }
 extern int power_supply_register(struct device *parent,
 				 struct power_supply *psy);
 extern void power_supply_unregister(struct power_supply *psy);
+extern int power_supply_powers(struct power_supply *psy, struct device *dev);
 
 /* For APM emulation, think legacy userspace. */
 extern struct class *power_supply_class;