kernel/reboot: Add register_platform_power_off()

Add platform-level registration helpers that will ease transition of the
arch/platform power-off callbacks to the new sys-off based API, allowing
us to remove the global pm_power_off variable in the future.
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/reboot.h

@@ -141,6 +141,9 @@ int devm_register_sys_off_handler(struct device *dev,
 				  int (*callback)(struct sys_off_data *data),
 				  void *cb_data);
 
+int register_platform_power_off(void (*power_off)(void));
+void unregister_platform_power_off(void (*power_off)(void));
+
 /*
  * Architecture independent implemenations of sys_reboot commands.
  */

kernel/reboot.c

@@ -469,6 +469,61 @@ int devm_register_sys_off_handler(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_register_sys_off_handler);
 
+static struct sys_off_handler *platform_power_off_handler;
+
+static int platform_power_off_notify(struct sys_off_data *data)
+{
+	void (*platform_power_power_off_cb)(void) = data->cb_data;
+
+	platform_power_power_off_cb();
+
+	return NOTIFY_DONE;
+}
+
+/**
+ *	register_platform_power_off - Register platform-level power-off callback
+ *	@power_off: Power-off callback
+ *
+ *	Registers power-off callback that will be called as last step
+ *	of the power-off sequence. This callback is expected to be invoked
+ *	for the last resort. Only one platform power-off callback is allowed
+ *	to be registered at a time.
+ *
+ *	Returns zero on success, or error code on failure.
+ */
+int register_platform_power_off(void (*power_off)(void))
+{
+	struct sys_off_handler *handler;
+
+	handler = register_sys_off_handler(SYS_OFF_MODE_POWER_OFF,
+					   SYS_OFF_PRIO_PLATFORM,
+					   platform_power_off_notify,
+					   power_off);
+	if (IS_ERR(handler))
+		return PTR_ERR(handler);
+
+	platform_power_off_handler = handler;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(register_platform_power_off);
+
+/**
+ *	unregister_platform_power_off - Unregister platform-level power-off callback
+ *	@power_off: Power-off callback
+ *
+ *	Unregisters previously registered platform power-off callback.
+ */
+void unregister_platform_power_off(void (*power_off)(void))
+{
+	if (platform_power_off_handler &&
+	    platform_power_off_handler->cb_data == power_off) {
+		unregister_sys_off_handler(platform_power_off_handler);
+		platform_power_off_handler = NULL;
+	}
+}
+EXPORT_SYMBOL_GPL(unregister_platform_power_off);
+
 static int legacy_pm_power_off_prepare(struct sys_off_data *data)
 {
 	if (pm_power_off_prepare)