powerpc/powernv: Add OPAL message notifier unregister function

Provide an unregister interface for the opal message notifiers
to be called when not needed like during driver unload/remove.

Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/opal.h

@@ -948,6 +948,8 @@ extern int opal_notifier_unregister(struct notifier_block *nb);
 
 extern int opal_message_notifier_register(enum OpalMessageType msg_type,
 						struct notifier_block *nb);
+extern int opal_message_notifier_unregister(enum OpalMessageType msg_type,
+					    struct notifier_block *nb);
 extern void opal_notifier_enable(void);
 extern void opal_notifier_disable(void);
 extern void opal_notifier_update_evt(uint64_t evt_mask, uint64_t evt_val);

arch/powerpc/platforms/powernv/opal.c

@@ -318,6 +318,13 @@ int opal_message_notifier_register(enum OpalMessageType msg_type,
 				&opal_msg_notifier_head[msg_type], nb);
 }
 
+int opal_message_notifier_unregister(enum OpalMessageType msg_type,
+				     struct notifier_block *nb)
+{
+	return atomic_notifier_chain_unregister(
+			&opal_msg_notifier_head[msg_type], nb);
+}
+
 static void opal_message_do_notify(uint32_t msg_type, void *msg)
 {
 	/* notify subscribers */