Doc/memory-hotplug.txt: corrections and callback function prototype

Documentation/memory-hotplug.txt describes that a callback function can
be added to the notification chain by calling hotplug_memory_notifier().
The function prototype of the callback function is mssing. This missing
information is added by the patch.

The description of the arguments of the callback function is
reworked.

The constants for the event types are corrected.

The possible return values are explained.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/memory-hotplug.txt

@@ -359,38 +359,51 @@ Need more implementation yet....
 --------------------------------
 8. Memory hotplug event notifier
 --------------------------------
-Memory hotplug has event notifier. There are 6 types of notification.
+Hotplugging events are sent to a notification queue.
 
-MEMORY_GOING_ONLINE
+There are six types of notification defined in include/linux/memory.h:
+
+MEM_GOING_ONLINE
   Generated before new memory becomes available in order to be able to
   prepare subsystems to handle memory. The page allocator is still unable
   to allocate from the new memory.
 
-MEMORY_CANCEL_ONLINE
+MEM_CANCEL_ONLINE
   Generated if MEMORY_GOING_ONLINE fails.
 
-MEMORY_ONLINE
+MEM_ONLINE
   Generated when memory has successfully brought online. The callback may
   allocate pages from the new memory.
 
-MEMORY_GOING_OFFLINE
+MEM_GOING_OFFLINE
   Generated to begin the process of offlining memory. Allocations are no
   longer possible from the memory but some of the memory to be offlined
   is still in use. The callback can be used to free memory known to a
   subsystem from the indicated memory block.
 
-MEMORY_CANCEL_OFFLINE
+MEM_CANCEL_OFFLINE
   Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
   the memory block that we attempted to offline.
 
-MEMORY_OFFLINE
+MEM_OFFLINE
   Generated after offlining memory is complete.
 
-A callback routine can be registered by
+A callback routine can be registered by calling
+
   hotplug_memory_notifier(callback_func, priority)
 
-The second argument of callback function (action) is event types of above.
+Callback functions with higher values of priority are called before callback
-The third argument is passed by pointer of struct memory_notify.
+functions with lower values.
+
+A callback function must have the following prototype:
+
+  int callback_func(
+    struct notifier_block *self, unsigned long action, void *arg);
+
+The first argument of the callback function (self) is a pointer to the block
+of the notifier chain that points to the callback function itself.
+The second argument (action) is one of the event types described above.
+The third argument (arg) passes a pointer of struct memory_notify.
 
 struct memory_notify {
        unsigned long start_pfn;
@@ -412,6 +425,18 @@ node loses all memory. If this is -1, then nodemask status is not changed.
 If status_changed_nid* >= 0, callback should create/discard structures for the
 node if necessary.
 
+The callback routine shall return one of the values
+NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
+defined in include/linux/notifier.h
+
+NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
+
+NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
+MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
+further processing of the notification queue.
+
+NOTIFY_STOP stops further processing of the notification queue.
+
 --------------
 9. Future Work
 --------------