drm: Update TTM initialization documentation

ttm_global_reference was renamed to drm_global_reference. This updates
the documentation to reflect that. While we are there, document the
drm_global_reference API and update the initialization interface
documentation.

Signed-off-by: Gabriel Krisman Bertazi <[e-mail]>
[danvet: Keep the warning, ttm docs are still massively inadequate.]
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/drm-mm.rst

···
 ------------------
 
     **Warning**
-
     This section is outdated.
 
-Drivers wishing to support TTM must fill out a drm_bo_driver
-structure. The structure contains several fields with function pointers
-for initializing the TTM, allocating and freeing memory, waiting for
-command completion and fence synchronization, and memory migration. See
-the radeon_ttm.c file for an example of usage.
+Drivers wishing to support TTM must pass a filled :c:type:`ttm_bo_driver
+<ttm_bo_driver>` structure to ttm_bo_device_init, together with an
+initialized global reference to the memory manager.  The ttm_bo_driver
+structure contains several fields with function pointers for
+initializing the TTM, allocating and freeing memory, waiting for command
+completion and fence synchronization, and memory migration.
 
-The ttm_global_reference structure is made up of several fields:
+The :c:type:`struct drm_global_reference <drm_global_reference>` is made
+up of several fields:
 
 .. code-block:: c
 
-              struct ttm_global_reference {
+              struct drm_global_reference {
                       enum ttm_global_types global_type;
                       size_t size;
                       void *object;
-                      int (*init) (struct ttm_global_reference *);
-                      void (*release) (struct ttm_global_reference *);
+                      int (*init) (struct drm_global_reference *);
+                      void (*release) (struct drm_global_reference *);
               };
 
 
···
 ttm_bo_global_release(), respectively. Also, like the previous
 object, ttm_global_item_ref() is used to create an initial reference
 count for the TTM, which will call your initialization function.
+
+See the radeon_ttm.c file for an example of usage.
+
+.. kernel-doc:: drivers/gpu/drm/drm_global.c
+   :export:
+
 
 The Graphics Execution Manager (GEM)
 ====================================

drivers/gpu/drm/drm_global.c

···
 	}
 }
 
+/**
+ * drm_global_item_ref - Initialize and acquire reference to memory
+ * object
+ * @ref: Object for initialization
+ *
+ * This initializes a memory object, allocating memory and calling the
+ * .init() hook. Further calls will increase the reference count for
+ * that item.
+ *
+ * Returns:
+ * Zero on success, non-zero otherwise.
+ */
 int drm_global_item_ref(struct drm_global_reference *ref)
 {
 	int ret = 0;
···
 	return ret;
 }
 EXPORT_SYMBOL(drm_global_item_ref);
+
+/**
+ * drm_global_item_unref - Drop reference to memory
+ * object
+ * @ref: Object being removed
+ *
+ * Drop a reference to the memory object and eventually call the
+ * release() hook.  The allocated object should be dropped in the
+ * release() hook or before calling this function
+ *
+ */
 
 void drm_global_item_unref(struct drm_global_reference *ref)
 {