drm: mm: add helper to unwind scan state

With the switch to implicit free space accounting one pointer
got unused when scanning. Use it to create a single-linked list
to ensure correct unwinding of the scan state.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_mm.c

@@ -460,6 +460,7 @@ void drm_mm_init_scan(struct drm_mm *mm, unsigned long size,
 	mm->scan_hit_start = 0;
 	mm->scan_hit_size = 0;
 	mm->scan_check_range = 0;
+	mm->prev_scanned_node = NULL;
 }
 EXPORT_SYMBOL(drm_mm_init_scan);
 
@@ -485,6 +486,7 @@ void drm_mm_init_scan_with_range(struct drm_mm *mm, unsigned long size,
 	mm->scan_start = start;
 	mm->scan_end = end;
 	mm->scan_check_range = 1;
+	mm->prev_scanned_node = NULL;
 }
 EXPORT_SYMBOL(drm_mm_init_scan_with_range);
 
@@ -514,6 +516,8 @@ int drm_mm_scan_add_block(struct drm_mm_node *node)
 	prev_node->hole_follows = 1;
 	list_del(&node->node_list);
 	node->node_list.prev = &prev_node->node_list;
+	node->node_list.next = &mm->prev_scanned_node->node_list;
+	mm->prev_scanned_node = node;
 
 	hole_start = drm_mm_hole_node_start(prev_node);
 	hole_end = drm_mm_hole_node_end(prev_node);

include/drm/drm_mm.h

@@ -72,6 +72,7 @@ struct drm_mm {
 	unsigned scanned_blocks;
 	unsigned long scan_start;
 	unsigned long scan_end;
+	struct drm_mm_node *prev_scanned_node;
 };
 
 static inline bool drm_mm_node_allocated(struct drm_mm_node *node)
@@ -86,6 +87,13 @@ static inline bool drm_mm_initialized(struct drm_mm *mm)
 #define drm_mm_for_each_node(entry, mm) list_for_each_entry(entry, \
 						&(mm)->head_node.node_list, \
 						node_list);
+#define drm_mm_for_each_scanned_node_reverse(entry, n, mm) \
+	for (entry = (mm)->prev_scanned_node, \
+		next = entry ? list_entry(entry->node_list.next, \
+			struct drm_mm_node, node_list) : NULL; \
+	     entry != NULL; entry = next, \
+		next = entry ? list_entry(entry->node_list.next, \
+			struct drm_mm_node, node_list) : NULL) \
 /*
  * Basic range manager support (drm_mm.c)
  */