drm: Declare that create drm_mm nodes with size 0 is illegal

At a higher level, all objects are created with definite size i.e. 0 is
illegal. In forthcoming patches, this assumption is dependent upon in
the drm_mm range manager, i.e. trying to create a drm_mm node with size
0 will have undefined behaviour. Add a couple of WARNs upon creating the
drm_mm node to prevent later bugs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470248788-30873-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_mm.c

@@ -250,6 +250,9 @@ int drm_mm_reserve_node(struct drm_mm *mm, struct drm_mm_node *node)
 	struct drm_mm_node *hole;
 	u64 hole_start, hole_end;
 
+	if (WARN_ON(node->size == 0))
+		return -EINVAL;
+
 	end = node->start + node->size;
 
 	/* Find the relevant hole to add our node to */
@@ -317,6 +320,9 @@ int drm_mm_insert_node_generic(struct drm_mm *mm, struct drm_mm_node *node,
 {
 	struct drm_mm_node *hole_node;
 
+	if (WARN_ON(size == 0))
+		return -EINVAL;
+
 	hole_node = drm_mm_search_free_generic(mm, size, alignment,
 					       color, sflags);
 	if (!hole_node)
@@ -419,6 +425,9 @@ int drm_mm_insert_node_in_range_generic(struct drm_mm *mm, struct drm_mm_node *n
 {
 	struct drm_mm_node *hole_node;
 
+	if (WARN_ON(size == 0))
+		return -EINVAL;
+
 	hole_node = drm_mm_search_free_in_range_generic(mm,
 							size, alignment, color,
 							start, end, sflags);