drm: Introduce an iterator over holes in the drm_mm range manager

This will be used i915 in forthcoming patches in order to measure the
largest contiguous chunk of memory available for enabling chipset
features.

v2: Try to make the macro marginally safer and more readable by not
depending upon the drm_mm_hole_node_end() being non-zero. Note that we
need to open code list_for_each() in order to update the hole_start,
hole_end variable on each iteration and keep the macro sane.

v3: Tidy up few BUG_ONs that fell foul of adding additional tests to
drm_mm_hole_node_start().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_mm.h

@@ -89,6 +89,29 @@ static inline bool drm_mm_initialized(struct drm_mm *mm)
 {
 	return mm->hole_stack.next;
 }
+
+static inline unsigned long __drm_mm_hole_node_start(struct drm_mm_node *hole_node)
+{
+	return hole_node->start + hole_node->size;
+}
+
+static inline unsigned long drm_mm_hole_node_start(struct drm_mm_node *hole_node)
+{
+	BUG_ON(!hole_node->hole_follows);
+	return __drm_mm_hole_node_start(hole_node);
+}
+
+static inline unsigned long __drm_mm_hole_node_end(struct drm_mm_node *hole_node)
+{
+	return list_entry(hole_node->node_list.next,
+			  struct drm_mm_node, node_list)->start;
+}
+
+static inline unsigned long drm_mm_hole_node_end(struct drm_mm_node *hole_node)
+{
+	return __drm_mm_hole_node_end(hole_node);
+}
+
 #define drm_mm_for_each_node(entry, mm) list_for_each_entry(entry, \
 						&(mm)->head_node.node_list, \
 						node_list)
@@ -99,6 +122,19 @@ static inline bool drm_mm_initialized(struct drm_mm *mm)
 	     entry != NULL; entry = next, \
 		next = entry ? list_entry(entry->node_list.next, \
 			struct drm_mm_node, node_list) : NULL) \
+
+/* Note that we need to unroll list_for_each_entry in order to inline
+ * setting hole_start and hole_end on each iteration and keep the
+ * macro sane.
+ */
+#define drm_mm_for_each_hole(entry, mm, hole_start, hole_end) \
+	for (entry = list_entry((mm)->hole_stack.next, struct drm_mm_node, hole_stack); \
+	     &entry->hole_stack != &(mm)->hole_stack ? \
+	     hole_start = drm_mm_hole_node_start(entry), \
+	     hole_end = drm_mm_hole_node_end(entry), \
+	     1 : 0; \
+	     entry = list_entry(entry->hole_stack.next, struct drm_mm_node, hole_stack))
+
 /*
  * Basic range manager support (drm_mm.c)
  */