drm: Add asserts to catch overflow in drm_mm_init() and drm_mm_init_scan()

A simple assert to ensure that we don't overflow start + size when
initialising the drm_mm, or its scanner.

In future, we may want to switch to tracking the value of ranges (rather
than size) so that we can cover the full u64, for example like resource
tracking.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-26-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_mm.c

@@ -729,6 +729,8 @@ void drm_mm_init_scan(struct drm_mm *mm,
 		      u64 alignment,
 		      unsigned long color)
 {
+	DRM_MM_BUG_ON(!size);
+
 	mm->scan_color = color;
 	mm->scan_alignment = alignment;
 	mm->scan_size = size;
@@ -764,6 +766,9 @@ void drm_mm_init_scan_with_range(struct drm_mm *mm,
 				 u64 start,
 				 u64 end)
 {
+	DRM_MM_BUG_ON(start >= end);
+	DRM_MM_BUG_ON(!size || size > end - start);
+
 	mm->scan_color = color;
 	mm->scan_alignment = alignment;
 	mm->scan_size = size;
@@ -882,6 +887,8 @@ EXPORT_SYMBOL(drm_mm_scan_remove_block);
  */
 void drm_mm_init(struct drm_mm *mm, u64 start, u64 size)
 {
+	DRM_MM_BUG_ON(start + size <= start);
+
 	INIT_LIST_HEAD(&mm->hole_stack);
 	mm->scanned_blocks = 0;