drm: Add consistency check for page-flipping

Driver implementations of the drm_crtc's .page_flip() function are
required to update the crtc->fb field on success to reflect that the new
framebuffer is now in use. This is important to keep reference counting
on the framebuffers balanced.

While at it, document this requirement to keep others from falling into
the same trap.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_crtc.c

@@ -3792,6 +3792,13 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 		/* Keep the old fb, don't unref it. */
 		old_fb = NULL;
 	} else {
+		/*
+		 * Warn if the driver hasn't properly updated the crtc->fb
+		 * field to reflect that the new framebuffer is now used.
+		 * Failing to do so will screw with the reference counting
+		 * on framebuffers.
+		 */
+		WARN_ON(crtc->fb != fb);
 		/* Unref only the old framebuffer. */
 		fb = NULL;
 	}