-static int add_span(struct vi_buffer *vb, int src, vi_addr start, unsigned long size)
+/* split_span splits the span sp. if size > 0 it moves the second part to sp+2,
+ * leaving an empty place at sp+1 for the new span. The start point of the
+ * second part is adjusted by size.
+ *
+ * It can't fail, because it's always called with the span array having at
+ * least two empty slots (see: add_span).
+ */
+void split_span(struct vi_buffer *vb, struct vi_span *sp, vi_addr at, unsigned long size)