dma-debug: add comment for failed to check map error

Add comment for failure to check a map error to help driver developers.

Link: http://lkml.kernel.org/r/1484622289-22085-1-git-send-email-miles.chen@mediatek.com
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/dma-debug.c

@@ -1155,6 +1155,11 @@ static void check_unmap(struct dma_debug_entry *ref)
 			   dir2name[ref->direction]);
 	}
 
+	/*
+	 * Drivers should use dma_mapping_error() to check the returned
+	 * addresses of dma_map_single() and dma_map_page().
+	 * If not, print this warning message. See Documentation/DMA-API.txt.
+	 */
 	if (entry->map_err_type == MAP_ERR_NOT_CHECKED) {
 		err_printk(ref->dev, entry,
 			   "DMA-API: device driver failed to check map error"