diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index 6188e19..753b723 100644
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -58,6 +58,18 @@
 	 *
 	 * Also note that these functions might sleep, so don't call them
 	 * in the atomic contexts!
+	 *
+	 * Return values for the get_ro callback should be:
+	 *   0 for a read/write card
+	 *   1 for a read-only card
+	 *   -ENOSYS when not supported (equal to NULL callback)
+	 *   or a negative errno value when something bad happened
+	 *
+	 * Return values for the get_ro callback should be:
+	 *   0 for a absent card
+	 *   1 for a present card
+	 *   -ENOSYS when not supported (equal to NULL callback)
+	 *   or a negative errno value when something bad happened
 	 */
 	void	(*set_ios)(struct mmc_host *host, struct mmc_ios *ios);
 	int	(*get_ro)(struct mmc_host *host);