diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index a7d5fa892be2..412de7354752 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -470,6 +470,7 @@ struct uart_port {
 	unsigned char		iotype;			/* io access style */
 	unsigned char		quirks;			/* internal quirks */
 
+#define UPIO_UNKNOWN		((unsigned char)~0U)	/* UCHAR_MAX */
 #define UPIO_PORT		(SERIAL_IO_PORT)	/* 8b I/O port access */
 #define UPIO_HUB6		(SERIAL_IO_HUB6)	/* Hub6 ISA card */
 #define UPIO_MEM		(SERIAL_IO_MEM)		/* driver-specific */