diff --git a/drivers/tty/serial/pmac_zilog.c b/drivers/tty/serial/pmac_zilog.c
index ba65a3bbd72a0..96e7aa479961c 100644
--- a/drivers/tty/serial/pmac_zilog.c
+++ b/drivers/tty/serial/pmac_zilog.c
@@ -213,6 +213,7 @@ static void pmz_interrupt_control(struct uart_pmac_port *uap, int enable)
 }
 
 static bool pmz_receive_chars(struct uart_pmac_port *uap)
+	__must_hold(&uap->port.lock)
 {
 	struct tty_port *port;
 	unsigned char ch, r1, drop, flag;