diff --git a/drivers/usb/serial/mos7840.c b/drivers/usb/serial/mos7840.c
index aeeb9cb..0b29c53 100644
--- a/drivers/usb/serial/mos7840.c
+++ b/drivers/usb/serial/mos7840.c
@@ -1722,6 +1722,7 @@
 	if (mos7840_port == NULL)
 		return -ENODEV;
 
+	/* FIXME: What locks the port registers ? */
 	mcr = mos7840_port->shadowMCR;
 	if (clear & TIOCM_RTS)
 		mcr &= ~MCR_RTS;