serial: Introduce UPSTAT_SYNC_FIFO for synchronised FIFOs

This change adds a flag to indicate that a UART is has an external means
of synchronising its FIFO, without needing CTSRTS or XON/XOFF.

This allows us to use the throttle/unthrottle callbacks, without having
to claim other methods of flow control.
Signed-off-by: NJeremy Kerr <jk@ozlabs.org>
Tested-by: NEddie James <eajames@linux.vnet.ibm.com>
Tested-by: NJoel Stanley <joel@jms.id.au>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -674,8 +674,8 @@ static void uart_send_xchar(struct tty_struct *tty, char ch)
 static void uart_throttle(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
+	upstat_t mask = UPSTAT_SYNC_FIFO;
 	struct uart_port *port;
-	upstat_t mask = 0;
 
 	port = uart_port_ref(state);
 	if (!port)
@@ -703,8 +703,8 @@ static void uart_throttle(struct tty_struct *tty)
 static void uart_unthrottle(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
+	upstat_t mask = UPSTAT_SYNC_FIFO;
 	struct uart_port *port;
-	upstat_t mask = 0;
 
 	port = uart_port_ref(state);
 	if (!port)

include/linux/serial_core.h

@@ -233,6 +233,7 @@ struct uart_port {
 #define UPSTAT_AUTORTS		((__force upstat_t) (1 << 2))
 #define UPSTAT_AUTOCTS		((__force upstat_t) (1 << 3))
 #define UPSTAT_AUTOXOFF		((__force upstat_t) (1 << 4))
+#define UPSTAT_SYNC_FIFO	((__force upstat_t) (1 << 5))
 
 	int			hw_stopped;		/* sw-assisted CTS flow state */
 	unsigned int		mctrl;			/* current modem ctrl settings */