SERIAL: core: add throttle/unthrottle callbacks for hardware assisted flow control

Add two callbacks for hardware assisted flow control; we need to know
when the tty layers want us to stop and restart due to their buffer
levels.

Call a driver specific throttle/unthrottle function if and only if the
driver indicates that it is using an enabled hardware assisted flow
control method, otherwise fall back to the non-hardware assisted
methods.

Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

2 files changed, 29 insertions, 4 deletions

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 9d8796e77188..098bb99c2b9f 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -610,27 +610,50 @@ 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;
+        struct uart_port *port = state->uart_port;
+        uint32_t mask = 0;
 
         if (I_IXOFF(tty))
+                mask |= UPF_SOFT_FLOW;
+        if (tty->termios.c_cflag & CRTSCTS)
+                mask |= UPF_HARD_FLOW;
+
+        if (port->flags & mask) {
+                port->ops->throttle(port);
+                mask &= ~port->flags;
+        }
+
+        if (mask & UPF_SOFT_FLOW)
                 uart_send_xchar(tty, STOP_CHAR(tty));
 
-        if (tty->termios.c_cflag & CRTSCTS)
-                uart_clear_mctrl(state->uart_port, TIOCM_RTS);
+        if (mask & UPF_HARD_FLOW)
+                uart_clear_mctrl(port, TIOCM_RTS);
 }
 
 static void uart_unthrottle(struct tty_struct *tty)
 {
         struct uart_state *state = tty->driver_data;
         struct uart_port *port = state->uart_port;
+        uint32_t mask = 0;
 
-        if (I_IXOFF(tty)) {
+        if (I_IXOFF(tty))
+                mask |= UPF_SOFT_FLOW;
+        if (tty->termios.c_cflag & CRTSCTS)
+                mask |= UPF_HARD_FLOW;
+
+        if (port->flags & mask) {
+                port->ops->unthrottle(port);
+                mask &= ~port->flags;
+        }
+
+        if (mask & UPF_SOFT_FLOW) {
                 if (port->x_char)
                         port->x_char = 0;
                 else
                         uart_send_xchar(tty, START_CHAR(tty));
         }
 
-        if (tty->termios.c_cflag & CRTSCTS)
+        if (mask & UPF_HARD_FLOW)
                 uart_set_mctrl(port, TIOCM_RTS);
 }
 
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index e2cda5d04e48..c6690a2a27fb 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -46,6 +46,8 @@ struct uart_ops {
         unsigned int    (*get_mctrl)(struct uart_port *);
         void            (*stop_tx)(struct uart_port *);
         void            (*start_tx)(struct uart_port *);
+        void            (*throttle)(struct uart_port *);
+        void            (*unthrottle)(struct uart_port *);
         void            (*send_xchar)(struct uart_port *, char ch);
         void            (*stop_rx)(struct uart_port *);
         void            (*enable_ms)(struct uart_port *);