serial: uart: add hw flow control support configuration

8250 uart driver currently supports only software assisted hw flow
control. The software assisted hw flow control maintains a hw_stopped
flag in the tty structure to stop and start transmission and use modem
status interrupt for the event to drive the handshake signals. This is
not needed if hw has flow control capabilities. This patch adds a
DT attribute for enabling hw flow control for a uart port. Also skip
stop and start if this flag is present in flag field of the port
structure.
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
CC: Rob Herring <robh+dt@kernel.org>
CC: Pawel Moll <pawel.moll@arm.com>
CC: Mark Rutland <mark.rutland@arm.com>
CC: Ian Campbell <ijc+devicetree@hellion.org.uk>
CC: Kumar Gala <galak@codeaurora.org>
CC: Randy Dunlap <rdunlap@infradead.org>
CC: Jiri Slaby <jslaby@suse.cz>
CC: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/of-serial.txt

@@ -37,6 +37,7 @@ Optional properties:
 - auto-flow-control: one way to enable automatic flow control support. The
   driver is allowed to detect support for the capability even without this
   property.
+- has-hw-flow-control: the hardware has flow control capability.
 
 Example:
 

drivers/tty/serial/8250/8250_core.c

@@ -2333,9 +2333,11 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
 	 * the trigger, or the MCR RTS bit is cleared.  In the case where
 	 * the remote UART is not using CTS auto flow control, we must
 	 * have sufficient FIFO entries for the latency of the remote
-	 * UART to respond.  IOW, at least 32 bytes of FIFO.
+	 * UART to respond.  IOW, at least 32 bytes of FIFO. Also enable
+	 * AFE if hw flow control is supported
 	 */
-	if (up->capabilities & UART_CAP_AFE && port->fifosize >= 32) {
+	if ((up->capabilities & UART_CAP_AFE && (port->fifosize >= 32)) ||
+	    (port->flags & UPF_HARD_FLOW)) {
 		up->mcr &= ~UART_MCR_AFE;
 		if (termios->c_cflag & CRTSCTS)
 			up->mcr |= UART_MCR_AFE;

drivers/tty/serial/of_serial.c

@@ -182,6 +182,10 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
 					  "auto-flow-control"))
 			port8250.capabilities |= UART_CAP_AFE;
 
+		if (of_property_read_bool(ofdev->dev.of_node,
+					  "has-hw-flow-control"))
+			port8250.port.flags |= UPF_HARD_FLOW;
+
 		ret = serial8250_register_8250_port(&port8250);
 		break;
 	}

drivers/tty/serial/serial_core.c

@@ -174,8 +174,12 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
 			if (tty->termios.c_cflag & CBAUD)
 				uart_set_mctrl(uport, TIOCM_RTS | TIOCM_DTR);
 		}
-
-		if (tty_port_cts_enabled(port)) {
+		/*
+		 * if hw support flow control without software intervention,
+		 * then skip the below check
+		 */
+		if (tty_port_cts_enabled(port) &&
+		    !(uport->flags & UPF_HARD_FLOW)) {
 			spin_lock_irq(&uport->lock);
 			if (!(uport->ops->get_mctrl(uport) & TIOCM_CTS))
 				tty->hw_stopped = 1;
@@ -2775,7 +2779,9 @@ void uart_handle_cts_change(struct uart_port *uport, unsigned int status)
 
 	uport->icount.cts++;
 
-	if (tty_port_cts_enabled(port)) {
+	/* skip below code if the hw flow control is supported */
+	if (tty_port_cts_enabled(port) &&
+	    !(uport->flags & UPF_HARD_FLOW)) {
 		if (tty->hw_stopped) {
 			if (status) {
 				tty->hw_stopped = 0;