tty: serial: Add CONSOLE_POLL support to SiFive UART

Add CONSOLE_POLL support for future KGDB porting.
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>

Changes since v1:
1. Fix the compile error reported by kbuild test robot
Link: https://lore.kernel.org/r/1584492027-23236-1-git-send-email-vincent.chen@sifive.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sifive.c

@@ -729,6 +729,29 @@ static const char *sifive_serial_type(struct uart_port *port)
 	return port->type == PORT_SIFIVE_V0 ? "SiFive UART v0" : NULL;
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+static int sifive_serial_poll_get_char(struct uart_port *port)
+{
+	struct sifive_serial_port *ssp = port_to_sifive_serial_port(port);
+	char is_empty, ch;
+
+	ch = __ssp_receive_char(ssp, &is_empty);
+	if (is_empty)
+		return NO_POLL_CHAR;
+
+	return ch;
+}
+
+static void sifive_serial_poll_put_char(struct uart_port *port,
+					unsigned char c)
+{
+	struct sifive_serial_port *ssp = port_to_sifive_serial_port(port);
+
+	__ssp_wait_for_xmitr(ssp);
+	__ssp_transmit_char(ssp, c);
+}
+#endif /* CONFIG_CONSOLE_POLL */
+
 /*
  * Early console support
  */
@@ -897,6 +920,10 @@ static const struct uart_ops sifive_serial_uops = {
 	.request_port	= sifive_serial_request_port,
 	.config_port	= sifive_serial_config_port,
 	.verify_port	= sifive_serial_verify_port,
+#ifdef CONFIG_CONSOLE_POLL
+	.poll_get_char	= sifive_serial_poll_get_char,
+	.poll_put_char	= sifive_serial_poll_put_char,
+#endif
 };
 
 static struct uart_driver sifive_serial_uart_driver = {