tty: serial: lantiq: implement earlycon support

This allows enabling earlycon for devices with a Lantiq serial console
by splitting lqasc_serial_port_write() from lqasc_console_write() and
re-using the new function for earlycon's write callback.

The kernel-parameter name matches the driver name ("lantiq"), similar
to how other drivers implement this.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -966,6 +966,12 @@
 			serial port must already be setup and configured.
 			Options are not yet supported.
 
+		lantiq,<addr>
+			Start an early, polled-mode console on a lantiq serial
+			(lqasc) port at the specified address. The serial port
+			must already be setup and configured. Options are not
+			yet supported.
+
 		lpuart,<addr>
 		lpuart32,<addr>
 			Use early console provided by Freescale LP UART driver

drivers/tty/serial/Kconfig

@@ -1161,6 +1161,7 @@ config SERIAL_LANTIQ
 	depends on LANTIQ
 	select SERIAL_CORE
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Support for console and UART on Lantiq SoCs.
 

drivers/tty/serial/lantiq.c

@@ -590,13 +590,20 @@ lqasc_console_putchar(struct uart_port *port, int ch)
 	ltq_w8(ch, port->membase + LTQ_ASC_TBUF);
 }
 
+static void lqasc_serial_port_write(struct uart_port *port, const char *s,
+				    u_int count)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&ltq_asc_lock, flags);
+	uart_console_write(port, s, count, lqasc_console_putchar);
+	spin_unlock_irqrestore(&ltq_asc_lock, flags);
+}
 
 static void
 lqasc_console_write(struct console *co, const char *s, u_int count)
 {
 	struct ltq_uart_port *ltq_port;
-	struct uart_port *port;
-	unsigned long flags;
 
 	if (co->index >= MAXPORTS)
 		return;
@@ -605,11 +612,7 @@ lqasc_console_write(struct console *co, const char *s, u_int count)
 	if (!ltq_port)
 		return;
 
-	port = &ltq_port->port;
-
-	spin_lock_irqsave(&ltq_asc_lock, flags);
-	uart_console_write(port, s, count, lqasc_console_putchar);
-	spin_unlock_irqrestore(&ltq_asc_lock, flags);
+	lqasc_serial_port_write(&ltq_port->port, s, count);
 }
 
 static int __init
@@ -659,6 +662,27 @@ lqasc_console_init(void)
 }
 console_initcall(lqasc_console_init);
 
+static void lqasc_serial_early_console_write(struct console *co,
+					     const char *s,
+					     u_int count)
+{
+	struct earlycon_device *dev = co->data;
+
+	lqasc_serial_port_write(&dev->port, s, count);
+}
+
+static int __init
+lqasc_serial_early_console_setup(struct earlycon_device *device,
+				 const char *opt)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = lqasc_serial_early_console_write;
+	return 0;
+}
+OF_EARLYCON_DECLARE(lantiq, DRVNAME, lqasc_serial_early_console_setup);
+
 static struct uart_driver lqasc_reg = {
 	.owner =	THIS_MODULE,
 	.driver_name =	DRVNAME,