serial: mps2-uart: add support for early console

This adds support early console for MPS2 UART which can be enabled via
earlycon=mps2,0x40004000
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NVladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -1452,6 +1452,7 @@ config SERIAL_MPS2_UART_CONSOLE
 	bool "MPS2 UART console support"
 	depends on SERIAL_MPS2_UART
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 
 config SERIAL_MPS2_UART
 	bool "MPS2 UART port"

drivers/tty/serial/mps2-uart.c

@@ -450,6 +450,34 @@ static struct console mps2_uart_console = {
 
 #define MPS2_SERIAL_CONSOLE (&mps2_uart_console)
 
+static void mps2_early_putchar(struct uart_port *port, int ch)
+{
+	while (readb(port->membase + UARTn_STATE) & UARTn_STATE_TX_FULL)
+		cpu_relax();
+
+	writeb((unsigned char)ch, port->membase + UARTn_DATA);
+}
+
+static void mps2_early_write(struct console *con, const char *s, unsigned int n)
+{
+	struct earlycon_device *dev = con->data;
+
+	uart_console_write(&dev->port, s, n, mps2_early_putchar);
+}
+
+static int __init mps2_early_console_setup(struct earlycon_device *device,
+					   const char *opt)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = mps2_early_write;
+
+	return 0;
+}
+
+OF_EARLYCON_DECLARE(mps2, "arm,mps2-uart", mps2_early_console_setup);
+
 #else
 #define MPS2_SERIAL_CONSOLE NULL
 #endif