serial: altera_uart: add earlycon support

Nios2 currently uses its own early printk implementation, rather than
using unified earlycon support to show boot messages on altera_uart.

Add earlycon support to altera_uart so that other archs may use it.
Also, this (together with the corresponding patch for altera_jtaguart)
will allow the early printk implementation in arch/nios2 to be removed
in a future patch.

Cc: Ley Foon Tan <[e-mail]>
Signed-off-by: Tobias Klauser <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/Kconfig

···
 	bool "Altera UART console support"
 	depends on SERIAL_ALTERA_UART=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Enable a Altera UART port to be the system console.
 

drivers/tty/serial/altera_uart.c

···
 
 #define	ALTERA_UART_CONSOLE	(&altera_uart_console)
 
+static void altera_uart_earlycon_write(struct console *co, const char *s,
+				       unsigned int count)
+{
+	struct earlycon_device *dev = co->data;
+
+	uart_console_write(&dev->port, s, count, altera_uart_console_putc);
+}
+
+static int __init altera_uart_earlycon_setup(struct earlycon_device *dev,
+					     const char *options)
+{
+	struct uart_port *port = &dev->port;
+
+	if (!port->membase)
+		return -ENODEV;
+
+	/* Enable RX interrupts now */
+	writel(ALTERA_UART_CONTROL_RRDY_MSK,
+	       port->membase + ALTERA_UART_CONTROL_REG);
+
+	if (dev->baud) {
+		unsigned int baudclk = port->uartclk / dev->baud;
+
+		writel(baudclk, port->membase + ALTERA_UART_DIVISOR_REG);
+	}
+
+	dev->con->write = altera_uart_earlycon_write;
+	return 0;
+}
+
+OF_EARLYCON_DECLARE(uart, "altr,uart-1.0", altera_uart_earlycon_setup);
+
 #else
 
 #define	ALTERA_UART_CONSOLE	NULL