tty: serial: atmel: add earlycon support

Add early console support which relies on the bootloader for the
initialization of the UART.
Please note, that the compatibles are taken from at91-usart MFD
driver.
Tested-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220217094620.1148571-1-michael@walle.ccSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -139,6 +139,7 @@ config SERIAL_ATMEL_CONSOLE
 	bool "Support for console on AT91 serial port"
 	depends on SERIAL_ATMEL=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Say Y here if you wish to use an on-chip UART on a Atmel
 	  AT91 processor as the system console (the system

drivers/tty/serial/atmel_serial.c

@@ -2673,6 +2673,30 @@ static struct console atmel_console = {
 	.data		= &atmel_uart,
 };
 
+static void atmel_serial_early_write(struct console *con, const char *s,
+				     unsigned int n)
+{
+	struct earlycon_device *dev = con->data;
+
+	uart_console_write(&dev->port, s, n, atmel_console_putchar);
+}
+
+static int __init atmel_early_console_setup(struct earlycon_device *device,
+					    const char *options)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = atmel_serial_early_write;
+
+	return 0;
+}
+
+OF_EARLYCON_DECLARE(atmel_serial, "atmel,at91rm9200-usart",
+		    atmel_early_console_setup);
+OF_EARLYCON_DECLARE(atmel_serial, "atmel,at91sam9260-usart",
+		    atmel_early_console_setup);
+
 #define ATMEL_CONSOLE_DEVICE	(&atmel_console)
 
 #else