serial: cadence: Add generic earlycon support

Add earlycon support for the cadence serial port.
This is based on recent patches:
"tty/serial: pl011: add generic earlycon support"
(sha1: 0d3c673e78)
"tty/serial: add arm/arm64 semihosting earlycon"
(sha1: d50d7269eb)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -921,6 +921,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	earlycon=	[KNL] Output early console device and options.
 
+		cdns,<addr>
+			Start an early, polled-mode console on a cadence serial
+			port at the specified address. The cadence serial port
+			must already be setup and configured. Options are not
+			yet supported.
+
 		uart[8250],io,<addr>[,options]
 		uart[8250],mmio,<addr>[,options]
 		uart[8250],mmio32,<addr>[,options]

drivers/tty/serial/Kconfig

@@ -1429,6 +1429,7 @@ config SERIAL_XILINX_PS_UART_CONSOLE
 	bool "Cadence UART console support"
 	depends on SERIAL_XILINX_PS_UART=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Enable a Cadence UART port to be the system console.
 

drivers/tty/serial/xilinx_uartps.c

@@ -1051,6 +1051,25 @@ static void cdns_uart_console_putchar(struct uart_port *port, int ch)
 	cdns_uart_writel(ch, CDNS_UART_FIFO_OFFSET);
 }
 
+static void cdns_early_write(struct console *con, const char *s, unsigned n)
+{
+	struct earlycon_device *dev = con->data;
+
+	uart_console_write(&dev->port, s, n, cdns_uart_console_putchar);
+}
+
+static int __init cdns_early_console_setup(struct earlycon_device *device,
+					   const char *opt)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = cdns_early_write;
+
+	return 0;
+}
+EARLYCON_DECLARE(cdns, cdns_early_console_setup);
+
 /**
  * cdns_uart_console_write - perform write operation
  * @co: Console handle