serial: xuartps: Add some register initialisation to cdns_early_console_setup()

Add initialisation of control register and baud rate to
cdns_early_console_setup(), required when running kernel standalone
without a boot loader. Baud rate is only initialised when specified in
earlycon command-line option, otherwise it is assumed this has been
set by a boot loader. Updated Documentation/kernel-parameters.txt
accordingly.
Signed-off-by: Scott Telford <stelford@cadence.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -1045,11 +1045,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			determined by the stdout-path property in device
 			tree's chosen node.
 
-		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.
+		cdns,<addr>[,options]
+			Start an early, polled-mode console on a Cadence
+			(xuartps) serial port at the specified address. Only
+			supported option is baud rate. If baud rate is not
+			specified, the serial port must already be setup and
+			configured.
 
 		uart[8250],io,<addr>[,options]
 		uart[8250],mmio,<addr>[,options]

drivers/tty/serial/xilinx_uartps.c

@@ -1165,9 +1165,34 @@ static void __init cdns_early_write(struct console *con, const char *s,
 static int __init cdns_early_console_setup(struct earlycon_device *device,
 					   const char *opt)
 {
-	if (!device->port.membase)
+	struct uart_port *port = &device->port;
+
+	if (!port->membase)
 		return -ENODEV;
 
+	/* initialise control register */
+	writel(CDNS_UART_CR_TX_EN|CDNS_UART_CR_TXRST|CDNS_UART_CR_RXRST,
+	       port->membase + CDNS_UART_CR);
+
+	/* only set baud if specified on command line - otherwise
+	 * assume it has been initialized by a boot loader.
+	 */
+	if (device->baud) {
+		u32 cd = 0, bdiv = 0;
+		u32 mr;
+		int div8;
+
+		cdns_uart_calc_baud_divs(port->uartclk, device->baud,
+					 &bdiv, &cd, &div8);
+		mr = CDNS_UART_MR_PARITY_NONE;
+		if (div8)
+			mr |= CDNS_UART_MR_CLKSEL;
+
+		writel(mr,   port->membase + CDNS_UART_MR);
+		writel(cd,   port->membase + CDNS_UART_BAUDGEN);
+		writel(bdiv, port->membase + CDNS_UART_BAUDDIV);
+	}
+
 	device->con->write = cdns_early_write;
 
 	return 0;