hvc: dcc: Add earlycon support

Add DCC earlycon support for early printks. The patch is useful for SoC
bringup where HW serial console is broken.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/41e2920a6348e65b2e986b0e65b66531e87cd756.1570543923.git.michal.simek@xilinx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_dcc.c

@@ -1,7 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2010, 2014 The Linux Foundation. All rights reserved.  */
 
+#include <linux/console.h>
 #include <linux/init.h>
+#include <linux/serial.h>
+#include <linux/serial_core.h>
 
 #include <asm/dcc.h>
 #include <asm/processor.h>
@@ -12,6 +15,31 @@
 #define DCC_STATUS_RX		(1 << 30)
 #define DCC_STATUS_TX		(1 << 29)
 
+static void dcc_uart_console_putchar(struct uart_port *port, int ch)
+{
+	while (__dcc_getstatus() & DCC_STATUS_TX)
+		cpu_relax();
+
+	__dcc_putchar(ch);
+}
+
+static void dcc_early_write(struct console *con, const char *s, unsigned n)
+{
+	struct earlycon_device *dev = con->data;
+
+	uart_console_write(&dev->port, s, n, dcc_uart_console_putchar);
+}
+
+static int __init dcc_early_console_setup(struct earlycon_device *device,
+					  const char *opt)
+{
+	device->con->write = dcc_early_write;
+
+	return 0;
+}
+
+EARLYCON_DECLARE(dcc, dcc_early_console_setup);
+
 static int hvc_dcc_put_chars(uint32_t vt, const char *buf, int count)
 {
 	int i;