tty: goldfish: Implement support for kernel 'earlycon' parameter

Add early console functionality to the Goldfish tty driver.

When 'earlycon' kernel command line parameter is used with no options,
the early console is determined by the 'stdout-path' property in device
tree's 'chosen' node. This is illustrated in the following device tree
source example:

Device tree example:

    chosen {
        stdout-path = "/goldfish_tty@1f004000";
    };

    goldfish_tty@1f004000 {
        interrupts = <0xc>;
        reg = <0x1f004000 0x0 0x1000>;
        compatible = "google,goldfish-tty";
    };
Signed-off-by: NMiodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: NGoran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: NAleksandar Markovic <aleksandar.markovic@imgtec.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/Kconfig

@@ -392,6 +392,9 @@ config PPC_EARLY_DEBUG_EHV_BC_HANDLE
 config GOLDFISH_TTY
 	tristate "Goldfish TTY Driver"
 	depends on GOLDFISH
+	select SERIAL_CORE
+	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Console and system TTY driver for the Goldfish virtual platform.
 

drivers/tty/goldfish.c

 /*
  * Copyright (C) 2007 Google, Inc.
  * Copyright (C) 2012 Intel, Inc.
+ * Copyright (C) 2017 Imagination Technologies Ltd.
  *
  * This software is licensed under the terms of the GNU General Public
  * License version 2, as published by the Free Software Foundation, and
@@ -24,6 +25,7 @@
 #include <linux/goldfish.h>
 #include <linux/mm.h>
 #include <linux/dma-mapping.h>
+#include <linux/serial_core.h>
 
 /* Goldfish tty register's offsets */
 #define	GOLDFISH_TTY_REG_BYTES_READY	0x04
@@ -440,6 +442,30 @@ static int goldfish_tty_remove(struct platform_device *pdev)
 	return 0;
 }
 
+static void gf_early_console_putchar(struct uart_port *port, int ch)
+{
+	__raw_writel(ch, port->membase);
+}
+
+static void gf_early_write(struct console *con, const char *s, unsigned int n)
+{
+	struct earlycon_device *dev = con->data;
+
+	uart_console_write(&dev->port, s, n, gf_early_console_putchar);
+}
+
+static int __init gf_earlycon_setup(struct earlycon_device *device,
+				    const char *opt)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = gf_early_write;
+	return 0;
+}
+
+OF_EARLYCON_DECLARE(early_gf_tty, "google,goldfish-tty", gf_earlycon_setup);
+
 static const struct of_device_id goldfish_tty_of_match[] = {
 	{ .compatible = "google,goldfish-tty", },
 	{},