tty: serial: meson: Implement earlycon support

Split off the bulk of the existing meson_serial_console_write()
implementation into meson_serial_port_write() for implementing
meson_serial_early_console_write().

Use "meson" as the earlycon driver name, courtesy of Nicolas.
Signed-off-by: NNicolas Saenz Julienne <nicolassaenzj@gmail.com>
Acked-by: NCarlo Caione <carlo@endlessm.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -1039,6 +1039,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			the driver will use only 32-bit accessors to read/write
 			the device registers.
 
+		meson,<addr>
+			Start an early, polled-mode console on a meson serial
+			port at the specified address. The serial port must
+			already be setup and configured. Options are not yet
+			supported.
+
 		msm_serial,<addr>
 			Start an early, polled-mode console on an msm serial
 			port at the specified address. The serial port

drivers/tty/serial/Kconfig

@@ -213,6 +213,7 @@ config SERIAL_MESON_CONSOLE
 	bool "Support for console on meson"
 	depends on SERIAL_MESON=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Say Y here if you wish to use a Amlogic MesonX UART as the
 	  system console (the system console is the device which

drivers/tty/serial/meson_uart.c

@@ -481,18 +481,13 @@ static void meson_console_putchar(struct uart_port *port, int ch)
 	writel(ch, port->membase + AML_UART_WFIFO);
 }
 
-static void meson_serial_console_write(struct console *co, const char *s,
-				       u_int count)
+static void meson_serial_port_write(struct uart_port *port, const char *s,
+				    u_int count)
 {
-	struct uart_port *port;
 	unsigned long flags;
 	int locked;
 	u32 val, tmp;
 
-	port = meson_ports[co->index];
-	if (!port)
-		return;
-
 	local_irq_save(flags);
 	if (port->sysrq) {
 		locked = 0;
@@ -516,6 +511,18 @@ static void meson_serial_console_write(struct console *co, const char *s,
 	local_irq_restore(flags);
 }
 
+static void meson_serial_console_write(struct console *co, const char *s,
+				       u_int count)
+{
+	struct uart_port *port;
+
+	port = meson_ports[co->index];
+	if (!port)
+		return;
+
+	meson_serial_port_write(port, s, count);
+}
+
 static int meson_serial_console_setup(struct console *co, char *options)
 {
 	struct uart_port *port;
@@ -554,6 +561,27 @@ static int __init meson_serial_console_init(void)
 }
 console_initcall(meson_serial_console_init);
 
+static void meson_serial_early_console_write(struct console *co,
+					     const char *s,
+					     u_int count)
+{
+	struct earlycon_device *dev = co->data;
+
+	meson_serial_port_write(&dev->port, s, count);
+}
+
+static int __init
+meson_serial_early_console_setup(struct earlycon_device *device, const char *opt)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = meson_serial_early_console_write;
+	return 0;
+}
+OF_EARLYCON_DECLARE(meson, "amlogic,meson-uart",
+		    meson_serial_early_console_setup);
+
 #define MESON_SERIAL_CONSOLE	(&meson_serial_console)
 #else
 #define MESON_SERIAL_CONSOLE	NULL