serial: nulldev: Add nulldev serial driver

Some device the serial console's initialization cannot run early during
the boot process. Hence, nulldev serial device is helpful on that
situation.

For example, if the serial module was implemented in FPGA. Serial
initialization is prohibited to run until the FPGA was programmed.

This commit is to adding nulldev serial driver. This will allows the
default console to be specified as a nulldev.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Keng Soon Cheah <keng.soon.cheah@ni.com>
Cc: Chen Yee Chew <chen.yee.chew@ni.com>

drivers/serial/Kconfig

@@ -414,6 +414,13 @@ config MXC_UART
 	  If you have a machine based on a Motorola IMX CPU you
 	  can enable its onboard serial port by enabling this option.
 
+config NULLDEV_SERIAL
+	bool "Null serial device"
+	help
+	  Select this to enable null serial device support. A null serial
+	  device merely acts as a placeholder for a serial device and does
+	  nothing for all it's operation.
+
 config PIC32_SERIAL
 	bool "Support for Microchip PIC32 on-chip UART"
 	depends on DM_SERIAL && MACH_PIC32

drivers/serial/Makefile

@@ -49,6 +49,7 @@ obj-$(CONFIG_BCM283X_MU_SERIAL) += serial_bcm283x_mu.o
 obj-$(CONFIG_MSM_SERIAL) += serial_msm.o
 obj-$(CONFIG_MVEBU_A3700_UART) += serial_mvebu_a3700.o
 obj-$(CONFIG_MPC8XX_CONS) += serial_mpc8xx.o
+obj-$(CONFIG_NULLDEV_SERIAL) += serial_nulldev.o
 
 ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_USB_TTY) += usbtty.o

drivers/serial/serial_nulldev.c

+/*
+ * Copyright (c) 2015 National Instruments
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <serial.h>
+
+static int nulldev_serial_setbrg(struct udevice *dev, int baudrate)
+{
+	return 0;
+}
+
+static int nulldev_serial_getc(struct udevice *dev)
+{
+	return -EAGAIN;
+}
+
+static int nulldev_serial_input(struct udevice *dev)
+{
+	return 0;
+}
+
+static int nulldev_serial_putc(struct udevice *dev, const char ch)
+{
+	return 0;
+}
+
+static const struct udevice_id nulldev_serial_ids[] = {
+	{ .compatible = "nulldev-serial" },
+	{ }
+};
+
+
+const struct dm_serial_ops nulldev_serial_ops = {
+	.putc = nulldev_serial_putc,
+	.getc = nulldev_serial_getc,
+	.setbrg = nulldev_serial_setbrg,
+};
+
+U_BOOT_DRIVER(serial_nulldev) = {
+	.name	= "serial_nulldev",
+	.id	= UCLASS_SERIAL,
+	.of_match = nulldev_serial_ids,
+	.ops	= &nulldev_serial_ops,
+};