Squashed commit of the following:

drivers/serial/uart_16550.c:  Add a configuration, analogous to the STM32 configuration option, to suppress the NuttX standard re-ordering for /dev/ttySN for special case of the 16550 UART.

    config/serial: UART 16550: Add CONFIG_SERIAL_UART_ARCH_MMIO option so the a memory mapped device doesn't need to provide uart_getreg() and uart_putreg() implementations.

    u16550_txempty() should check UART_LSR_TEMT to avoid some data left in the transmit FIFO

arch/arm/src/moxart/moxart_16550.c

@@ -58,16 +58,6 @@
  * Public Functions
  ****************************************************************************/
 
-uart_datawidth_t uart_getreg(uart_addrwidth_t base, unsigned int offset)
-{
-  return *((volatile uart_addrwidth_t *)base + offset);
-}
-
-void uart_putreg(uart_addrwidth_t base, unsigned int offset, uart_datawidth_t value)
-{
-  *((volatile uart_addrwidth_t *)base + offset) = value;
-}
-
 void uart_decodeirq(int irq, FAR void *context)
 {
   int i;

configs/moxa/nsh/defconfig

@@ -86,6 +86,7 @@ CONFIG_SCHED_CHILD_STATUS=y
 CONFIG_SCHED_HAVE_PARENT=y
 CONFIG_SCHED_HPWORK=y
 CONFIG_SCHED_WAITPID=y
+CONFIG_SERIAL_UART_ARCH_MMIO=y
 CONFIG_STANDARD_SERIAL=y
 CONFIG_START_DAY=28
 CONFIG_START_MONTH=6

drivers/serial/Kconfig-16550

@@ -5,6 +5,16 @@
 
 if 16550_UART
 
+config 16550_SERIAL_DISABLE_REORDERING
+	bool "Disable reordering of ttySx devices."
+	default n
+	---help---
+		NuttX per default reorders the serial ports (/dev/ttySx) so that the
+		console is always on /dev/ttyS0. If more than one UART is in use this
+		can, however, have the side-effect that all port mappings
+		(hardware USART1 -> /dev/ttyS0) change if the console is moved to another
+		UART.  This option disables that re-ordering for 16550 UARTs.
+
 config 16550_UART0
 	bool "16550 UART0"
 	default n
@@ -307,6 +317,10 @@ config 16550_SUPRESS_CONFIG
 		that configures the 16550_UART.  In that case, you may want to
 		just leave the existing console configuration in place.  Default: n
 
+config SERIAL_UART_ARCH_MMIO
+	bool "Platform access register through the memory mapping"
+	default n
+
 config SERIAL_UART_ARCH_IOCTL
 	bool "Platform has own custom IOCTL"
 	default n

drivers/serial/uart_16550.c

@@ -2,7 +2,7 @@
  * drivers/serial/uart_16550.c
  * Serial driver for 16550 UART
  *
- *   Copyright (C) 2011, 2013, 2017 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2011, 2013, 2017-2018 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -143,7 +143,7 @@ static char g_uart3rxbuffer[CONFIG_16550_UART3_RXBUFSIZE];
 static char g_uart3txbuffer[CONFIG_16550_UART3_TXBUFSIZE];
 #endif
 
-/* This describes the state of the LPC17xx uart0 port. */
+/* This describes the state of the 16550 uart0 port. */
 
 #ifdef CONFIG_16550_UART0
 static struct u16550_s g_uart0priv =
@@ -178,7 +178,7 @@ static uart_dev_t g_uart0port =
 };
 #endif
 
-/* This describes the state of the LPC17xx uart1 port. */
+/* This describes the state of the 16550 uart1 port. */
 
 #ifdef CONFIG_16550_UART1
 static struct u16550_s g_uart1priv =
@@ -213,7 +213,7 @@ static uart_dev_t g_uart1port =
 };
 #endif
 
-/* This describes the state of the LPC17xx uart1 port. */
+/* This describes the state of the 16550 uart1 port. */
 
 #ifdef CONFIG_16550_UART2
 static struct u16550_s g_uart2priv =
@@ -248,7 +248,7 @@ static uart_dev_t g_uart2port =
 };
 #endif
 
-/* This describes the state of the LPC17xx uart1 port. */
+/* This describes the state of the 16550 uart1 port. */
 
 #ifdef CONFIG_16550_UART3
 static struct u16550_s g_uart3priv =
@@ -283,6 +283,39 @@ static uart_dev_t g_uart3port =
 };
 #endif
 
+
+/* Which UART with be tty0/console and which tty1? tty2? tty3? */
+
+#ifdef CONFIG_16550_SERIAL_DISABLE_REORDERING
+
+#  if defined(CONFIG_16550_UART0_SERIAL_CONSOLE)
+#    define CONSOLE_DEV     g_uart0port    /* UART0=console */
+#  elif defined(CONFIG_16550_UART1_SERIAL_CONSOLE)
+#    define CONSOLE_DEV     g_uart1port    /* UART1=console */
+#  elif defined(CONFIG_16550_UART2_SERIAL_CONSOLE)
+#    define CONSOLE_DEV     g_uart2port    /* UART2=console */
+#  elif defined(CONFIG_16550_UART3_SERIAL_CONSOLE)
+#    define CONSOLE_DEV     g_uart3port    /* UART3=console */
+#  endif
+
+#  ifdef CONFIG_16550_UART0
+#    define TTYS0_DEV       g_uart0port
+#  endif
+
+#  ifdef CONFIG_16550_UART1
+#    define TTYS1_DEV       g_uart1port
+#  endif
+
+#  ifdef CONFIG_16550_UART2
+#    define TTYS2_DEV       g_uart2port
+#  endif
+
+#  ifdef CONFIG_16550_UART3
+#    define TTYS3_DEV       g_uart3port
+#  endif
+
+#else  /* CONFIG_16550_SERIAL_DISABLE_REORDERING */
+
 /* Which UART with be tty0/console and which tty1? tty2? tty3? */
 
 #  if defined(CONFIG_16550_UART0_SERIAL_CONSOLE)
@@ -444,6 +477,8 @@ static uart_dev_t g_uart3port =
 #    endif
 #  endif
 
+#endif /* CONFIG_16550_SERIAL_DISABLE_REORDERING */
+
 /****************************************************************************
  * Private Functions
  ****************************************************************************/
@@ -454,7 +489,11 @@ static uart_dev_t g_uart3port =
 
 static inline uart_datawidth_t u16550_serialin(FAR struct u16550_s *priv, int offset)
 {
+#ifdef CONFIG_SERIAL_UART_ARCH_MMIO
+  return *((FAR volatile uart_addrwidth_t *)priv->uartbase + offset);
+#else
   return uart_getreg(priv->uartbase, offset);
+#endif
 }
 
 /****************************************************************************
@@ -464,7 +503,11 @@ static inline uart_datawidth_t u16550_serialin(FAR struct u16550_s *priv, int of
 static inline void u16550_serialout(FAR struct u16550_s *priv, int offset,
                                     uart_datawidth_t value)
 {
+#ifdef CONFIG_SERIAL_UART_ARCH_MMIO
+  *((FAR volatile uart_addrwidth_t *)priv->uartbase + offset) = value;
+#else
   uart_putreg(priv->uartbase, offset, value);
+#endif
 }
 
 /****************************************************************************
@@ -1000,7 +1043,7 @@ static bool u16550_txready(struct uart_dev_s *dev)
 static bool u16550_txempty(struct uart_dev_s *dev)
 {
   FAR struct u16550_s *priv = (FAR struct u16550_s *)dev->priv;
-  return ((u16550_serialin(priv, UART_LSR_OFFSET) & UART_LSR_THRE) != 0);
+  return ((u16550_serialin(priv, UART_LSR_OFFSET) & UART_LSR_TEMT) != 0);
 }
 
 /****************************************************************************

include/nuttx/serial/uart_16550.h

@@ -345,8 +345,11 @@ typedef uint32_t uart_addrwidth_t;
  *
  ************************************************************************************/
 
+#ifndef CONFIG_SERIAL_UART_ARCH_MMIO
 uart_datawidth_t uart_getreg(uart_addrwidth_t base, unsigned int offset);
 void uart_putreg(uart_addrwidth_t base, unsigned int offset, uart_datawidth_t value);
+#endif
+
 int uart_ioctl(struct file *filep, int cmd, unsigned long arg);
 
 #endif /* CONFIG_16550_UART */