arch/tiva: Serial TIOCxBRK BSD-compatible BREAK support

In the lower half UART driver for Tiva architecture (TM4C12x), adding
the TIOCxBRK ioctl calls, which allow an application to transmit a BSD
compatible line BREAK. TIOCSBRK starts the BREAK and TIOCCBRK ends it.

This architecture supports BSD-style BREAK in hardware. We write to
the BRK bit (bit 0) of the UART Line Control register (UARTLCRH) to
start the BREAK, which begins after the UART finishes shifting out the
current character in progress, if any, including its stop bit(s), and
continues indefinitely until we write to the BRK bit again to stop the
BREAK.

* arch/arm/src/tiva/Kconfig
  (config TIVA_UART_BREAKS): New. Appears as CONFIG_TIVA_UART_BREAKS
   in code.

* arch/arm/src/tiva/common/tiva_serial.c
  (struct up_dev_s): Add new field 'brk' to indicate line break in
   progress when built with CONFIG_TIVA_UART_BREAKS.
  (up_ioctl): Add cases for TIOCSBRK to turn BSD-compatible break on
   unconditionally and TIOCCBRK to turn break off unconditionally.
  (up_txint): Block enabling TX interrupt if line break in progress.
   This is similar to the STM32F7 implementation.

arch/arm/src/tiva/Kconfig

@@ -1081,6 +1081,19 @@ config TIVA_HCIUART_SW_TXFLOW
 
 endmenu # HCI UART Driver Configuration
 
+config TIVA_UART_BREAKS
+	bool "Add TIOxSBRK to support sending Breaks"
+	depends on TIVA_UART0 || TIVA_UART1 || TIVA_UART2 || TIVA_UART3 || TIVA_UART4 || TIVA_UART5 || TIVA_UART6 || TIVA_UART7
+	default n
+	---help---
+		Add TIOCxBRK routines to send a BSD compatible line break.
+		TIOCSBRK will start the break and TIOCCBRK will end the break.
+		This implementation uses the BRK bit (bit 0) of the UART Line
+		Control register (UARTLCRH) to send the break. The break begins
+		after the UART finishes shifting out the current character in
+		progress, if any, including its stop bit(s), and continues
+		indefinitely until stopped by software.
+
 config TIVA_SSI0
 	bool "SSI0"
 	default n

arch/arm/src/tiva/common/tiva_serial.c

@@ -301,6 +301,9 @@ struct up_dev_s
 #ifdef CONFIG_SERIAL_OFLOWCONTROL
   bool     oflow;     /* output flow control (CTS) enabled */
 #endif
+#ifdef CONFIG_TIVA_UART_BREAKS
+  bool     brk;       /* true: Line break in progress */
+#endif
 };
 
 /****************************************************************************
@@ -1099,11 +1102,12 @@ static int up_interrupt(int irq, void *context, void *arg)
 
 static int up_ioctl(struct file *filep, int cmd, unsigned long arg)
 {
-#if defined(CONFIG_SERIAL_TIOCSERGSTRUCT) || defined(CONFIG_SERIAL_TERMIOS)
+#if defined(CONFIG_SERIAL_TIOCSERGSTRUCT) || defined(CONFIG_SERIAL_TERMIOS) \
+    || defined(CONFIG_TIVA_UART_BREAKS)
   struct inode      *inode = filep->f_inode;
   struct uart_dev_s *dev   = inode->i_private;
 #endif
-#if defined(CONFIG_SERIAL_TERMIOS)
+#if defined(CONFIG_SERIAL_TERMIOS) || defined(CONFIG_TIVA_UART_BREAKS)
   struct up_dev_s   *priv  = (struct up_dev_s *)dev->priv;
 #endif
   int                ret   = OK;
@@ -1242,6 +1246,52 @@ static int up_ioctl(struct file *filep, int cmd, unsigned long arg)
       break;
 #endif /* CONFIG_SERIAL_TERMIOS */
 
+#ifdef CONFIG_TIVA_UART_BREAKS
+    case TIOCSBRK:  /* BSD compatibility: Turn break on, unconditionally */
+      {
+        irqstate_t flags;
+        uint32_t tx_break;
+
+        flags = enter_critical_section();
+
+        /* Disable any further tx activity */
+
+        priv->brk = true;
+        up_txint(dev, false);
+
+        /* Send a break signal */
+
+        tx_break = up_serialin(priv, TIVA_UART_LCRH_OFFSET);
+        tx_break |= UART_LCRH_BRK;
+        up_serialout(priv, TIVA_UART_LCRH_OFFSET, tx_break);
+
+        leave_critical_section(flags);
+      }
+      break;
+
+    case TIOCCBRK:  /* BSD compatibility: Turn break off, unconditionally */
+      {
+        irqstate_t flags;
+        uint32_t tx_break;
+
+        flags = enter_critical_section();
+
+        /* Stop sending the break signal */
+
+        tx_break = up_serialin(priv, TIVA_UART_LCRH_OFFSET);
+        tx_break &= ~UART_LCRH_BRK;
+        up_serialout(priv, TIVA_UART_LCRH_OFFSET, tx_break);
+
+        /* Enable further tx activity */
+
+        priv->brk = false;
+        up_txint(dev, true);
+
+        leave_critical_section(flags);
+      }
+      break;
+#endif /* CONFIG_TIVA_UART_BREAKS */
+
     default:
       ret = -ENOTTY;
       break;
@@ -1351,6 +1401,16 @@ static void up_txint(struct uart_dev_s *dev, bool enable)
       /* Set to receive an interrupt when the TX fifo is half emptied */
 
 #ifndef CONFIG_SUPPRESS_SERIAL_INTS
+#  ifdef CONFIG_TIVA_UART_BREAKS
+      /* Do not enable TX interrupt if line break in progress */
+
+      if (priv->brk)
+        {
+          leave_critical_section(flags);
+          return;
+        }
+#  endif
+
       priv->im |= UART_IM_TXIM;
       up_serialout(priv, TIVA_UART_IM_OFFSET, priv->im);