drivers/usbdev: Correct input flow control logic when watermarks are not enabled. Problem not by and change based on suggestion by Juha Niskanen.

drivers/serial/Kconfig

@@ -518,7 +518,7 @@ if SERIAL_IFLOWCONTROL_WATERMARKS
 config SERIAL_IFLOWCONTROL_LOWER_WATERMARK
 	int "RX lower Watermark (percent)"
 	default 10
-	range 0 100
+	range 1 99
 	---help---
 		Call the rxflowcontrol method then there are this amount (or or less)
 		data buffered in the serial drivers RX buffer.  This is expressed
@@ -528,7 +528,7 @@ config SERIAL_IFLOWCONTROL_LOWER_WATERMARK
 config SERIAL_IFLOWCONTROL_UPPER_WATERMARK
 	int "RX upper Watermark (percent)"
 	default 90
-	range 0 100
+	range 1 99
 	---help---
 		Call the rxflowcontrol method then there are this amount (or more)
 		data buffered in the serial drivers RX buffer.  This is expressed

drivers/serial/serial.c

@@ -70,6 +70,24 @@
 
 #define uart_putc(ch) up_putc(ch)
 
+/* Check watermark levels */
+
+#if defined(CONFIG_SERIAL_IFLOWCONTROL) && \
+    defined(CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS)
+#  if CONFIG_SERIAL_IFLOWCONTROL_LOWER_WATERMARK < 1
+#    warning CONFIG_SERIAL_IFLOWCONTROL_LOWER_WATERMARK too small
+#  endif
+#  if CONFIG_SERIAL_IFLOWCONTROL_UPPER_WATERMARK > 99
+#    warning CONFIG_SERIAL_IFLOWCONTROL_UPPER_WATERMARK too large
+#  endif
+#  if CONFIG_SERIAL_IFLOWCONTROL_LOWER_WATERMARK >= CONFIG_SERIAL_IFLOWCONTROL_UPPER_WATERMARK
+#    warning CONFIG_SERIAL_IFLOWCONTROL_LOWER_WATERMARK too large
+#    warning Must be less than CONFIG_SERIAL_IFLOWCONTROL_UPPER_WATERMARK
+#  endif
+#endif
+
+/* Timing */
+
 #define HALF_SECOND_MSEC 500
 #define HALF_SECOND_USEC 500000L
 
@@ -995,7 +1013,7 @@ static ssize_t uart_read(FAR struct file *filep, FAR char *buffer, size_t buflen
       (void)uart_rxflowcontrol(dev, nbuffered, false);
     }
 #else
-  /* If the RX  buffer empty */
+  /* Is the RX buffer empty? */
 
   if (rxbuf->head == rxbuf->tail)
     {

drivers/usbdev/cdcacm.c

@@ -515,7 +515,10 @@ static int cdcacm_recvpacket(FAR struct cdcacm_dev_s *priv,
     }
 
 #ifdef CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS
-  /* Pre-calcuate the watermark level that we will need to test against. */
+  /* Pre-calcuate the watermark level that we will need to test against.
+   * Note that the range of the the upper watermark is from 1 to 99 percent
+   * and that the actual capacity of the RX biffer is (recv->size - 1).
+   */
 
   watermark = (CONFIG_SERIAL_IFLOWCONTROL_UPPER_WATERMARK * recv->size) / 100;
 #endif
@@ -535,8 +538,8 @@ static int cdcacm_recvpacket(FAR struct cdcacm_dev_s *priv,
 
   while (nexthead != recv->tail && nbytes < reqlen)
     {
-#ifdef CONFIG_SERIAL_IFLOWCONTROL
+#if defined(CONFIG_SERIAL_IFLOWCONTROL) && \
-#ifdef CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS
+    defined(CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS)
       unsigned int nbuffered;
 
       /* How many bytes are buffered */
@@ -565,21 +568,6 @@ static int cdcacm_recvpacket(FAR struct cdcacm_dev_s *priv,
               break;
             }
         }
-#else
-      /* Check if RX buffer is full and allow serial low-level driver to pause
-       * processing. This allows proper utilization of hardware flow control.
-       */
-
-      if (nexthead == rxbuf->tail)
-        {
-          if (cdcuart_rxflowcontrol(&priv->serdev, recv->size, true))
-            {
-              /* Low-level driver activated RX flow control, exit loop now. */
-
-              break;
-            }
-        }
-#endif
 #endif
 
       /* Copy one byte to the head of the circular RX buffer */
@@ -604,10 +592,23 @@ static int cdcacm_recvpacket(FAR struct cdcacm_dev_s *priv,
 
   recv->head = currhead;
 
+#if defined(CONFIG_SERIAL_IFLOWCONTROL) && \
+    !defined(CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS)
+  /* Check if RX buffer became full and allow serial low-level driver to
+   * pause processing. This allows proper utilization of hardware flow
+   * control when there are no watermarks.
+   */
+
+ if (nexthead == recv->tail)
+   {
+     (void)cdcuart_rxflowcontrol(&priv->serdev, recv->size, true);
+   }
+#endif
+
   /* If data was added to the incoming serial buffer, then wake up any
    * threads is waiting for incoming data. If we are running in an interrupt
-   * handler, then the serial driver will not run until the interrupt handler
+   * handler, then the serial driver will not run until the interrupt
-   * returns.
+   * handler returns.
    */
 
   if (nbytes > 0)