From eaaa69da0a591105437d883ec480a01454cf28d0 Mon Sep 17 00:00:00 2001
From: Gregory Nutt <gnutt@nuttx.org>
Date: Tue, 26 Jul 2016 06:47:53 -0600
Subject: [PATCH] serial.c: Fix an error in FIONWRITE calculation

---
 drivers/serial/serial.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c
index 49d6a970b7..2b6d7ec8e5 100644
--- a/drivers/serial/serial.c
+++ b/drivers/serial/serial.c
@@ -946,7 +946,7 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
 
               /* Determine the number of bytes waiting in the TX buffer */
 
-              if (dev->xmit.head < dev->xmit.tail)
+              if (dev->xmit.tail <= dev->xmit.head)
                 {
                   count = dev->xmit.head - dev->xmit.tail;
                 }