diff --git a/apps/drivers/px4fmu/fmu.cpp b/apps/drivers/px4fmu/fmu.cpp
index a995f6214d..2e3b130a9c 100644
--- a/apps/drivers/px4fmu/fmu.cpp
+++ b/apps/drivers/px4fmu/fmu.cpp
@@ -500,7 +500,7 @@ PX4FMU::pwm_ioctl(file *filp, int cmd, unsigned long arg)
 		/* FALLTHROUGH */
 	case PWM_SERVO_GET(0):
 	case PWM_SERVO_GET(1): {
-			channel = cmd - PWM_SERVO_SET(0);
+			channel = cmd - PWM_SERVO_GET(0);
 			*(servo_position_t *)arg = up_pwm_servo_get(channel);
 			break;
 		}
diff --git a/nuttx/drivers/serial/serial.c b/nuttx/drivers/serial/serial.c
index c650da5db3..28c657af05 100644
--- a/nuttx/drivers/serial/serial.c
+++ b/nuttx/drivers/serial/serial.c
@@ -660,9 +660,9 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
 
   int ret = dev->ops->ioctl(filep, cmd, arg);
 
-  /* Append any higher level TTY flags */
+  /* If the low-level handler didn't handle the call, see if we can handle it here */
 
-  if (ret == OK)
+  if (ret == -ENOTTY)
     {
       switch (cmd)
         {
@@ -686,8 +686,43 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
             irqrestore(state);
 
             *(int *)arg = count;
+            ret = 0;
+
+            break;
           }
 
+          case FIONWRITE:
+          {
+            int count;
+            irqstate_t state = irqsave();
+
+            /* determine the number of bytes free in the buffer */
+
+            if (dev->xmit.head < dev->xmit.tail)
+              { 
+                count = dev->xmit.tail - dev->xmit.head - 1;
+              }
+            else
+              {
+                count = dev->xmit.size - (dev->xmit.head - dev->xmit.tail) - 1;
+              }
+
+            irqrestore(state);
+
+            *(int *)arg = count;
+            ret = 0;
+
+            break;
+          }
+        }
+    }
+
+  /* Append any higher level TTY flags */
+
+  else if (ret == OK)
+    {
+      switch (cmd)
+        {
 #ifdef CONFIG_SERIAL_TERMIOS
           case TCGETS:
           {
diff --git a/nuttx/include/nuttx/fs/ioctl.h b/nuttx/include/nuttx/fs/ioctl.h
index 08f62e1648..6d60c2ee97 100644
--- a/nuttx/include/nuttx/fs/ioctl.h
+++ b/nuttx/include/nuttx/fs/ioctl.h
@@ -110,6 +110,10 @@
                                            * OUT: Bytes readable from this fd
                                            */
 
+#define FIONWRITE       _FIOC(0x0005)     /* IN:  Location to return value (int *)
+                                           * OUT: Bytes writable to this fd
+                                           */
+
 /* NuttX file system ioctl definitions **************************************/
 
 #define _DIOCVALID(c)   (_IOC_TYPE(c)==_DIOCBASE)