termios: Notify driver about inactive transmit

Returning this state in the return value leads to race conditions on
SMP.  The inactive state notification must be inside the critical
section.

cpukit/libcsupport/src/termios.c

@@ -1332,6 +1332,7 @@ rtems_termios_refill_transmitter (struct rtems_termios_tty *tty)
       wakeUpWriterTask = true;
     }
 
+    (*tty->device.write) (tty->minor, NULL, 0);
     nToSend = 0;
   } else {
     len = tty->t_dqlen;
@@ -1351,6 +1352,7 @@ rtems_termios_refill_transmitter (struct rtems_termios_tty *tty)
        * Buffer has become empty
        */
       tty->rawOutBufState = rob_idle;
+      (*tty->device.write) (tty->minor, NULL, 0);
       nToSend = 0;
 
       /*
@@ -1367,6 +1369,7 @@ rtems_termios_refill_transmitter (struct rtems_termios_tty *tty)
       /* set flag, that output has been stopped */
       tty->flow_ctrl |= FL_OSTOP;
       tty->rawOutBufState = rob_busy; /*apm*/
+      (*tty->device.write) (tty->minor, NULL, 0);
       nToSend = 0;
     } else {
       /*

doc/bsp_howto/console.t

@@ -408,6 +408,12 @@ static ssize_t my_driver_interrupt_write(int minor, const char *buf, size_t n)
      * characters in the device data structure.
      */
 
+    /*
+     * Termios will set n to zero to indicate that the transmitter is
+     * now inactive.  The output buffer is empty in this case.  The
+     * driver may disable the transmit interrupts now.
+     */
+
     return 0;
 @}
 @end group