For those coming here from Ubuntu or derivatives, I've uploaded a patched version of qt5 serialport package to my ppa: https://launchpad.net/~hyozd/+archiv...tu/serialplot/ . I basically back-ported fixes from Qt 5.6. Here is the diff:

Qt Code: 
  1. --- a/src/serialport/qserialport_unix.cpp
  2. +++ b/src/serialport/qserialport_unix.cpp
  3. @@ -208,14 +208,11 @@
  4.  
  5.  void QSerialPortPrivate::close()
  6.  {
  7. -    if (settingsRestoredOnClose) {
  8. -        if (::tcsetattr(descriptor, TCSANOW, &restoredTermios) == -1)
  9. -            setError(getSystemError());
  10. -    }
  11. +    if (settingsRestoredOnClose)
  12. +        ::tcsetattr(descriptor, TCSANOW, &restoredTermios);
  13.  
  14.  #ifdef TIOCNXCL
  15. -    if (::ioctl(descriptor, TIOCNXCL) == -1)
  16. -        setError(getSystemError());
  17. +        ::ioctl(descriptor, TIOCNXCL);
  18.  #endif
  19.  
  20.      if (readNotifier) {
  21. @@ -228,8 +225,7 @@
  22.          writeNotifier = Q_NULLPTR;
  23.      }
  24.  
  25. -    if (qt_safe_close(descriptor) == -1)
  26. -        setError(getSystemError());
  27. +    qt_safe_close(descriptor);
  28.  
  29.      lockFileScopedPointer.reset(Q_NULLPTR);
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