QSerialPort::waitForBytesWritten() does not work properly

Now, in Qt5.2.1 QTBUG-33987 seems to be fixed, I have no longer the problem that waitForReadyRead returns true although the following readAll() returns an empty QByteArray.

Unfortunately, the method waitForBytesWritten seems to be broken now. Sometimes, not always, the function timeouts and therefore returns false. At the moment I have two solutions:

1. Don't call waitForBytesWritten. I've a handshake based communication, so every write causes a response, that I check by calling waitForReadyRead. Instead of
   write -> waitForBytesWritten -> waitForReadyRead -> readAll
   I just use it this way now:
   write -> waitForReadyRead -> readAll
2. Return to Qt5.2.0