Our application uses QSerialPort to communication with our READ (hardware our company produces).
Since the upgrade to Qt 5.13.1 the communication no longer works. readyRead() is not emitted until a lot of data has been written, but our commands are usually just a few bytes long.
We used Qt 5.9.0 before and all worked fine.
Tested it with Qt 5.12.4 and Qt 5.13.0 - with both at least the Qt Terminal example works fine.

The behavior can easily be reproduced with the Qt Terminal example - it is unusable with Qt 5.13.1.
Our serial port settings are:

	m_serial->setBaudRate(QSerialPort::Baud115200);
	m_serial->setDataBits(QSerialPort::Data8);
	m_serial->setParity(QSerialPort::NoParity);
	m_serial->setStopBits(QSerialPort::OneStop);
	m_serial->setFlowControl(QSerialPort::NoFlowControl);

It appears
Commit: Emit _q_notify only if there's no notification pending
Change-Id: I0ee4522db63844bdd37169d3a31bde2d6f92f383
is a regression.