Avoiding Timeout on QSerialPort when handling high speed data - c++

Question

I'm working on a windows application that receives data from a sensor at 600Hz. In two out of five cases, my IO thread reads the 4 bytes of data from the sensor successfully and passes it on to the GUI thread.
The problem is three out of five times, QSerialPort has inexplicable timeouts where QSerialPort's waitForReadyRead() returns false and serial.errorString() has a timeout error. In which case it will never read data. If I read from the serial port despite the timeout error I will read 2000+ bytes of data in the next waitForReadyRead which will be delivered in chunks which renders the realtime data reception aspect of my application obsolete.
I've tried using the readyRead() signal of the serial port but it exhibits the same behaviour ie. if the timeout error appears, no readyRead() signal is ever fired.
UPDATE: I am able to reproduce the issue with Qt's terminal example ([QT_INSTALL_EXAMPLES]/serialport/terminal) which uses a non-blocking read. The frequency of the bug is considerably less but it's definitely still there.
UPDATE: Using Serial Port Monitor, I can see that when it gets stuck, the Qt Terminal Example gets stuck on IOCTL_SERIAL_WAIT_ON_MASK, my example gets stuck on IRP_MJ_WRITE DOWN just after the IOCT_SERIAL_WAIT_ON_MASK. This never happens with other terminal softwares leading me to think the problem is definitely with Qt.
Pastebin of Serial Port Monitor Output
void IOThread::run(){
QSerialPort serial;
serial.setPortName(portname)
serial.setBaudRage(QSerialPort::Baud115200);
serial.setStopBits(QSerialPort::OneStop)
serial.setParity(QSerialPort::NoParity);
serial.setDataBits(QSerialPort::Data8);
serial.setFlowControl(QSerialPort::NoFlowControl);
if(!serial.open(QIODevice::ReadWrite)
{
qDebug() << "Error Opening Port";
return;
}
else
{
qDebug() << "Error Message: " << serial.errorString() // prints "Unknown Error"
}
while(true)
{
if(serial.waitForReadyRead(1000))
{
qDebug() << "Normal read";
reception_buffer = serial.readAll();
}
else
{
qDebug() << "Timeout";
/* serial.readAll() here will read nothing but force next read to read huge chunk of data */
continue;
}
}
// Process data...
}

Answer 1

Try if this makes any difference:
while (true) {
QByteArray reception_buffer;
if (serial.waitForReadyRead(1000)) {
reception_buffer = serial.readAll();
while (serial.waitForReadyRead(10)) {
reception_buffer += serial.readAll();
}
qDebug() << "reception_buffer ready";
}
else {
qDebug() << "Timeout";
}
}
If you want to prevent from timeouting from waitForReadyRead you can set:
if(serial.waitForReadyRead(-1))
bool QSerialPort::waitForReadyRead(int msecs = 30000) will timeout after msecs milliseconds; the default timeout is 30000 milliseconds. If msecs is -1, the function will not time out.

Answer 2

gets stuck on IOCTL_SERIAL_WAIT_ON_MASK
Most likelly a problem is in your HW or driver. QSP use asynchronous notification, based on WaitCommEvent. If WaitCommEvent get stuck - then a problem is in your device or driver (most likelly).

Answer 3

Thanks to the guys at QSerialPort, this bug in Qt 5.10.1 is solved by applying this patch: https://codereview.qt-project.org/#/c/225277/
"QSP may ignore all the read events when the data comes to the device
within opening. In this case, even re-opening of a device does not
help. Reason is that the QWinOverlappedIoNotifier is enabled after
than the startAsyncCommunication() called, that probably, leads to
ignoring for all EV_RXCHAR events. A workaround is to enable the
notifier before than any of I/O operation called." - Denis Shienkov, QSerialPort Maintainer