QSerialPort: delayed signal on new received bytes

McLion

[quote author="kuzulis" date="1405088316"]
Move the QSerialPort to separate QThread and set "thread priority": http://qt-project.org/doc/qt-4.8/qthread.html#setPriority.

[/quote]

Sounds like quite some piece of work since I have no experience with QThread .. never used before.

Thanks

McLion

What I don't get is the difference between QextSerialPort and QSerialPort. since they both rely on the native Linux IO functions (AFAIK, wrapped by QIO device) how comes they have such a huge difference in emitting the readyRead signal?

I am not really sure putting the serial port in a seperate thread is the best solution. It is also recommended "Best Practice" to not put it in a separate thread for asynchronous communication. When I see that QextSerialPort does not have this delay and emits the readyRead like immediately, I strongly feel that something with the QSerialPort code is wrong.

kuzulis

bq. I strongly feel that something with the QSerialPort code is wrong.

You can itself try to find a problem place. The main places (difference) at which I would to check, is:

Try to add this code (from the QextSerialPort "sources":https://code.google.com/p/qextserialport/source/browse/src/qextserialport_unix.cpp ,line 86):

@
const long vdisable = ::fpathconf(fd, _PC_VDISABLE);
currentTermios.c_cc[VINTR] = vdisable;
currentTermios.c_cc[VQUIT] = vdisable;
currentTermios.c_cc[VSTART] = vdisable;
currentTermios.c_cc[VSTOP] = vdisable;
currentTermios.c_cc[VSUSP] = vdisable;
@

at initialization of device.

Try to change the QSocketNotifier's event handling from the direct events to the signal/slot. The QSerialPort override the QSocketNotifier::event() method, but the QextSerialPort use the QSocketNotifier::activated(int) signal.
Maybe something excessively in QSerialPortPrivate::readNotification() method. Maybe is reasonable to simplify it up to:

@
// Always buffered, read data from the port into the read buffer
qint64 newBytes = readBuffer.size();
qint64 bytesToRead = policy == QSerialPort::IgnorePolicy ? ReadChunkSize : 1;

if (readBufferMaxSize && bytesToRead > (readBufferMaxSize - readBuffer.size())) {
    bytesToRead = readBufferMaxSize - readBuffer.size();
    if (bytesToRead == 0) {
        // Buffer is full. User must read data from the buffer
        // before we can read more from the port.
        return false;
    }
}

char *ptr = readBuffer.reserve(bytesToRead);
const qint64 readBytes = readFromPort(ptr, bytesToRead);

if (readBytes <= 0) {
    QSerialPort::SerialPortError error = decodeSystemError();
    if (error != QSerialPort::ResourceError)
        error = QSerialPort::ReadError;
    else
        setReadNotificationEnabled(false);
    q->setError(error);
    readBuffer.chop(bytesToRead);
    return false;
}

readBuffer.chop(bytesToRead - qMax(readBytes, qint64(0)));

newBytes = readBuffer.size() - newBytes;

// If read buffer is full, disable the read port notifier.
if (readBufferMaxSize && readBuffer.size() == readBufferMaxSize)
    setReadNotificationEnabled(false);

// only emit readyRead() when not recursing, and only if there is data available
const bool hasData = newBytes > 0;

if (!emittedReadyRead && hasData) {
    emittedReadyRead = true;
    emit q->readyRead();
    emittedReadyRead = false;
}

if (!hasData)
    setReadNotificationEnabled(true);

@

McLion

I'll read through that.
One additional question: I assume it doesn't make a difference if I use
@QByteArray read(qint64 maxSize)@

instead of
@virtual qint64 readData(char * data, qint64 maxSize)@

to read out data, though readData seems to read from a ringbuffer and read reads from IODevice?

kuzulis

The readData(() it is protected method of QIODevice. You can't use it in your user code. The common chain is following: read() -> readData() -> read from ring buffer.

The implementation of QSerialPort::readData() always read data from the internal ring buffer, but implementation of QextSerialPort try to read from the internal buffer + read directly from device.

So, it make difference between read() and readData(). :)

McLion

Arrgghh ... trying to modify code of QSerialPort as you suggested.
Last week I could build successfully on Windows and Linux, now I crash even building on windows with a
@DEVINST was not declared in this scope @

kuzulis

Try to replace the DEVINST by DWORD. Most likely your compiler do not contains the "cfgmgr32.h" file (or file has other place).

McLion

Solved the DEVINST issue and added the termios settings. No change in the behaviour. There are only 3 diffs left from stty --a --F /dev/ttyS1 between QextSerialPort and QSerialPort. The following 3 option are deactivated (-) with QextSerialPort and activated with QSerialPort:
@ignpar
echoe
echok
@

and I don't think that this will make the difference. Now going to look into your further suggestions.

McLion

Found that I have bytes dropping with QSerial as well. Seems to be some issue with underlying native linux or alike.
Coming back ...

kuzulis

bq. Found that I have bytes dropping with QSerial as well.

There can't be.

Btw: this is an other issue not related with this thread.

McLion

You're right, it's not exactly related to the topic of this thread.
I just wanted to update this thread with the fact that I am not further investigating the delayed readyRead as long as I have bytes dropping.
I actually implemented QSerial on my 4.6.3 because I thought that the bytes dropping was an issue of QextSerialPort. As it turns out, the issue is somewhere else (HW / underlying Linux).