是什么导致Mono下SerialPort.Write中的零星错误？

Question

I'm using a USB connection (FTDI) from Linux to an Arduino Nano under Mono. This mostly works but after a while I get, at a seemingly random time, a TimeoutException attempting to Write to the SerialPort. Apparently (see the FIXME and associated bug report ), any error in Write will result in a TimeoutException, and there is no delay in timing, so I think it is fair to assume that it isn't actually a timeout, but just an error.

Mono code:

// FIXME: this reports every write error as timeout
if (write_serial (fd, buffer, offset, count, write_timeout) < 0)
    throw new TimeoutException("The operation has timed-out");

Here (see write_serial) is the Mono wrapper being called, which appears to turn any error result from "poll" or "write" into a simple -1 error result.

Mono Helper:

int
write_serial (int fd, guchar *buffer, int offset, int count, int timeout)
{
    struct pollfd pinfo;
    guint32 n;

    pinfo.fd = fd;
    pinfo.events = POLLOUT;
    pinfo.revents = POLLOUT;

    n = count;

    while (n > 0)
    {
        ssize_t t;

        if (timeout != 0) {
            int c;

            while ((c = poll (&pinfo, 1, timeout)) == -1 && errno == EINTR)
                ;
            if (c == -1)
                return -1;
        }       

        do {
            t = write (fd, buffer + offset, n);
        } while (t == -1 && errno == EINTR);

        if (t < 0)
            return -1;

        offset += t;
        n -= t; 
    }

    return 0;
}

I've had no problems communicating from Windows/.NET to the same device and using the same write code (you'll see below).

This leaves me at a bit of a loss as to how to even diagnose this without writing some extensive C tests that I hope would reproduce the same IO patterns and timing as my current code does. There are port sniffers, but that would merely capture the data, not diagnose and error. Is there perhaps a way to catch and log IO errors in the Linux kernel?

My code is pretty straight forward. Here is the relevant snippet:

    _port = new SerialPort(portName, Configuration.BaudRate);
    _port.NewLine = "\n";
    _port.ReadTimeout = Configuration.StartupCommandTimeout; // 7000ms
    _port.WriteTimeout = Configuration.StartupCommandTimeout;
    _port.Open();

    _port.WriteLine(command);

This always happens away from the main thread, though may not always occur on the same thread that the port was constructed, read from, and otherwise written to.

Using Mono version 3.2.8, though the code below is from Master as doesn't appeared to have changed. I'm running Ubuntu 15.10 on ARM.

Answer 1

In my control system I have many threads executing simultaneously. By giving increased thread priority (ThreadPriority.AboveNormal) to the actor (essentially thread) which services the serial port, I seem to have minimized or possibly eliminated the problem. Perhaps Linux is more sensitive to serial port service starvation issues than Windows, or it is the varying hardware.