Re: input lag on /dev/ttyACM0?

[Jeffrey Walton <noloader@gmail.com>]

On Wed, Oct 29, 2025 at 1:04 AM Arina Lynn Grace
<arinalynngrace@protonmail.com> wrote:
>
> Good evening,
>
> I'm trying to get serial data from a microcontroller board (Adafruit Feather RP2350) for debug purposes. My test code sends out an incrementing number every x seconds. If x == 1 second, it works no problem with any serial monitor (such as tio, minicom, or the Arduno IDE). However, if x >= 2.3 seconds, the data stops after a message or two. The serial port doesn't disconnect when the issue occurs, I only get disconnect alerts when I reset or physically unplug the board. I have tried multiple microcontroller boards from multiple brands, I've switched cables, USB ports, nothing works. This issue occurs on my Debian 12 Framework 13 laptop and my partner's Debian 13 Thinkpad T480s, but not my Debian 10 server, my freshly reflashed Raspberry Pi 3, or my WIndows 10 PC. Those computers accept the data just fine. I know the code is running properly on the boards because I set the built in LED to flash every time a serial message is sent out. The common factor seems to be our laptops.
>
> I have checked lsof /dev/ttyACM0 to rule out other software grabbing the serial port, I've checked udev and stty rules (well, my partner did) to the best of my (well, her) ability, everything is identical between the different computers serial settings. My partner updated her kernel to the latest version and it still occurs.
>
> I'm out of ideas, and this issue has completely stopped my personal projects. Does anyone have any ideas what could be going wrong?

Some code or a minimal reproducer would probably be helpful.

One thing I do after opening /dev/ttyACM0 is set TIOCEXCL on the file
descriptor to ensure other programs, like NetworkManager or
ModemManager, do not open the device:

    const char device[] = "/dev/ttyACM0";
    fd = open(device, O_RDWR | O_NOCTTY | O_SYNC);
    if (fd == -1) {
        log_error("Failed to open device %s: %s\n", device, strerror(errno));
        goto finished;
    }

    if (ioctl(fd, TIOCEXCL, NULL) == -1) {
        log_warn("Failed to set TIOCEXCL on device: %s\n", strerror(errno));
    }

Jeff