We are trying to establish communication between a ttyACMx UART device & a PC which runs CentOS8.

We are using gtkterm to send data to the UART device.
The version of gtkterm is :
[root@localhost arunodaya.s]# gtkterm --help
GTKTerm version 1.0
(c) Julien Schmitt

Kernal version is :
[root@localhost arunodaya.s]# uname -a
Linux localhost.localdomain 4.18.0-147.8.1.el8_1.x86_64 #1 SMP Thu Apr 9 13:49:54 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

If we connect a loop back device (ttyUSBx), we are able to communicate using gtkterm.

We tried in a machine with fedora 31 & found the same problem.

The same device (UART) is communicating fine in an ubuntu 18 (LTS version).

O/P log of strace when using gtkterm

0 openat(AT_FDCWD, "/dev/ttyACM0", O_RDWR|O_NOCTTY|O_NONBLOCK) = 10
1 flock(10, LOCK_EX|LOCK_NB) = 0
2 ioctl(10, TCGETS, {B115200 -opost -isig -icanon -echo ...}) = 0
3 ioctl(10, SNDCTL_TMR_START or TCSETS, {B115200 -opost -isig -icanon -echo ...}) = 0
4 ioctl(10, TCGETS, {B115200 -opost -isig -icanon -echo ...}) = 0
5 ioctl(10, TCFLSH, TCOFLUSH) = 0
6 ioctl(10, TCFLSH, TCIFLUSH) = 0
7 fstat(10, {st_mode=S_IFCHR|0660, st_rdev=makedev(166, 0), ...}) = 0
8 fcntl(10, F_GETFL) = 0x8802 (flags O_RDWR|O_NONBLOCK|O_LARGEFILE)
9 write(4, "\1\0\0\0\0\0\0\0", 8) = 8
10 fstat(10, {st_mode=S_IFCHR|0660, st_rdev=makedev(166, 0), ...}) = 0
11 fcntl(10, F_GETFL) = 0x8802 (flags O_RDWR|O_NONBLOCK|O_LARGEFILE)
12 write(4, "\1\0\0\0\0\0\0\0", 8) = 8
13 write(4, "\1\0\0\0\0\0\0\0", 8) = 8
14 poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}, {fd=5, events=POLLIN}], 3, 0) = 1 ([{fd=4, revents=POLLIN}])
15 read(4, "\5\0\0\0\0\0\0\0", 16) = 8
16 poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}, {fd=5, events=POLLIN}], 3, 0) = 0 (Timeout)
17 poll([{fd=3, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=3, revents=POLLOUT}])

Kindly suggest us a solution.

Just to verify, did you also try other ways to communicate, like, other software like minicom, screen? did you also compare the relevant strace output for both working(ubuntu) and not working systems?

We tried with minicom, we had the same issue. We compared the strace output, the difference is this extra line which has timeout for all file descriptors. We are not sure whether this will lead to this behavior.

"16 poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}, {fd=5, events=POLLIN}], 3, 0) = 0 (Timeout)"

Is it all on the SAME machine with SAME device?

The root cause of the issue is with auto initialization of Modem(ttyACM) and the way it is handled. We added a sniffer in between the OS and the device & found that, before we send any data, the OS itself sends "AT" command for modem initialization. This was rendering the ttyACM device to go into undefined mode. The device in question is not a modem but is enumerating as one (Virtual COM Port).

We have disabled the ModemManager service and now as it looks like it is working normally. Need to find a more graceful way of handling this.

No, we checked with different configuration (same device but different machine)

That's very informative, thanks.
Maybe there's a way to disable handling of modems in there by default instead of shutting the whole thing down. There must be.

If we Change the ModemManager.service to start in strict instead of default, this issue can be overcome as well.