HI All I append "console=ttyS0,115200n8r" to my kernel command line and reboot my system. # cat /proc/tty/driver/serial serinfo:1.0 driver revision: 0: uart:16550A port:000003F8 irq:4 tx:0 rx:0 RTS|DTR 1: uart:unknown port:000002F8 irq:3 2: uart:unknown port:000003E8 irq:4 3: uart:unknown port:000002E8 irq:3 then I echo 1 > /dev/ttyS0, but it hangs.(If I don't use r, it works well.) Does it need my console connected with other machine? Also, many services become timeout when I use systemctl restart/status/stop(tx,rx data doesn't increase). I just want to check whether it is a usage problem(my system environmet is centos8). My machine is physical machine and virtual machine doesn't have this problem(it has RTS|CTS|DTR|DSR|CD flag). Best Regards Yang Xu
