the usage of hardware flow control in serial console

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

'echo' is horrible at testing serial port communication. Why not use a real serial terminal program that knows how to set the line settings?

Something needs to be on the other end of that serial connection for this to work properly, right?

Why do you want to use "r" here? What problem are you trying to solve?

thanks,

greg k-h