the usage of hardware flow control in serial console

the usage of hardware flow control in serial console

[xuyang2018.jy]

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

Re: the usage of hardware flow control in serial console

[Greg KH]

On Tue, Jun 01, 2021 at 02:39:41AM +0000, xuyang2018.jy@fujitsu.com wrote:
> 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.)

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

> Does it need my console connected with other machine?

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

> 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).

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

thanks,

greg k-h