[CentOS] CentOS7 - Serial Console and Flow Control

[Lamar Owen]

On 10/16/2015 08:23 AM, Mike - st257 wrote:
> On Thu, Oct 15, 2015 at 5:50 PM, <m.roth at 5-cent.us> wrote:
>
>> You did take out "quiet", too?
> I did not.
> I would expect what shows up on the VGA console to be identical on the
> serial console.
> ...
> To make matters more complex, this is an offsite box for which I've
> implemented full disk encryption.  And if I don't get the LUKS passphrase
> prompt on the serial console, well I'm in a bind...
For the LUKS prompt to show in text mode you do have to remove the 
quiet.  I'd be interested in finding out if the /etc/default/grub that I 
posted works for you (with the proper change in UUID's of course).

The key things for me to get this working was getting the bit rate 
correct (the GRUB serial port settings do not propagate to the kernel; 
you have to set all the serial port settings on the kernel command 
line); you have to make sure the bit rate of the kernel's serial console 
and your SoL BMC are the same (and your original problem description 
made it sound like they were not set up the same); the documentation to 
set this up is found in 
/usr/share/doc/kernel-doc-3.10.0/Documentation/serial-console.txt which 
is part of the kernel-doc package.

For hardware (RTS) flow control at the kernel (and assuming your SoL 
uses 115200 for the bit rate, and assuming you want the VGA to still be 
a console), you might want the parameters:
console=ttyS0,115200n8r console=tty0

You don't actually have to keep the VGA as a console, incidentally, for 
either GRUB or the kernel.  I have run a machine that way before.  I 
personally am not a fan of using flow control on a console; I tend to 
just set the bit rates lower (I have gotten burned before by RTS/CTS 
flow control on a device console).  But that's just my personal 
preference; the 'r' at the end of the parameters for the serial console 
selects RTS/CTS flow control.

By default the kernel will set up the serial console for 9600n8 
operation, regardless of what you set it for GRUB or the serial console 
redirect in your BMC setup.

I'll reiterate that systemd does the Right Thing for the case of a 
serial console, and spawns a getty with the same parameters as you set 
on the kernel's console line; you do not need to create a separate 
.service file for the serial console's getty, in my experience.