CentOS - Oct 2015 - CentOS7 - Serial Console and Flow Control

Mike - st257 wrote:
> Hello List,
>
> I'm ironing out details to upgrade a few systems to CentOS7.
>
> My servers have BMC with Serial over LAN support. In C5 and C6, I
determined how to have BIOS/POST, kernel, and serial console access. I'm
reading up on the method to accomplish the pieces with
C7.
>
> Presently SoL output works, so I see BIOS/POST messages and the GRUB
boot list.
> My changes to enable serial redirection for the kernel do not appear to
work.
>
> I've made the following changes to GRUB2's /etc/grub/default config
file: -- removed rhgb
<snip>
You did take out "quiet", too?

        mark

On Thu, Oct 15, 2015 at 5:50 PM, <m.roth at 5-cent.us> wrote:
> Mike - st257 wrote:
> > Hello List,
> >
> > I'm ironing out details to upgrade a few systems to CentOS7.
> >
> > My servers have BMC with Serial over LAN support. In C5 and C6, I
> determined how to have BIOS/POST, kernel, and serial console access.
I'm
> reading up on the method to accomplish the pieces with C7.
> >
> > Presently SoL output works, so I see BIOS/POST messages and the GRUB
> boot list.
> > My changes to enable serial redirection for the kernel do not appear
to
> work.
> >
> > I've made the following changes to GRUB2's /etc/grub/default
config
> file: -- removed rhgb
> <snip>
> 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.

I'll give that a shot today.

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...

>
>         mark
>
>
>
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos
>


-- 
---~~.~~---
Mike
//  SilverTip257  //

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.

On 10/16/2015 09:14 AM, Lamar Owen wrote:
> [It is documented that] 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.
>
Replying to myself with a correction to my statement and a 
clarification.... while it is documented this way, I do remember having 
a bit rate mismatch without specifying a bit rate of 9600, so it's 
possible the default bit rate was changed upstream.  And it's possible I 
that I am remembering incorrectly, and just put the parameter there anyway.

Mike - st257 wrote:
> On Thu, Oct 15, 2015 at 5:50 PM, <m.roth at 5-cent.us> wrote:
>> Mike - st257 wrote:
<snip>
>> > My servers have BMC with Serial over LAN support. In C5 and C6, I
>> determined how to have BIOS/POST, kernel, and serial console access.
I'm
>> reading up on the method to accomplish the pieces with C7.
>> >
>> > Presently SoL output works, so I see BIOS/POST messages and the
GRUB
>> boot list.
>> > My changes to enable serial redirection for the kernel do not
appear
>> to work.
>> >
>> > I've made the following changes to GRUB2's
/etc/grub/default config
>> file: -- removed rhgb
>> <snip>
>> 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.
>
> I'll give that a shot today.
<snip>
Do. That hides everything before the o/s switches root. It's *possible*
that it's hiding where you're being prompted for the LUKS password.
Personally, I'd have left / unencrypted....

     mark