Syslinux - Jan 2017 - How to show boot prompt and edit kernel cmdline from serial?

For testing purposses, I'm trying to hook on QEmu an ISO image that
start automatically with syslinux (just a DEFAULT label and no TIMEOUT
option) and change the kernel arguments to redirect the console to a
serial port instead of the screen.

So far I've seen that the SERIAL statement give me a welcome message
and the loading of the kernel and initramfs on the serial port (but
curiously, not the "Loading NodeOS" message that I put, just directly
the kernel string...), but the system start automatically. I've read
in the docs that the boot prompt can be forced to be shown if the
caps_lock or the scroll_lock keys are set, but QEmu doen't offer a way
to have them enabled at boot time and by using the monitor seems to be
too complicated (and since syslinux is booting automatically, too much
time constrained too). So I ask: is there a way to force to show the
boot prompt on the serial port on demand? Maybe a magic characters
sequence I send over the serial port itself after I receive the
syslinux welcome message?


-- 
"Si quieres viajar alrededor del mundo y ser invitado a hablar en un
monton de sitios diferentes, simplemente escribe un sistema operativo
Unix."
? Linus Tordvals, creador del sistema operativo Linux

> For testing purposses, I'm trying to hook on QEmu an ISO image that
> start automatically with syslinux (just a DEFAULT label and no TIMEOUT
> option) and change the kernel arguments to redirect the console to a
> serial port instead of the screen.
> 
> So far I've seen that the SERIAL statement give me a welcome message
> and the loading of the kernel and initramfs on the serial port (but
> curiously, not the "Loading NodeOS" message that I put, just
directly
> the kernel string...), but the system start automatically. I've read
> in the docs that the boot prompt can be forced to be shown if the
> caps_lock or the scroll_lock keys are set, but QEmu doen't offer a way
> to have them enabled at boot time and by using the monitor seems to be
> too complicated (and since syslinux is booting automatically, too much
> time constrained too). So I ask: is there a way to force to show the
> boot prompt on the serial port on demand? Maybe a magic characters
> sequence I send over the serial port itself after I receive the
> syslinux welcome message?
> 
> 
> -- 
 
What's wrong with using "PROMPT 1" in your syslinux.cfg?

Generally speaking, posting the entire cfg might help.

Regards,
Ady.

> What's wrong with using "PROMPT 1" in your syslinux.cfg?
Nothing at all, in fact the solution I'm using is it combined with
TIMEOUT 1, and it works :-) It's not fully nice on regular usage to
have a useless "boot: " prompt for just one 1/10 seconds, but at least
it gives my test environment an opportunity to hook there, write
"NodeOS console=ttyS0" and start doing the boot on the terminal
instead of doing the regular boot on the QEmu window :-)

> Generally speaking, posting the entire cfg might help.
Just for the record, my (somewhat hacky) working syslinux.cfg file is:


SERIAL 0
PROMPT 1
TIMEOUT 1

DEFAULT NodeOS
LABEL NodeOS
  SAY Booting NodeOS...
  LINUX  bzImage
  INITRD initram.gz
  APPEND vga=0x318 ip=dhcp root=$USERSFS


It's really a template, the $USERSFS string is changed for the actual
UUID of the partition after it's being generated.


-- 
"Si quieres viajar alrededor del mundo y ser invitado a hablar en un
monton de sitios diferentes, simplemente escribe un sistema operativo
Unix."
? Linus Tordvals, creador del sistema operativo Linux