Re: Console Hang on SMP Secondary CPU Bringup

[John Ogness <john.ogness@linutronix.de>]

On 2026-10-02, Derek Barbosa <debarbos@redhat.com> wrote:
>> To be sure it is timing related due to the nbcon atomic printing, you
>> could try short-circuiting:
>> 
>> 	drivers/tty/serial/8250/8250_port.c:serial8250_console_write_atomic()
>> 
>> so that it immediately does
>> 
>> 	return true;
>> 
>
> I will be running more trials of the reproducer to confirm, but so
> far, we still hang. I wasn't able to extract some traces, hopefully
> will be able to soon...

That is surprising.

For the v6.6-rt you can manually add:

#define CONFIG_SERIAL_8250_LEGACY_CONSOLE

to drivers/tty/serial/8250/8250.h and this should cause the driver to
run in legacy mode. If you add this macro to 8250.h, does the hang go
away?

> FWIW, Clark suggested it may be worth setting keep_bootcon in the
> commandline.

If you use keep_bootcon, then the 8250 will operate in a special mode
that is compatible with the bootcons. It is not a solution, even if it
worked.

> I'm off to LPC, so I will be radio silent for another day or two while
> I have these spin on the Azure VMs.

Sebastian and I are also off to LPC, so we can continue the discussion
in person. :-)

John