RE: Problem with nbcon console and amba-pl011 serial port

John Ogness
2025-06-03T10:13:34Z

Hi Toshiyuki,

On 2025-06-03, "Toshiyuki Sato (Fujitsu)" <fj6611ie@fujitsu.com> wrote:
>> 4. pr_emerg() has a high logging level, and it effectively steals the console
>> from the "pr/ttyAMA0" task, which I believe is intentional in the nbcon design.
>> Down in pl011_console_write_thread(), the "pr/ttyAMA0" task is doing
>> nbcon_enter_unsafe() and nbcon_exit_unsafe() around each character
>> that it outputs. When pr_emerg() steals the console, nbcon_exit_unsafe()
>> returns 0, so the "for" loop exits. pl011_console_write_thread() then
>> enters a busy "while" loop waiting to reclaim the console. It's doing this
>> busy "while" loop with interrupts disabled, and because of the panic,
>> it never succeeds. Whatever CPU is running "pr/ttyAMA0" is effectively
>> stuck at this point.
>>
>> 5. Meanwhile panic() continues, calling panic_other_cpus_shutdown(). On
>> ARM64, other CPUs are stopped by sending them an IPI. Each CPU receives
>> the IPI and calls the PSCI function to stop itself. But the CPU running
>> "pr/ttyAMA0" is looping forever with interrupts disabled, so it never
>> processes the IPI and it never stops. ARM64 doesn't have a true NMI that
>> can override the looping with interrupts disabled, so there's no way to
>> stop that CPU.
>>
>> 6. The failure to stop the "pr/ttyAMA0" CPU then causes downstream
>> problems, such as when loading and running a kdump kernel.

[...]

> After reproducing the issue,
> I plan to try a workaround that forcibly terminates the nbcon_reacquire_nobuf
> loop in pl011_console_write_thread if other_cpu_in_panic is true.
> Please comment if you have any other ideas.

For panic, if it is OK to leave uap->clk enabled and not restore REG_CR,
then it should be fine to just return. But only for panic.

So something like:

while (!nbcon_enter_unsafe(wctxt)) {
if (other_cpu_in_panic())
return;
nbcon_reacquire_nobuf(wctxt);
}

(And other_cpu_in_panic() will need to be made generally available.)

John Ogness