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

Hi Toshiyuki,

On 2025-06-03, "Toshiyuki Sato (Fujitsu)" [off-list ref] wrote:

quoted

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