Problem with nbcon console and amba-pl011 serial port

[John Ogness]

On 2025-06-04, Petr Mladek <pmladek at suse.com> wrote:
> On Wed 2025-06-04 04:11:10, Toshiyuki Sato (Fujitsu) wrote:
>> > On 2025-06-03, John Ogness <john.ogness at linutronix.de> wrote:
>> > > On 2025-06-03, "Toshiyuki Sato (Fujitsu)" <fj6611ie at 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.
>
> I am a bit surprised that it never succeeds. The panic CPU takes over
> the ownership but it releases it when the messages are flushed. And
> the original owner should be able to reacquire it in this case.

The problem is that other_cpu_in_panic() will return true forever, which
will cause _all_ acquires to fail forever. Originally we did allow
non-panic to take over again after panic releases ownership. But IIRC we
removed that capability because it allowed us to reduce a lot of
complexity. And now nbcon_waiter_matches() relies on "Lower priorities
are ignored during panic() until reboot."

John Ogness