[PATCH] powerpc/crash: Allow direct printing on kexec

[Ryo Takakura]

Hi John!

On 2024-10-20, John Ogness <john.ogness at linutronix.de> wrote:
>On 2024-10-20, Ryo Takakura <ryotkkr98 at gmail.com> wrote:
>> Let legacy consoles print directly so that we can see messages on kexec, as
>> the commit b6cf8b3f3312 ("printk: add lockless ringbuffer") turned printk
>> ring buffer lockless and there should be no worries panicked CPU 
>> deadlocking writing into ringbuffer after shutting down non-panicked CPU.
>
>It is correct that the ringbuffer is now lockless. But the legacy
>console drivers are not. Allowing them to print directly in panic can
>lead to similar effects that commit af2876b501e4 ("powerpc/crash: Use
>NMI context for printk when starting to crash") was working around.

Oh I see.
I wasn't taking the locks acquired by console drivers into account...
Thanks once again for the feedback!

>Note that although printk is deferred, it is only the printing that is
>deferred. The messages are landing in the ringbuffer immediately. So
>they would be available to kdump and crash kernels.

I also agree to rather skip printing and leave it to kdump and 
crash kernels considering the chance of pointed out risk on panic.

>Rather than removing the deferring, it would be better to convert the
>console you are using to the new NBCON API. Then it would be able to
>print direct and safe during panic. (printk_deferred does not affect
>NBCON consoles.) What console driver are you using that you want to see
>the messages on?

I was working on qemu ppc64 this time but I am usually working on 
Raspberry Pi 4 (mostly for fun and study) which uses either of 
bcm2835-aux-uart or amba-pl011. It would be really nice to see them 
working as nbcon!
I am thinking of taking a look at [0] but If there were any other 
references, I would really like to look into as well.

>John Ogness

Sincerely,
Ryo Takakura

[0] https://lore.kernel.org/lkml/87wn3zsz5x.fsf@jogness.linutronix.de/