Re: [PATCH] MIPS: SGI-IP27: print the NMI dump on an nbcon console

[John Ogness]

On 2026-10-01, Imre Kaloz <kaloz@xxxxxxxxxx> wrote:
> nmi_dump() has no caller in Linux. install_cpu_nmi_handler() stores
> its address in the per-CPU NMI vector the PROM keeps in low memory,
> and the PROM jumps to it when the system controller asserts NMI on
> the CPUs: the "nmi" command at the L1 (or the MMSC on an Origin
> 2000), typically used to get a dump out of a hung machine.
>
> Every CPU that takes the NMI enters nmi_dump(). The first one to get
> nmi_lock waits until all online CPUs have arrived, prints the saved
> state of every CPU and ends with the NI_PORT_RESET write. The others
> spin on nmi_lock, which is never released. No CPU goes back to the
> interrupted context

Thanks for the explanation. I guess it should have been obvious, also
because @nmi_lock is locked and never unlocked.

> @@ -183,6 +184,12 @@ static void nmi_dump(void)
>  	 */
>  	arch_spin_lock(&nmi_lock);
>
> +	/*
> +	 * No CPU leaves the NMI handler before the hub reset below, so an
> +	 * nbcon console's printer thread would never print the dump.
> +	 */
> +	nbcon_cpu_emergency_enter();
> +

Note that putting the CPU into emergency state does not cause any
printing. A later printk call is required.

Also note that only _this_ CPU is put into the emergency state. So any
printk's on other CPUs will not be visible until _this_ CPU performs a
printk.

In order to make sure that any backlog is flushed, I suggest also
adding:

	printk_trigger_flush();

after the loop waiting for the other CPUs, just before triggering the
reset.

John