Re: [PATCH printk] printk/nbcon: Change nbcon_irq_work to IRQ_WORK_LAZY

[Sebastian Andrzej Siewior]

On 2026-08-27 20:53:38 [+0206], John Ogness wrote:
> For some platforms it is a problem to queue irq_work when entering
> cpuidle states. Since nbcon uses irq_work for waking the printing
> kthreads, any printk() calls when entering cpuidle states can lead
> to the affected hardware hanging. Tegra20 and Tegra30 are examples
> of such platforms. Avoiding raising the irq_work IRQ has shown to
> circumvent the problem.
>
> Change the nbcon_irq_work to be IRQ_WORK_LAZY, thus not raising
> an IRQ upon irq_work queuing. The irq_work is then handled on the
> next interrupt (worst case, kernel tick). This additional delay is
> acceptable because nbcon_irq_work is only responsible for
> non-emergency deferred printing, which is delayed anyway. This also
> has the benefit of not needing to raise an IRQ for each printk()
> call.

This makes sense regardless of the Tegra issue. I'm still curious what
happens on Tegra ;)

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

> Link: https://lore.kernel.org/lkml/f3757a75-0ba1-4558-bf57-f19ab7e59a4c@xxxxxxxxxx
> Fixes: 76f258bf3f2a ("printk: nbcon: Introduce printer kthreads")
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> ---
>  kernel/printk/nbcon.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index a5921a84a80ed..ad79e30afd647 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1782,7 +1782,7 @@ bool nbcon_alloc(struct console *con)
>  	}
>  
>  	rcuwait_init(&con->rcuwait);
> -	init_irq_work(&con->irq_work, nbcon_irq_work);
> +	con->irq_work = IRQ_WORK_INIT_LAZY(nbcon_irq_work);
>  	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_prev_seq), -1UL);
>  	nbcon_state_set(con, &state);
>  
> 
> base-commit: ffe0486b139e45cd9c9ca2584f04a1910fe4f8a6

Sebastian