Re: [PATCH tty v11 1/2] serial: 8250: Switch to nbcon console, take 2

[Jon Hunter]

Hi John,

On 22/08/2026 01:32, John Ogness wrote:
> On 2026-08-21, Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
> > I replaced the WARN with a pr_err() and I see the same issue.
>
> Thanks for testing. But just to be certain, can you restore the original
> code (with the warning) and apply this horrible hack to force atomic
> printing? I want to be certain that the problem goes away when nbcon
> deferred printing is not active. For thet test you should boot normally
> (without keep_bootcon).
>
> ===== BEGIN HACK =====
> diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
> index 85fbf1801cbe0..5806815fa11b2 100644
> --- a/kernel/printk/internal.h
> +++ b/kernel/printk/internal.h
> @@ -202,6 +202,7 @@ static inline void printk_get_console_flush_type(struct console_flush_type *ft)
>   				ft->nbcon_offload = true;
>   			else
>   				ft->nbcon_atomic = true;
> +			ft->nbcon_atomic = true;
>   		}
>   
>   		/* Legacy consoles are flushed directly when possible. */
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 4b03b019cd5ee..152d9ac28176d 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1185,6 +1185,7 @@ static bool nbcon_kthread_should_wakeup(struct console *con, struct nbcon_contex
>   
>   	if (kthread_should_stop())
>   		return true;
> +	return false;
>   
>   	/*
>   	 * Block the kthread when the system is in an emergency or panic mode.
> ===== END HACK =====
>
> I am expecting that this will work and the warning will be cleanly
> visible on the UART console.

With this I see a little more output on the console, but it still
appears to hang later and never fully boots.

Boot log here: https://pastebin.com/Xe1vmeUj
 
> If the above test was successful, I am wondering if cpuidle should be
> disabling the deferred printing as this seems quite similar to suspend.
>
> Could you perform another test using the following hack _instead_ of the
> above hack? This is probably a bit extreme since it does more than just
> disable deferring, but it could help us figure out what is going on.
>
> ===== BEGIN HACK2 =====
> diff --git a/drivers/cpuidle/cpuidle-tegra.c b/drivers/cpuidle/cpuidle-tegra.c
> index aca907a62bb5d..0fb449e23f677 100644
> --- a/drivers/cpuidle/cpuidle-tegra.c
> +++ b/drivers/cpuidle/cpuidle-tegra.c
> @@ -15,6 +15,7 @@
>   #define pr_fmt(fmt)	"tegra-cpuidle: " fmt
>   
>   #include <linux/atomic.h>
> +#include <linux/console.h>
>   #include <linux/cpuidle.h>
>   #include <linux/cpumask.h>
>   #include <linux/cpu_pm.h>
> @@ -165,6 +166,8 @@ static __cpuidle int tegra_cpuidle_state_enter(struct cpuidle_device *dev,
>   {
>   	int err;
>   
> +	console_suspend_all();
> +
>   	/*
>   	 * CC6 state is the "CPU cluster power-off" state.  In order to
>   	 * enter this state, at first the secondary CPU cores need to be
> @@ -176,7 +179,7 @@ static __cpuidle int tegra_cpuidle_state_enter(struct cpuidle_device *dev,
>   	if (index == TEGRA_CC6) {
>   		err = tegra_cpuidle_coupled_barrier(dev);
>   		if (err)
> -			return err;
> +			goto out;
>   	}
>   
>   	local_fiq_disable();
> @@ -204,6 +207,8 @@ static __cpuidle int tegra_cpuidle_state_enter(struct cpuidle_device *dev,
>   	cpu_pm_exit();
>   	tegra_pm_clear_cpu_in_lp2();
>   	local_fiq_enable();
> +out:
> +	console_resume_all();
>   
>   	return err ?: index;
>   }
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 3fcdf4b4e2e53..8d126b5068684 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2787,7 +2787,7 @@ void console_suspend_all(void)
>   	struct console *con;
>   
>   	if (console_suspend_enabled)
> -		pr_info("Suspending console(s) (use no_console_suspend to debug)\n");
> +		pr_info_once("Suspending console(s) (use no_console_suspend to debug)\n");
>   
>   	/*
>   	 * Flush any console backlog and then avoid queueing irq_work until
> ===== END HACK2 =====


I tried the above anyway, but this is hitting a 'scheduling while atomic' bug ...

[    2.083585] BUG: scheduling while atomic: swapper/0/0/0x00000002
[    2.084118] Modules linked in:
[    2.099178]
[    2.106346] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 7.2.0-next-20260817-00003-ga0c285a6f831 #23 PREEMPT
[    2.106413] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
[    2.106440] Call trace:
[    2.106468]  unwind_backtrace from show_stack+0x10/0x14
[    2.106596]  show_stack from dump_stack_lvl+0x54/0x68
[    2.106714]  dump_stack_lvl from __schedule_bug+0x50/0x64
[    2.106815]  __schedule_bug from __schedule+0x9e0/0x1348
[    2.106897]  __schedule from schedule+0x2c/0xf8
[    2.106962]  schedule from schedule_timeout+0xbc/0x114
[    2.107049]  schedule_timeout from __down_common+0x14c/0x3ac
[    2.107137]  __down_common from down+0x4c/0x60
[    2.107213]  down from __pr_flush+0x294/0x3d0
[    2.107284]  __pr_flush from console_suspend_all+0x44/0xc8
[    2.107342]  console_suspend_all from tegra_cpuidle_state_enter+0x18/0x33c
[    2.107406]  tegra_cpuidle_state_enter from tegra_cpuidle_enter+0x168/0x184
[    2.107470]  tegra_cpuidle_enter from cpuidle_enter_state+0x80/0x610
[    2.107533]  cpuidle_enter_state from cpuidle_enter_state_coupled+0x3ac/0x400
[    2.107637]  cpuidle_enter_state_coupled from cpuidle_enter+0x3c/0x54
[    2.107736]  cpuidle_enter from do_idle+0x150/0x340
[    2.107809]  do_idle from cpu_startup_entry+0x28/0x2c
[    2.107861]  cpu_startup_entry from secondary_start_kernel+0x118/0x120
[    2.107959]  secondary_start_kernel from 0x1160a0
[    2.108085] bad: scheduling from the idle thread!

Cheers
Jon

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nvpublic