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

[John Ogness <john.ogness@linutronix.de>]

On 2026-08-21, Jon Hunter <jonathanh@nvidia.com> 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.

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 =====

John Ogness