Re: [PATCH v2 3/4] printk: nbcon: move printk_delay to console emiting code

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

On 2026-07-07, Petr Mladek <pmladek@suse.com> wrote:
> To summarize this discussion:
>
>   + Benedikt would prefer to add the synchronous mode before
>     moving the delay from the printk() caller to the console emit
>     code path.
>
>   + The delay in the printk() caller code path did not guarantee
>     the output but it increased the chance to see it.
>
>   + The synchronous mode will be even more reliable than the delay
>     in printk() caller path.
>
> Please, let me know if I did not get it right,
>
> John, did you have any plan how to add the synchronous mode, please?
> Does it look complicated?
>
> I guess that we would somehow need to "mis-use" the emergency
> priority and force it everywhere by some global system setting.

The synchronous mode would rely on the driver being nbcon. I envision
something like this:

---- BEGIN SYNC IDEA ----
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2fe9a963c823a..beb5bb3c037b5 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2456,6 +2456,8 @@ asmlinkage int vprintk_emit(int facility, int level,
 
 	if (ft.nbcon_atomic)
 		nbcon_atomic_flush_pending();
+	else if (have_nbcon_console_sync)
+		nbcon_atomic_flush_sync_pending();
 
 	if (ft.nbcon_offload)
 		nbcon_kthreads_wake();
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 4b03b019cd5ee..8f8f29f757614 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1200,7 +1200,7 @@ static bool nbcon_kthread_should_wakeup(struct console *con, struct nbcon_contex
 	cookie = console_srcu_read_lock();
 
 	flags = console_srcu_read_flags(con);
-	if (console_is_usable(con, flags, false)) {
+	if (!(flags & CON_SYNC) &&  console_is_usable(con, flags, false)) {
 		/* Bring the sequence in @ctxt up to date */
 		ctxt->seq = nbcon_seq_read(con);
 
@@ -1654,7 +1654,7 @@ static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
  *					write_atomic() callback
  * @stop_seq:			Flush up until this record
  */
-static void __nbcon_atomic_flush_pending(u64 stop_seq)
+static void __nbcon_atomic_flush_pending(u64 stop_seq, bool sync_only)
 {
 	struct console *con;
 	int cookie;
@@ -1666,6 +1666,9 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq)
 		if (!(flags & CON_NBCON))
 			continue;
 
+		if (sync_only && !(flags & CON_SYNC))
+			continue;
+
 		if (!console_is_usable(con, flags, true))
 			continue;
 
@@ -1688,7 +1691,12 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq)
  */
 void nbcon_atomic_flush_pending(void)
 {
-	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), false);
+}
+
+void nbcon_atomic_flush_sync_pending(void)
+{
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), true);
 }
 
 /**
@@ -1701,7 +1709,7 @@ void nbcon_atomic_flush_pending(void)
 void nbcon_atomic_flush_unsafe(void)
 {
 	panic_nbcon_allow_unsafe_takeover = true;
-	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb), false);
 	panic_nbcon_allow_unsafe_takeover = false;
 }
 
---- END SYNC IDEA ----

BTW: The sync mode is the final piece so that PeterZ can start using
mainline code for debugging by serial instead of his own workaround [0].

John

[0] https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=debug/experimental