If a non-boot console is registering and boot consoles exist, the
consoles are flushed before being unregistered. This allows the
non-boot console to continue where the boot console left off.

If for whatever reason flushing fails, the lowest seq found from
any of the enabled boot consoles is used. Until now con->seq was
checked. However, if it is an nbcon boot console, the function
nbcon_seq_read() must be used to read seq because con->seq is
always 0.

Check if it is an nbcon boot console and if so call
nbcon_seq_read() to read seq.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/printk.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1685a71f3f71..696a9d76c09c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3391,11 +3391,20 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
 
 				newcon->seq = prb_next_seq(prb);
 				for_each_console(con) {
-					if ((con->flags & CON_BOOT) &&
-					    (con->flags & CON_ENABLED) &&
-					    con->seq < newcon->seq) {
-						newcon->seq = con->seq;
+					u64 seq;
+
+					if (!((con->flags & CON_BOOT) &&
+					      (con->flags & CON_ENABLED))) {
+						continue;
 					}
+
+					if (con->flags & CON_NBCON)
+						seq = nbcon_seq_read(con);
+					else
+						seq = con->seq;
+
+					if (seq < newcon->seq)
+						newcon->seq = seq;
 				}
 			}
 
-- 
2.39.2

On Sun 2024-02-18 20:03:04, John Ogness wrote:
> If a non-boot console is registering and boot consoles exist, the
> consoles are flushed before being unregistered. This allows the
> non-boot console to continue where the boot console left off.
> 
> If for whatever reason flushing fails, the lowest seq found from
> any of the enabled boot consoles is used. Until now con->seq was
> checked. However, if it is an nbcon boot console, the function
> nbcon_seq_read() must be used to read seq because con->seq is
> always 0.

This says that con->seq is always 0.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3391,11 +3391,20 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
>  
>  				newcon->seq = prb_next_seq(prb);
>  				for_each_console(con) {
> -					if ((con->flags & CON_BOOT) &&
> -					    (con->flags & CON_ENABLED) &&
> -					    con->seq < newcon->seq) {
> -						newcon->seq = con->seq;
> +					u64 seq;
> +
> +					if (!((con->flags & CON_BOOT) &&
> +					      (con->flags & CON_ENABLED))) {
> +						continue;
>  					}
> +
> +					if (con->flags & CON_NBCON)
> +						seq = nbcon_seq_read(con);
> +					else
> +						seq = con->seq;
> +
> +					if (seq < newcon->seq)
> +						newcon->seq = seq;

But this sets con->seq to some value even for nbcon consoles.

It would make more sense to use nbcon_seq_force() for nbcon
consoles here and remove the copying from nbcon_init().

Otherwise, it looks good.

Best Regards,
Petr