linux-kernel - Re: [PATCH 1/7] printk: Make console_{suspend,resume} handle CON

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Message-ID: <aEq9_kOoLSQwuYBq@pathway.suse.cz>
Date: Thu, 12 Jun 2025 13:46:06 +0200
From: Petr Mladek <pmladek@...e.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Jason Wessel <jason.wessel@...driver.com>,
	Daniel Thompson <danielt@...nel.org>,
	Douglas Anderson <dianders@...omium.org>,
	Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	kgdb-bugreport@...ts.sourceforge.net, linux-um@...ts.infradead.org
Subject: Re: [PATCH 1/7] printk: Make console_{suspend,resume} handle
 CON_SUSPENDED

On Fri 2025-06-06 23:53:43, Marcos Paulo de Souza wrote:
> Since commit 9e70a5e109a4 ("printk: Add per-console suspended state") the
> CON_SUSPENDED flag was introced, and this flag was being checked on
> console_is_usable function, which returns false if the console is suspended.
> 
> No functional changes.

I double checked potential functional changes. In particular, I
checked where the CON_ENABLED and CON_SUSPENDED flags were used.

Both flags seems to have the same effect in most situations,
for example, in console_is_usable() or console_unblank().

But there seems to be two exceptions: kdb_msg_write() and
show_cons_active(). These two functions check only
the CON_ENABLED flag. And they think that the console is
usable when the flag is set.

The change in this patch would change the behavior of the two
functions during suspend. It is later fixed by the 3rd and 4th
patch. But it might cause regressions during bisections.

It is probably not a big deal because the system is not much
usable during the suspend anyway. But still, I would feel more
comfortable if we prevented the "temporary" regression.

I see two possibilities:

   1. Merge the 3rd and 4th patch into this one. It would change
      the semantic in a single patch.

   2. First update kdb_msg_write() and show_cons_active()
      to check both CON_ENABLE and CON_SUSPENDED flags.

The 1st solution probably makes more sense because we are going
to remove the CON_ENABLE flag in the end. And even the merged
patch is small enough.

Best Regards,
Petr