On Wed 2021-01-06 13:04:57, Sergey Senozhatsky wrote:

On (21/01/05 17:49), Petr Mladek wrote:

quoted

The following change solved the problem for me as well. It causes
that ttynull is initialized after stdiocons console.

diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
index eced70ec54e1..602af4d30bd4 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c

@@ -121,7 +121,6 @@ static void __exit ttynull_exit(void)
 	tty_port_destroy(&ttynull_port);
 }
 
-module_init(ttynull_init);
-module_exit(ttynull_exit);
+late_initcall_sync(ttynull_init);
 
 MODULE_LICENSE("GPL v2");

But I am not completely sure that it is the right solution.

Wow, hmm, puzzled. Why does it help?

I have been as well. But it seems that I got it, see below.

quoted

It is strange. Console should get registered only when
it was added by add_preferred_console(). It means that
ttynull_init() should not register by default.

[..]

quoted

Some clue might be in stderr_console. It has
to be explicitly unregistered to avoid staying as
the default console, see unregister_stderr() in
arch/um/drivers/stderr_console.c

Hmm... Some random thoughts:

Looking at arch/um/drivers/stderr_console.c - it doesn't have tty
driver and it doesn't register one.

stderr_console.c is used only during early boot.

stdio_console.c is the console that is supposed to print kunit
test results. And it has tty driver.

But I still don't understand why the initcall patch helped.
Can you shed some light on it?

The trick is that both stdio_init() and ttynull_init():

  + call register_console()
  + the console has tty driver

The first one is registered as a fallback when there is
no preferred console (has_preferred_console()).

It means that late_initcall_sync(ttynull_init) makes sense.
We need to call register_console() from ttynull_init() so that
it is registered when defined on the command line. But it should
be the last chance to register a fallback console with tty binding.

Alternative solution is to ignore ttynull as the fallback console
in register_console(). I mean the following:

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ffdd0dc7ec6d..cdb77903b0af 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c

@@ -2816,8 +2816,12 @@ void register_console(struct console *newcon)
 	 *	See if we want to use this console driver. If we
 	 *	didn't select a console we take the first one
 	 *	that registers here.
+	 *
+	 *	Ignore ttynull console. It should be used only
+	 *	when explicitly configured or as an ultimate
+	 *	fallback when no better console gets registered at all.
 	 */
-	if (!has_preferred_console) {
+	if (!has_preferred_console && strcmp(newcon->name, "ttynull") != 0) {
 		if (newcon->index < 0)
 			newcon->index = 0;
 		if (newcon->setup == NULL ||

Best Regards,
Petr

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