Re: kunit stopped working

[Petr Mladek <pmladek@suse.com>]

On Wed 2021-01-06 13:04:57, Sergey Senozhatsky wrote:
> On (21/01/05 17:49), Petr Mladek wrote:
> > 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.

> > 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.
> [..]
> > 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