linux-kernel - Re: [PATCH] printk: handle blank console arguments passed in.

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Date:   Tue, 6 Oct 2020 18:35:14 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Shreyas Joshi <shreyas.joshi@...mp.com>, rostedt@...dmis.org,
        shreyasjoshi15@...il.com, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] printk: handle blank console arguments passed in.

On Tue 2020-10-06 15:43:28, Petr Mladek wrote:
> On Tue 2020-10-06 03:45:00, Guenter Roeck wrote:
> > On 10/6/20 2:52 AM, Petr Mladek wrote:
> > > And it makes perfect sense to disable all consoles or drop all defined
> > > by dts. But I would prefer to make it more obvious way, for
> > > example by parameters like:
> > > 
> > >    + console=none
> > >    + no-console
> > >    + no-dtd-console
> > >    + no-default-console
> > > 
> > Again, the problem isn't limited to dts provided consoles, or at least
> > that was my understanding. I am still trying to understand how default
> > consoles are defined, so I may get something wrong. Anyway, personally I
> > liked "console=", but that is just me. Anything else should work for us
> > as long as it is backward compatible (which excludes the no-xxx options).
> 
> Here is my understanding:
> 
> The consoles can be defined by scpr, dts, and on the command line. It
> is anyone calling add_preferred_console().
> 
> Then the various devices call register_console(). They are registered
> only when they match any console in console_cmdline[] array, see
> try_enable_new_console().
> 
> > Whatever is decided, I'd like to have it made official and documented to
> > avoid a similar problem in the future.

Sigh, it is even bigger mess than I expected. There is a magic
variable "console_set_on_cmdline". It used, for example, in
of_console_check() to prevent using the default console from dts.

It is used on few more locations to prevent the default console.
But there are other locations when add_preferred_console() is
called without checking this variable.

As a result, "console=" has different effect on different systems.

I tend to revert the problematic patch now.

And I would try to clean up this mess for-5.11. There is a big chance
that people used the empty console= only on systems where it disabled
all default consoles. I would try to make it the official global
behavior. But this would need some longer testing in linux-next, ...

Best Regards,
Petr