linux-kernel - Re: [PATCH 1/1] Revert "init/console: Use ttynull as a fallback when there is no console"

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Message-ID: <20210107174638.GH4077@smile.fi.intel.com>
Date:   Thu, 7 Jan 2021 19:46:38 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Shreyas Joshi <shreyas.joshi@...mp.com>,
        shreyasjoshi15@...il.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Vineet Gupta <vgupta@...opsys.com>,
        Thomas Meyer <thomas@...3r.de>,
        David Gow <davidgow@...gle.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Shuah Khan <skhan@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Revert "init/console: Use ttynull as a fallback when
 there is no console"

On Thu, Jan 07, 2021 at 05:44:00PM +0100, Petr Mladek wrote:
> This reverts commit 757055ae8dedf5333af17b3b5b4b70ba9bc9da4e.
> 
> The commit caused that ttynull was used as the default console
> on many systems. It happened when there was no console configured
> on the command line and ttynull_init() was the first initcall
> calling register_console().
> 
> The commit fixed a historical problem that have been there for ages.
> The primary motivation was the commit 3cffa06aeef7ece30f6
> ("printk/console: Allow to disable console output by using console=""
> or console=null"). It provided a clean solution
> for a workaround that was widely used and worked only by chance.
> 
> This revert causes that the console="" or console=null command line
> options will again work only by chance. These options will cause that
> a particular console will be preferred and the default (tty) ones
> will not get enabled. There will be no console registered at
> all. As a result there won't be stdin, stdout, and stderr for
> the init process. But it worked exactly this way even before.
> 
> The proper solution has to fulfill many conditions:
> 
>   + Register ttynull only when explicitly required or as
>     the ultimate fallback.
> 
>   + ttynull must get associated with /dev/console but it must
>     not become preferred console when used as a fallback.
>     Especially, it must still be possible to replace it
>     by a better console later.
> 
> Such a change requires clean up of the register_console() code.
> Otherwise, it would be even harder to follow. Especially, the use
> of has_preferred_console and CON_CONSDEV flag is tricky. The clean
> up is risky. The ordering of consoles is not well defined. And
> any changes tend to break existing user settings.
> 
> Do the revert at the least risky solution for now.

Shouldn't it have Fixes tag along with Reported-by ones and explanation what
was the actual problem reported?

-- 
With Best Regards,
Andy Shevchenko