linux-kernel - Re: [PATCH v9 3/3] printk: fix double printing with earlycon

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Message-ID: <02d08d4a-95b2-b0f2-0319-f41e16e4c8b3@linaro.org>
Date:   Mon, 15 May 2017 00:01:48 +0300
From:   Aleksey Makarov <aleksey.makarov@...aro.org>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sabrina Dubroca <sd@...asysnail.net>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Hurley <peter@...leysoftware.com>,
        Jiri Slaby <jslaby@...e.com>,
        Robin Murphy <robin.murphy@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Nair, Jayachandran" <Jayachandran.Nair@...ium.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH v9 3/3] printk: fix double printing with earlycon



On 05/12/2017 04:46 PM, Petr Mladek wrote:
> On Fri 2017-05-12 14:57:29, Petr Mladek wrote:
>> On Thu 2017-05-11 17:41:58, Sergey Senozhatsky wrote:
>>> On (05/11/17 17:24), Sergey Senozhatsky wrote:
>>>> On (05/09/17 10:29), Sabrina Dubroca wrote:
>>>> [..]
>>>>> That's caused a change of behavior in my qemu setup, with this cmdline
>>>>>
>>>>>      root=/dev/sda1 console=ttyS1 console=ttyS0
>>>>>
>>>>> Before, the kernel logs appeared on ttyS1, and I logged in with ttyS0
>>>>> (with my setup, ttyS1 is a file and ttyS0 is unix socket). Now, the
>>>>> kernel logs go to ttyS0. I need to swap the two console= parameters to
>>>>> restore behavior.
> 
> Do you actually need to define console=ttyS0 on the cmdline?

Exactly.  You should specify a console on the command line only
if you want kernel logs on it.  It's kernel bug that one of this
consoles does not receive kernel logs, see
Documentation/admin-guide/serial-console.rst

> IMHO, if register_console() was called for the unix socket, it
> would make logs appear on both ttyS0 and ttyS1. It seems
> that register_console() is called only for the console that
> stores logs into the file.

It's not quite accurate sentence.  It's qemu who deals with file/socket and
it is transparent to kernel.

>>>>> adding
>>>>> console=tty0 anywhere on that cmdline makes the logs appear on both
>>>>> tty0 and one ttyS* (but only one of them, and the ordering of the
>>>>> ttyS* matters).
> 
> I guess that it worked this way before. I mean that the logs appeared
> on both tty0 and one of ttyS*. The only difference should be that
> the patch changed the selected ttyS*. So this is still the same problem.
> 
> 
>> Hmm, I have no idea how to fix this. This is the case where
>> a registered console matches more entries from the command line.
>> The fix that caused this regression fixed exactly this situation
>> and we wanted to make the preferred console first.
>>
>> I am not sure if we broke some backward compatibility or actually made
>> it more predictable in the long term.
> 
> I think that we actually fixed a very old bug. The last mentioned
> console= should be the preferred one and the logs are finally
> printed there. Or do I miss anything?

Last mentioned 'console=' (preferred console) is the console that
should become /dev/console.  Its driver is returned by console_device().
In other respects the last mentioned console is not special,
so I believe it is irrelevant to the report.

Thank you
Aleksey Makarov