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

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Date:   Fri, 12 May 2017 00:13:57 +0300
From:   Aleksey Makarov <aleksey.makarov@...aro.org>
To:     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>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v9 3/3] printk: fix double printing with earlycon



On 05/11/2017 11:41 AM, 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.
>>>
>>> There might be some other problem (in qemu?) though, because 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).
>>
>> thanks for the report.
>>
>> so we have ttyS1 first and ttyS0 last.
>> after commit in question, register_console() iterates console_cmdline
>> in reverse order so we see ttyS0 first, then we hit `if (newcon->index < 0)'
>> condition, set newcon to ttyS0, because we iterate in reverse order now, and
>> break out. so we enable ttyS0, instead of ttyS1.
>>
>> previously, we iterated console_cmdline from index 0 and saw ttyS1 first.
>> so the same `if (newcon->index < 0)' condition would set newcone to ttyS1,
>> and, thus, we would enable ttyS1, not ttyS0.
> 
> Alexey,
> can we have preferred console at offset 0 (not at console_cmdline_cnt - 1)
> and restore the previous register_console() iteration order?

I don't quite understand what is the problem.  Give me more time please.
I hope I will be able to look at this on the weekend.

Thank you
Aleksey Makarov