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

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Date:   Thu, 11 May 2017 17:24:33 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sabrina Dubroca <sd@...asysnail.net>
Cc:     Aleksey Makarov <aleksey.makarov@...aro.org>,
        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 (05/09/17 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.

	-ss