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

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Date:   Thu, 11 May 2017 17:41:58 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Aleksey Makarov <aleksey.makarov@...aro.org>
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>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v9 3/3] printk: fix double printing with earlycon

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?

	-ss