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Date:   Thu, 18 May 2017 17:49:18 +0200
From:   Petr Mladek <>
To:     Aleksey Makarov <>
Cc:     Sergey Senozhatsky <>,
        Sabrina Dubroca <>,,,
        Sudeep Holla <>,
        Greg Kroah-Hartman <>,
        Peter Hurley <>,
        Jiri Slaby <>,
        Robin Murphy <>,
        Steven Rostedt <>,
        "Nair, Jayachandran" <>,
        Sergey Senozhatsky <>
Subject: Re: [PATCH v9 3/3] printk: fix double printing with earlycon

On Sun 2017-05-14 23:37:50, Aleksey Makarov wrote:
> On 05/12/2017 03:57 PM, 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.
> >>>>
> >>>>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).
> >>>
> The problem is that when we are registering a new console,
> we walk over the `console_cmdline` list and match _only one_ of
> the specified consoles, contrary to what stated in
> Documentation/admin-guide/serial-console.rst:
> 	You can specify multiple console= options on the kernel
> 	command line. Output will appear on *all* of them.

and from the other mail:

> 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.

Thanks a lot for explanation. I missed these pieces.

But this also means that your commit cf39bf58afdaabc0b
("printk: fix double printing with earlycon") helps only
when the duplicate of the boot console is defined as the preferred

Well, the reverse order of searching the console_cmdline
might help also in other cases but it is weird approach.

> I emphasized all here.  Moreover, it is impossible to fix this
> without deep reworking of all the console framework.

IMHO, the same is true also for fixing the bug with double
printing correctly. The current fix helps in some situations
but it might break others. The question is how many people will
see the good and "bad" effects.

BTW: I wonder if we really need to add consoles defined
by ACPI SPCR table into the console_cmdline array. I am even
more curious when seeing the following code in

int __init parse_spcr(bool earlycon)
	if (earlycon)

	err = add_preferred_console(uart, 0, opts + strlen(uart) + 1);

It seems that we setup (register) the early console before
we add it to the console_cmdline array. Do we really need
to call add_preferred_console() for these early consoles?

If we do not call add_preferred_console() here, it should fix
the duplicate output as well. Or do I still miss something?

Best Regards,

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