linux-kernel - Re: [RFC PATCH v2 0/3] Prefer working VT console over SPCR and device-tree chosen stdout-path

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Date:   Fri, 1 May 2020 18:07:10 +0300
From:   Alper Nebi Yasak <alpernebiyasak@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Petr Mladek <pmladek@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Biggers <ebiggers@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Grzegorz Halat <ghalat@...hat.com>,
        Lukas Wunner <lukas@...ner.de>,
        Nicolas Pitre <nico@...xnic.net>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [RFC PATCH v2 0/3] Prefer working VT console over SPCR and
 device-tree chosen stdout-path



On 01/05/2020 16:16, Andy Shevchenko wrote:
> On Fri, May 1, 2020 at 2:11 PM Alper Nebi Yasak
> <alpernebiyasak@...il.com> wrote:
>> I'm assuming "by default" here means "without console arguments"
>> regardless of firmware requests. This paragraph (with small changes) is
>> repeated on many other Kconfig descriptions (drivers/tty/serial/Kconfig,
>> drivers/tty/serial/8250/Kconfig, arch/sparc/Kconfig from grepping for
>> '/dev/tty0' on **/Kconfig).
>>
>>  From Documentation/admin-guide/serial-console.rst:
>>
>>> You can specify multiple console= options on the kernel command line.
>>> [...]
>>> Note that you can only define one console per device type (serial, video).
>>>
>>> If no console device is specified, the first device found capable of
>>> acting as a system console will be used. At this time, the system
>>> first looks for a VGA card and then for a serial port. So if you don't
>>> have a VGA card in your system the first serial port will automatically
>>> become the console.
>>
>> and later on:
>>
>>> Note that if you boot without a ``console=`` option (or with
>>> ``console=/dev/tty0``), ``/dev/console`` is the same as ``/dev/tty0``.
>>> In that case everything will still work.
> 
> I'm wondering if behaviour is changed if you put console=tty1 instead
> of console=tty0.

Just tested again with the QEMU aarch64 VM. Comparing console=tty1 and
console=tty0 cases: /proc/consoles has tty1 instead of tty0 (both also
has ttyAMA0), and `echo '/dev/console is here' >>/dev/console` goes to
vt1 instead of the currently visible vt. Same difference before and
after this patchset.