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 14:08:56 +0300
From:   Alper Nebi Yasak <alpernebiyasak@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Petr Mladek <pmladek@...e.com>,
        linux-serial@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        linux-arm-kernel@...ts.infradead.org, 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 04:30, Sergey Senozhatsky wrote:> Well, if there is a "mandated console", then why would we prefer
> any other console?

>From what I understand, the firmware provides serial console settings to
be used as the preferred _serial_ console (where it would be OK to
switch to graphical consoles later on) and the kernel currently
understands that such a console should be the preferred _system_ console
(always preferred over even graphical ones). By "mandated" I'm referring
to the kernel's current behavior, not to (in my understanding) the
firmware's intentions.

Even if the firmware/specifications is really asking the kernel to (tell
userspace programs to) always use the serial console instead of the
framebuffer console, while on e.g. a laptop-like device intended to be
used with a keyboard and display -- is that the correct thing to do?

>From the userspace, under the conditions:

- CONFIG_VT_CONSOLE is enabled
- There is a working graphics adapter and a display
- There is no console argument given in the kernel command line

I expect that:

- tty0 is included in the /proc/consoles list [1]
- tty0 is the preferred console and /dev/console refers to it [2]

With SPCR both are false, and with stdout-path only the second is false.
Again, I'm OK with these being false during earlier stages until
graphics start working, but I'm arguing they should be true after then.

In the patches I tried to keep these serial consoles still enabled and
preferred during early stages of boot, by trying to switch to vt only
after a real working graphical backend for it is initialized.

I mean, if my expectations are unreasonable and the current kernel
behaviour is considered correct, these patches would be conceptually
wrong; so please tell me if I got anything right/wrong in all this.


[1] From the descripion of CONFIG_VT_CONSOLE:

> [...] If you answer Y here, a virtual terminal (the device used to
> interact with a physical terminal) can be used as system console.
> [...] you should say Y here unless you want the kernel messages be
> output only to a serial port [...]

and by "as a prerequisite of [2]"


[2] From the descripion of CONFIG_VT_CONSOLE:

> If you do say Y here, by default the currently visible virtual
> terminal (/dev/tty0) will be used as system console. You can change
> that with a kernel command line option such as "console=tty3" which
> would use the third virtual terminal as system console. [...]

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.