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:   Thu, 30 Apr 2020 22:32:04 +0300
From:   Alper Nebi Yasak <alpernebiyasak@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.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>,
        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 30/04/2020 19:44, Andy Shevchenko wrote:
> First of all I see only cover letter and one out of 3 patches.

Apologies, the tool I've used to send the patches (U-Boot's patman)
Cc-ed the scripts/get_maintainer.pl output per-patch, instead of
per-series as I had assumed it would. This was the first time I tried
it, I'll keep that in mind.

Here are links to all four emails:

https://lore.kernel.org/linux-serial/20200430161438.17640-1-alpernebiyasak@gmail.com/
https://lore.kernel.org/linux-serial/20200430161438.17640-2-alpernebiyasak@gmail.com/
https://lore.kernel.org/linux-serial/20200430161438.17640-3-alpernebiyasak@gmail.com/
https://lore.kernel.org/linux-serial/20200430161438.17640-4-alpernebiyasak@gmail.com/

Or I can resend the last two patches to you, or resend all the parts to
everyone again.

>> eventually figured out that the kernel preferred the serial port
>> (inaccessible to me) over the built-in working display/keyboard and was
>> probably asking there.
> 
> "probably". Please, confirm that first.
> Also, without command line it's hard to say what you have asked kernel to do.

I was trying to boot a Debian userspace with cryptsetup, with the kernel
command line:

	root=/dev/mapper/sda3_crypt quiet splash

The Debian initramfs handles most of the work (the password prompt,
device mounts, etc.).

When I used the same kernel/initramfs/rootfs on a QEMU aarch64 VM, it
only prompted on the serial console instead the framebuffer. I'm
assuming the same thing happens on my hardware as well.

I can also ask the Debian initramfs to launch a shell by adding "break"
to the command line, which won't be printed on my device's screen unless
I also add "console=tty0". That shell also only appears on the serial
console on the QEMU aarch64 VM, unless I again add "console=tty0".

This is my primary computer and I'd prefer not dismantling it, so my
findings above are the best I believe I can do to confirm it now. I'm 
hoping other people would be interested in this, and would test more 
than I can.

>> Running plymouth in the initramfs solves that specific problem, but
> 
> What is plymouth?

Plymouth is a userspace program that's famous for showing a splash
animation during boot, but in this context: it handles user-interaction
that might need to happen while the initramfs is running, by printing
messages and prompts and reading user input to/from all consoles.

>>    ------------------+-----------------------+-----------------------+
>>    Chromebook Plus   | tty0     -WU (EC p  ) | tty0     -WU (EC p  ) |
>>    (w/o either)      |                       |                       |
>>    ------------------+-----------------------+-----------------------+
> 
> either == SPCR or stdout-path?

As in "When the device has no SPCR _and_ no chosen stdout-path".

>> This patchset tries to ensure that VT is preferred in those conditions
>> even in the presence of firmware-mandated serial consoles.
> 
> This sounds completely wrong. serial should be preferred over vt due to very
> debugging on early stages and SPCR is exactly for that.

I'm saying that from a userspace perspective, and the patches explicitly
try to switch to the vt only after a real framebuffer is initialized. So
if I did it right, it would still use SPCR/stdout-path's console during
the early stages. (I admit I haven't adjusted to talking within a kernel 
context yet).

In all honesty, I'm not sure if this is even considered a kernel bug,
let alone my patches a correct solution; hence the RFC PATCH as an
attempt at demonstrating this can be "fixed" in kernel.

> Maybe you should figure out the real root cause?

Thanks for the reply. Any ideas on what else could be causing this 
behaviour?