linux-kernel - Re: Non-working serial console

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Message-ID: <477007d7-e573-56dd-90de-877eeebd5c4c@molgen.mpg.de>
Date:   Sat, 4 Jun 2022 11:21:12 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Non-working serial console

Dear Greg,


Am 04.06.22 um 11:04 schrieb Greg KH:
> On Thu, Jun 02, 2022 at 11:09:15PM +0200, Paul Menzel wrote:

>> Am 02.06.22 um 18:50 schrieb Paul Menzel:
>>
>>> Since a while I noticed, output to the serial console with
>>> `console=ttyS0,115200n8` does not work with the attached configuration
>>> `defconfig-non-working-serial.txt` created by `make savedefconfig`.
>>> Only, when with `earlyprintk=ttyS0,115200,keep` the serial console
>>> starts working. I am able to reproduce it in QEMU. It’s reproducible
>>> with Linus’ latest master branch.
>>>
>>>       $ git log --oneline --no-decorate -1
>>>       8ab2afa23bd19 Merge tag 'for-5.19/fbdev-1' of git://git.kernel.org/pub/scm/linux/kernel/git/deller/linux-fbdev
>>>       $ qemu-system-x86_64 --version
>>>       QEMU emulator version 5.1.0
>>>       Copyright (c) 2003-2020 Fabrice Bellard and the QEMU Project developers
>>>       $ qemu-system-x86_64 -kernel arch/x86/boot/bzImage -append "console=ttyS0,115200n8" -serial file:/dev/shm/kernel.txt -curses
>>>
>>> With `earlyprintk=` it works:
>>>
>>>       $ qemu-system-x86_64 -kernel arch/x86/boot/bzImage -append "earlyprintk=serial console=ttyS0,115200n8" -serial file:/dev/shm/kernel.txt -curses
>>>
>>> Strangely, I found a different configuration, where it works, but I
>>> didn’t see what configuration option makes the difference.
>>>
>>> Can you reproduce the problem with `defconfig-no-working-serial.txt`?
>>
>> It turns out, the non-working configuration build the serial 8250 driver as
>> a module (`CONFIG_SERIAL_8250=m`) instead of building it into the Linux
>> kernel. Building it into the Linux kernel and using
>> `CONFIG_SERIAL_8250_CONSOLE=y` fixes my issue.
> 
> That makes sense, you need the console to be able to properly send data
> out to it :)

Indeed.

I am still confused, that `earlyprintk=ttyS0,115200,keep` works though 
despite `CONFIG_SERIAL_8250=m`. Doesn’t that mean, that Linux 
nevertheless contains some code to initialize the serial console, and 
send data to it?

> Was this a Kconfig change somewhere recently that we messed up the
> defaults for?  Any chance you can use 'git bisect' to track down the
> offending change?

No, I guess I made the change several months back to make the Linux 
kernel image smaller to decrease the boot time. (I have to do 
measurements again.)


Kind regards,

Paul