linux-kernel - Non-working serial console

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Date:   Thu, 2 Jun 2022 18:50:50 +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: Non-working serial console

Dear Linux folks,

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`?

Kind regards,

Paul
View attachment "defconfig-working-serial.txt" of type "text/plain" (19843 bytes)

View attachment "defconfig-non-working-serial.txt" of type "text/plain" (26209 bytes)