linux-kernel - Re: Non-working serial console

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Message-ID: <YpsgHpRfz0JQcm9D@kroah.com>
Date:   Sat, 4 Jun 2022 11:04:30 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Non-working serial console

On Thu, Jun 02, 2022 at 11:09:15PM +0200, Paul Menzel wrote:
> Dear Linux folks,
> 
> 
> 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 :)

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?

thanks,

greg k-h