Re: Non-working serial console

[Paul Menzel]

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