Messages in this thread | | | Date | Sat, 4 Jun 2022 11:21:12 +0200 | Subject | Re: Non-working serial console | From | Paul Menzel <> |
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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
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