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.
```
$ diff -u .config.old .config
--- .config.old 2022-05-31 18:40:31.329017225 +0200
+++ .config 2022-06-02 22:53:06.317175267 +0200
@@ -1814,6 +1814,7 @@
CONFIG_UEFI_CPER=y
CONFIG_UEFI_CPER_X86=y
+CONFIG_EFI_EARLYCON=y
CONFIG_EFI_CUSTOM_SSDT_OVERLAYS=y
# CONFIG_EFI_DISABLE_RUNTIME is not set
# CONFIG_EFI_COCO_SECRET is not set
@@ -2554,11 +2555,13 @@
#
# Serial drivers
#
-CONFIG_SERIAL_8250=m
+CONFIG_SERIAL_EARLYCON=y
+CONFIG_SERIAL_8250=y
# CONFIG_SERIAL_8250_DEPRECATED_OPTIONS is not set
CONFIG_SERIAL_8250_PNP=y
# CONFIG_SERIAL_8250_16550A_VARIANTS is not set
# CONFIG_SERIAL_8250_FINTEK is not set
+CONFIG_SERIAL_8250_CONSOLE=y
CONFIG_SERIAL_8250_DMA=y
# CONFIG_SERIAL_8250_PCI is not set
CONFIG_SERIAL_8250_NR_UARTS=32
@@ -2575,7 +2578,8 @@
#
# CONFIG_SERIAL_KGDB_NMI is not set
# CONFIG_SERIAL_UARTLITE is not set
-CONFIG_SERIAL_CORE=m
+CONFIG_SERIAL_CORE=y
+CONFIG_SERIAL_CORE_CONSOLE=y
CONFIG_CONSOLE_POLL=y
# CONFIG_SERIAL_JSM is not set
# CONFIG_SERIAL_LANTIQ is not set
@@ -2590,7 +2594,7 @@
# CONFIG_SERIAL_SPRD is not set
# end of Serial drivers
-CONFIG_SERIAL_MCTRL_GPIO=m
+CONFIG_SERIAL_MCTRL_GPIO=y
# CONFIG_SERIAL_NONSTANDARD is not set
# CONFIG_N_GSM is not set
# CONFIG_NOZOMI is not set
```
The question is, if `earlyprintk=ttyS0,115200(,keep)` worked before with
`CONFIG_SERIAL_8250=m`, why the driver is needed, and `console=ttyS0,…`
cannot fall back to the earlyprintk driver.
Kind regards,
Paul
