RPI, baudrate for serial console

Hardware
1

I have RPI 3B+, it runs IPfire core 184 and I connected serial console.

  • I modified /boot/config.txt, I added line enable_uart=1
  • I modified /boot/uEnv.txt, it has line SERIAL-CONSOLE=ON

I use screen /dev/ttyUSB0 115200 command on my Linux PC, to connect to the serial console. It works but only from the point when kernel starts to boot.

There is a lot of garbage printed in the initial stage, maybe that different baudrate is used but I failed to find what is correct value for that part. Maybe it is just a random garbage. Any idea?

It looks like this (reboot):

Deactivating all swap files/partitions...                              [  OK  ]
Bringing down the loopback interface...                                [  OK  ]
Prepare for reboot...
reboot: Restarting system
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Booting Linux on physical CPU 0x0000000000 [0x410fd034]
Linux version 6.6.15-ipfire (root@arm64-01.zrh.ipfire.org) (gcc (GCC) 13.2.0, GNU ld (GNU Binutils) 2.41) #1 SMP PREEMPT_DYNAMIC Fri Feb  9 18:47:53 GMT 2024
KASLR disabled due to lack of seed
Machine model: Raspberry Pi 3 Model B+
efi: UEFI not found.
2

This is a hardware issue on the 3b+ (software uart) that u-boot has the wrong (unstable) baudrate because a timer is not supported. There is easy way to fix this without breaking the compatiblity with other arm boards.