I'm experimenting with upstream kernel and now I try to get serial console working. I'm getting only garbage symbols when I try to communicate:
I think it is important to notice that I only get garbage when I send something or when RPi send something back, not constantly.
It is also not a baud rate problem because same baud rate is valid when I use default RPi kernel. Therefore it is also not a cable problem, because serial console works well using default kernel or Raspbian OS.
I found out that different parameters like core_freq or enable_uart could help, so I tried them with no success.
Here is my config.txt:
initramfs initramfs-linux.img followkernel
root=UUID=[deleted] rw rootwait console=ttyS1,115200 console=tty1 plymouth.enable=0
Kernel version 5.7