Issues attempting to use Raspberry Pi 4 serial console on mainline

[Nathan Chancellor <natechancellor@gmail.com>]

Hi all,

Thank you for all of the hard work that has been done on supporting the
Raspberry Pi 4 upstream. It has been a great platform so far for testing
various clang technologies on actual hardware.

I am investigating a panic that occurs when running a guest under KVM
when clang's Shadow Call Stack is enabled and it would be handy to grab
the kernel panic via serial console as the device panics and I lose my
ssh connection. I picked up a USB to TTL cable and I am able to get
connected with the following config.txt options and cmdline.txt options:

$ head -n4 /boot/config.txt
enable_uart=1
kernel=Image
os_prefix=custom-mainline-arm64/
upstream_kernel=1

$ cat /boot/custom-mainline-arm64/cmdline.txt
console=ttyS1,115200 console=tty1 root=PARTUUID=45a8dd8a-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait plymouth.ignore-serial-consoles

However, when I connect to the serial console via PuTTY, I get nothing
but garbage output: https://imgur.com/a/ekFlLYq

As I understand it, that is due to the mini UART not being as good as a
regular PL011. On the downstream kernel, one would apply
'dtoverlay=disable-bt' which would free up the first PL011 to be used as
the primary UART but the device tree overlays do not work on a mainline
kernel. Is there an easy way to disable Bluetooth via the device tree?
If not, is there any recommended or documented way to use the mini UART
successfully? I have seen information around using 'core_freq=...' but
the documentation says that does not work for the Raspberry Pi 4.

Any guidance or documentation that you could point me to would be much
appreciated.

Cheers,
Nathan

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