Hi, I needed 921600 baud and found the same problem as you. Attached is a overlay that has worked for me for all the speeds up to 921600. Hopefully you can use this as a starting point to try higher bauds. Instructions for adding overlays are here: https://docs.armbian.com/User-Guide_Allwinner_overlays/ if you're not already aware.
The problem is outlined here: https://github.com/armbian/linux/blob/sun8i/drivers/tty/serial/sunxi-uart.c#L358 - the ABP2 bus is normally 24MHz. I chose 30MHz for my case but you might have to try 54545454Hz or some of the others in that table.
If you do
sudo cat /sys/kernel/debug/clk/clk_summary
you can see i2c and scr0(??) also share that clock - if you see any issues with those after this change it's likely the clock rate change has caused it.
@MarshallMCH, @lpercifield, @Bongho Lee FYI in case any of you are still wanting this 7 years later 😆
Hope that helps,
Greg
uart-baud-upto-921600.dts
