@philrandal I’ve just started a thread about device tree overlays which are needed for pps.
planning a how-to post on the whole project later on 
the serial port is a bit special cause it is also used via sbi and the riscv-sbi-hvc kernel driver. to just use it as a serial port for gps we disabled logins and kernel messages on ttyS0 and hvc0. (if hvc0 is in use setting the baudrate on ttyS0 crashes the kernel…)