I have replied a similarly question, for enable more UART ports, follow then (read all):
https://forum.radxa.com/t/uart2-is-busy-uart4-does-not-works/1237/4?u=adryanalencar
- First you need enable other UART peripherals (described in link)
- Setup then in `/boot/uEnv.txt boot settings file
Note: the linux kernel remap a serial ports on
/dev/ttySX. X represents the index of uart, sequentially by number of enabled drivers;
verbosity=7
fdtfile=rockchip/rk3308-rock-pi-s.dtb
console=ttyS0,1500000n8 <====== Change console UART here
overlays=rk3308-uart0, rk3308-uart2 <====== You can add UARTs drivers on there
rootuuid=ee60d2e1-358c-467c-acc8-a147c5b38695
initrdsize=0x5c96f8
kernelversion=4.4.143-55-rockchip-g6b7accbc999b
initrdimg=initrd.img-4.4.143-55-rockchip-g6b7accbc999b
kernelimg=vmlinuz-4.4.143-55-rockchip-g6b7accbc999b