Hi! I solved this problem.
We can enable uart function in startup settings, enable uart overlay you want to use.
I believe this works for all rock cards.
Find all available uart peripherals executing this command:
find /boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/ | grep uart
the output is similiars with this:
root@rockpis:/# find /boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/ | grep uart
/boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/overlay/rk3308-console-on-uart0.dtbo
/boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/overlay/rk3308-console-on-uart1.dtbo
/boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/overlay/rk3308-console-on-uart2.dtbo
/boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/overlay/rk3308-uart0.dtbo
/boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/overlay/rk3308-uart1.dtbo
/boot/dtbs/4.4.143-55-rockchip-g6b7accbc999b/rockchip/overlay/rk3308-uart2.dtbo
And the available ports is:
rk3308-uart0.dtbo
rk3308-uart1.dtbo
rk3308-uart2.dtbo
if that doesn’t work, find the folder avaiable into /boot/dtbs and replace it in command directory path ( /boot/dtbs/{AVAILABLE FOLDER}/rockchip/)
Now whe can edit the /boot/uEnv.txt file:
verbosity=7
fdtfile=rockchip/rk3308-rock-pi-s.dtb
console=ttyS0,1500000n8
overlays=rk3308-uart0, rk3308-uart2 <====== Your uart ports can write on this line separated by a comma
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
Save it and reboot the system.