@大法师 Many thanks for your reply, but unoccupied serial ports accessible from GPIOs include uart3,6. There left only uart6 if uart3 can't be functional. Is it possible to activate uart3-m2-xfer?

It seems that the DTB file has changed a lot in the 20.04 ubuntu image. I can only find uart3-m1-xfer instead of the needed uart3-m2-xfer in DTS file converted from rk3588s-cp4.dtb. What should I do in order to use uart3 under this circumstance?

		uart2 {

			uart2m0-xfer {
				rockchip,pins = <0x00 0x0e 0x0a 0x167 0x00 0x0d 0x0a 0x167>;
				phandle = <0x136>;
			};
		};

		uart3 {

			uart3m1-xfer {
				rockchip,pins = <0x03 0x0e 0x0a 0x167 0x03 0x0d 0x0a 0x167>;
				phandle = <0x137>;
			};
		};

		uart4 {

			uart4m1-xfer {
				rockchip,pins = <0x03 0x18 0x0a 0x167 0x03 0x19 0x0a 0x167>;
				phandle = <0x138>;
			};
		};

		uart5 {

			uart5m1-xfer {
				rockchip,pins = <0x03 0x15 0x0a 0x167 0x03 0x14 0x0a 0x167>;
				phandle = <0x139>;
			};
		};

@george Is there any way to use GPIOs without sudo priviledge, such as in a ros package?