Here below an example of device tree configuration used on modifications to standard DAVE's kit for the ORCA SOM:

&uart2 uart4 { pinctrl-names = "default"; pinctrl-0 = <&pinctrl_uart2pinctrl_uart4>; assigned-clocks = <&clk IMX8MP_CLK_UART4>; assigned-clock-parents = <&clk IMX8MP_SYS_PLL1_80M>; fsl,uart-has-rtscts; status = "okay";

pinctrl_uart2 pinctrl_uart4: uart2grp uart4grp { fsl,pins = < MX8MP_IOMUXC_UART2_RXD__UART2_DCE_RX 0x49 MX8MP_IOMUXC_ECSPI2_SCLK__UART4_DCE_RX 0x140 MX8MP_IOMUXC_UART2_TXD__UART2_DCE_TX 0x49 MX8MP_IOMUXC_ECSPI2_MOSI__UART4_DCE_TX 0x140 MX8MP_IOMUXC_ECSPI2_MISO__UART4_DCE_CTS 0x140 MX8MP_IOMUXC_ECSPI2_SS0__UART4_DCE_RTS 0x140 >; }; 

[ 0.754248755511] 30860000.serial: ttymxc0 at MMIO 0x30860000 (irq = 26, base_baud = 5000000) is a IMX[ 0.761203762356] 30880000.serial: ttymxc2 at MMIO 0x30880000 (irq = 27, base_baud = 5000000) is a IMX[ 0.769995771151] 30890000.serial: ttymxc1 at MMIO 0x30890000 (irq = 28, base_baud = 1500000) is a IMX[ 0.779648] printk: console [ttymxc1] enabled[ 0.798672] 30a60000.serial: ttymxc3 at MMIO 0x30a60000 (irq = 35, base_baud = 5000000) is a IMX

root@desk-mx8mp:~# stty -F /dev/ttymxc2 ttymxc3 115200 -echoroot@desk-mx8mp:~# cat /dev/ttymxc2 ttymxc3 &

root@desk-mx8mp:~# echo "Test loopback" > /dev/ttymxc2ttymxc3