DIAG implements routing of diagnostics related messages between host and various subsystems.
Ensure that your kernel is built with CONFIG_CONFIGFS_FS and CONFIG_USB_CONFIGFS_F_FS and that configfs is mounted in /sys/kernel/config.
G1="/sys/kernel/config/usb_gadget/g1"
mkdir $G1
mkdir $G1/strings/0x409
mkdir $G1/functions/ffs.diag
mkdir $G1/configs/c.1
mkdir $G1/configs/c.1/strings/0x409
echo 0xVID > $G1/idVendor
echo 0xPID > $G1/idProduct
echo SERIAL > $G1/strings/0x409/serialnumber
echo MANUFACTURER > $G1/strings/0x409/manufacturer
echo PRODUCT > $G1/strings/0x409/product
echo "diag_dun" > $G1/configs/c.1/strings/0x409/configuration
ln -s $G1/functions/ffs.diag $G1/configs/c.1
mkdir /dev/ffs-diag
mount -t functionfs diag /dev/ffs-diag
diag-router &
sleep 1
echo 6a00000.dwc3 > $G1/UDC