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How to Implement an USB 2.0 / USB 3.0 Mass Storage Gadget

How to Implement an USB 2.0 / USB 3.0 Mass Storage Gadget

Owned by vincent.shih
Last updated: Mar 27, 2025
4 min read

This document demonstrates how to implement an USB 2.0 / USB 3.0 mass storage gadget on SP7350 platforms.

 

Table of Contents

Kernel Setup

Enter the kernel configuration by running the following command in the top directory of project.

# make kconfig

 

  1. Select submenus for USB 2.0 controller : Device Drivers > USB support > USB Gadget Support > USB Peripheral Controller. Select <*> for “Sunplus USB 2.0 Device Controller“.

image-20240708-071332.png

 

  1. Select submenus for USB OTG Transceive driver : Device Drivers > USB support > USB Physical Layer drivers. Select <*> for “Sunplus USB OTG Transceive driver“.

image-20240708-071051.png

  1. Select submenus for USB 3.0 controller : Device Drivers > USB support. Select <*> for “DesignWare USB3 DRD Core Support“ and select “Dual Role mode“ for “Deware USB3 DRD Core Support“.

image-20240703-083817.png

 

  1. Select submenus for USB Gadget configfs : Device Drivers > USB support > USB Gadget Support. Select <M> for “USB Gadget functions configurable through configfs“ and [*] for “Mass storage“.

image-20240703-084023.png

 

  1. Execute the make command to build the Linux image.

Create a backing storage file

The backing storage file must be prepared before the gadget uses it.

  1. Use the following command to create 32MB files (backing_file_u2 and backing_file_u3) filled with zeros.

# dd bs=1M count=32 if=/dev/zero of=/backing_file_u2 32+0 records in 32+0 records out 33554432 bytes (34 MB, 32 MiB) copied, 0.0850515 s, 395 MB/s # dd bs=1M count=32 if=/dev/zero of=/backing_file_u3 32+0 records in 32+0 records out 33554432 bytes (34 MB, 32 MiB) copied, 0.0836486 s, 401 MB/s

 

  1. Use the following command to create the FAT file system in backing_file_u2 and backing_file_u3.

# mkfs.fat /backing_file_u2 mkfs.fat 4.1 (2017-01-24) # mkfs.fat /backing_file_u3 mkfs.fat 4.1 (2017-01-24)

 

ConfigFS

Configure mass storage gadget through configfs saved as a file named usb_mass_storage.

#!/bin/bash USB2_UMS_EN=on USB3_UMS_EN=on # install the drivers modprobe libcomposite modprobe usb_f_mass_storage # disable debug message echo 0 > /sys/module/sunplus_udc/parameters/dmsg # create /sys/kernel/config if [ ! -d /sys/kernel/config ] then mkdir -p /sys/kernel/config fi # mount configfs if ! mountpoint -q /sys/kernel/config then mount -t configfs none /sys/kernel/config fi if [ $USB2_UMS_EN = on ] then UDC_NAME="f8102800.usb" BACKING_FILE=/backing_file_u2 # create gadget cd /sys/kernel/config/usb_gadget mkdir g1 cd g1 echo "64" > bMaxPacketSize0 echo "0x200" > bcdUSB # create configuration mkdir -p configs/c.1 mkdir -p configs/c.1/strings/0x409 echo 120 > configs/c.1/MaxPower mkdir strings/0x0409 echo "1234" > strings/0x0409/serialnumber echo "Sunplus" > strings/0x0409/manufacturer echo "SP7350 USB2.0 Mass Storage Gadget" > strings/0x0409/product # create functions mkdir functions/mass_storage.usb0 echo "$BACKING_FILE" > functions/mass_storage.usb0/lun.0/file # link functions and configuration ln -s functions/mass_storage.usb0 configs/c.1 # activate echo $UDC_NAME > UDC fi if [ $USB3_UMS_EN = on ] then UDC_NAME="f80a1000.dwc3" BACKING_FILE=/backing_file_u3 # create gadget cd /sys/kernel/config/usb_gadget if [ $USB2_UMS_EN = on ] then mkdir g2 cd g2 else mkdir g1 cd g1 fi echo "64" > bMaxPacketSize0 echo "0x300" > bcdUSB # create configuration mkdir configs/c.1 mkdir configs/c.1/strings/0x409 echo 120 > configs/c.1/MaxPower mkdir strings/0x0409 echo "5678" > strings/0x0409/serialnumber echo "Sunplus" > strings/0x0409/manufacturer echo "SP7350 USB3.0 Mass Storage Gadget" > strings/0x0409/product # create functions mkdir functions/mass_storage.usb0 echo "$BACKING_FILE" > functions/mass_storage.usb0/lun.0/file # link functions and configuration ln -s functions/mass_storage.usb0 configs/c.1 # activate echo $UDC_NAME > UDC fi

 

Implementation

After updating the image to the SP7350 platform, booting the system and copying backing_file_u2 and backing_file_u3 to it, use sh usb_mass_storage command to configure USB 2.0 and USB 3.0 as mass storage devices.

image-20240708-065620.png

 

After connecting the mass storage gadget to a Windows PC through an USB cable, “E:\” and “F:\”will show up in “Portable Devices” of “Device Manager” and 2 “USB Mass Storage Device“ in “Universal Serial Bus controllers“.

image-20240801-034342.png

 

Clear ConfigFS Configuration

The following script can be used to clear configfs configuration of the gadget.

#!/bin/bash addr1="g1" addr2="g2" arr=("addr1" "addr2") GADGET=/sys/kernel/config/usb_gadget for i in "${arr[@]}"; do if [ -d $GADGET/${!i} ] then if [ -d $GADGET/${!i}/configs/c.1 ] then if [ -d $GADGET/${!i}/configs/c.1/mass_storage.usb0 ] then rm $GADGET/${!i}/configs/c.1/mass_storage.usb0 fi if [ -d $GADGET/${!i}/configs/c.1/strings/0x409 ] then rmdir $GADGET/${!i}/configs/c.1/strings/0x409 fi rmdir $GADGET/${!i}/configs/c.1 fi if [ -d $GADGET/${!i}/functions/mass_storage.usb0 ] then rmdir $GADGET/${!i}/functions/mass_storage.usb0 fi if [ -d $GADGET/${!i}/strings/0x0409 ] then rmdir $GADGET/${!i}/strings/0x0409 fi rmdir $GADGET/${!i} fi done if grep -q "^"usb_f_mass_storage"" /proc/modules then rmmod usb_f_mass_storage fi if grep -q "^"libcomposite"" /proc/modules then rmmod libcomposite fi

 

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