1.16. USB support

1.16.1. USB host support

barebox has support for both USB host and USB device mode. USB devices take a long time to probe, so they are not probed automatically. Probing has to be triggered using the usb - (re-)detect USB devices or detect - detect devices command. USB devices in barebox are not hot-pluggable. It is expected that USB devices are not disconnected while barebox is running. USB Networking

barebox supports r8152, ASIX-compatible devices and the SMSC95xx. After detection, the device shows up as an extra network device (e.g. eth1) and can be used like a regular network device.

To use a USB network device together with the ifup - bring a network interface up command, add the following to /env/network/eth0-discover:



Alternatively, a detect -a (all) can be forced in ifup by setting global.net.ifup_force_detect=1. USB mass storage

barebox supports USB mass storage devices. After probing them with the usb - (re-)detect USB devices command, they show up as /dev/diskx and can be used like any other device.

1.16.2. USB device support

barebox supports several different USB gadget drivers:

  • Device Firmware Upgrade (DFU)

  • Android Fastboot

  • USB mass storage

  • serial gadget

The recommended way to use USB gadget is with the usbgadget - Create USB Gadget multifunction device command. While there are individual commands for dfu - device firmware update and usbserial - serial gadget enable/disable, the usbgadget - Create USB Gadget multifunction device commands supports registering composite gadgets, which exports multiple functions at once. This happens in the “background” without impacting use of the shell. Partition description

The USB gadget commands for Android Fastboot, DFU and the mass storage gadget take a partition description which describes which barebox partitions are exported via USB. The partition description is referred to as <desc> in the command help texts. It has the general form partition(name)flags,partition(name)flags,....

The partition field is the partition as accessible in barebox. This can be a path in /dev/, but could also be a regular file.

The name field is the name under which the partition shall be exported. This is the name under which the partition can be found with the host tool.

Several flags are supported, each denoted by a single character:

  • s Safe mode. The file is downloaded completely before it is written (DFU specific)

  • r Readback. The partition is allowed to be read back (DFU specific)

  • c The file shall be created if it doesn’t exist. Needed when a regular file is exported.

  • u The partition is a MTD device and shall be flashed with a UBI image.

  • o The partition is optional, i.e. if it is not available at initialization time, it is skipped

    instead of aborting the initialization. This is currently only supported for fastboot.



Board code authors are encouraged to provide a default environment containing partitions with descriptive names. For boards where this is not specified, there exist a number of partition specifiers for automatically generating entries:

  • block exports all registered block devices (e.g. eMMC and SD)

  • auto currently equivalent to block. May be extended to other flashable

    devices, like EEPROMs, MTD or UBI volumes in future

Example usage of exporting registered block devices, barebox update handlers and a single file that is created on flashing:

detect -a # optional. Detects everything, so auto can register it
usbgadget -A auto,/tmp/fitimage(fitimage)c -b DFU

USB Device Firmware Upgrade (DFU) is an official USB device class specification of the USB Implementers Forum. It provides a vendor-independent way to update the firmware of embedded devices. The current specification is version 1.1 and can be downloaded here: http://www.usb.org/developers/devclass_docs/DFU_1.1.pdf

On the barebox side, the update is handled with the usbgadget - Create USB Gadget multifunction device or the dfu - device firmware update command.

On the host side, the tool dfu-util can be used to update the partitions. It is available for most distributions and typically supports the following options:

dfu-util -h
Usage: dfu-util [options] ...
  -h --help                     Print this help message
  -V --version                  Print the version number
  -v --verbose                  Print verbose debug statements
  -l --list                     List the currently attached DFU capable USB devices
  -e --detach                   Detach the currently attached DFU capable USB devices
  -d --device vendor:product    Specify Vendor/Product ID of DFU device
  -p --path bus-port. ... .port Specify path to DFU device
  -c --cfg config_nr            Specify the Configuration of DFU device
  -i --intf intf_nr             Specify the DFU Interface number
  -a --alt alt                  Specify the Altsetting of the DFU Interface
                                by name or by number
  -t --transfer-size            Specify the number of bytes per USB Transfer
  -U --upload file              Read firmware from device into <file>
  -D --download file            Write firmware from <file> into device
  -R --reset                    Issue USB Reset signalling once we're finished
  -s --dfuse-address address    ST DfuSe mode, specify target address for
                                raw file download or upload. Not applicable for
                                DfuSe file (.dfu) downloads

To update the kernel for the above example, you would use something like the following:

dfu-util -D arch/arm/boot/zImage -a kernel

The dfu-util command automatically finds DFU-capable devices. If there are multiple devices found, you need to identify one with the -d/-p options. USB serial console

barebox can provide a serial console over USB. This can be initialized with the usbserial - serial gadget enable/disable command. Once the host is plugged in it should show a new serial device, on Linux for example /dev/ttyACM0. Android Fastboot support

barebox has support for the android fastboot protocol. There is no dedicated command for initializing the fastboot protocol, instead it is integrated into the Multifunction Composite Gadget, see usbgadget - Create USB Gadget multifunction device for a usage description.

The Fastboot gadget supports the following commands:

  • fastboot flash

  • fastboot getvar

  • fastboot boot

  • fastboot reboot

fastboot flash additionally supports image types UBI and Barebox. For UBI Images and a MTD device as target, ubiformat is called. For a Barebox image with an available barebox update handler for the fastboot exported device, the barebox_update is called (exported as bbu-<update_handler_name> fastboot partition).

The barebox Fastboot gadget supports the following non standard extensions:

  • fastboot getvar all Shows a list of all variables, including exported partitions

  • fastboot oem getenv <varname> Shows a barebox environment variable

  • fastboot oem setenv <varname>=<value> Sets a barebox environment variable

  • fastboot oem exec <cmd> executes a shell command. Note the output can’t be seen on the host, but the fastboot command returns successfully when the barebox command was successful and it fails when the barebox command fails.

Example booting kernel/devicetree/initrd with fastboot

In Barebox start the fastboot gadget:

usbgadget -A /kernel(kernel)c,/initrd(initrd)c,/devicetree(devicetree)c

On the host you can use this script to start a kernel with kernel, devicetree and initrd:

#!/usr/bin/env bash

set -e
set -v

if [ "$#" -lt 3 ]
        echo "USAGE: $0 <KERNEL> <DT> <INITRD> [<ARGS>]"
        exit 0


shift 3

fastboot -i 7531 flash kernel $kernel
fastboot -i 7531 flash devicetree $dt
fastboot -i 7531 flash initrd $initrd

fastboot -i 7531 oem exec 'global linux.bootargs.fa'$ct'=rdinit=/sbin/init'
if [ $# -gt 0 ]
        for i in $*
                fastboot -i 7531 oem exec 'global linux.bootargs.fa'$ct'='"\"$i\""
                ct=$(($ct + 1))
timeout -k 5 3 fastboot -i 7531 oem exec -- bootm -o /devicetree -r /initrd /kernel USB Mass storage gadget

Example exporting barebox block devices to a USB host:

usbgadget -S /dev/mmc0(emmc),/dev/mmc1(sd) USB Composite Multifunction Gadget

With the Composite Multifunction Gadget it is possible to create a USB device with multiple functions. A useful combination is creating a Fastboot gadget and a USB serial console. This combination can be created with:

usbgadget -A /dev/mmc2.0(root),/dev/mmc2.1(data) -a

The -A option will create a Fastboot function providing /dev/mmc2.0 as root partition and /dev/mmc2.1 as data partition. The -a option will create a USB CDC ACM compliant serial device.

Unlike the dfu - device firmware update command the usbgadget command returns immediately after creating the gadget. The gadget can be removed with usbgadget -d.

1.16.3. USB OTG support

barebox does not have true USB OTG support. However, barebox supports some USB cores in both host and device mode. If these are specified for otg in the device tree (dr_mode = “otg”;) barebox registers a OTG device which can be used to decide which mode shall be used. The device has a mode parameter which by default has the value otg. setting this to host or peripheral puts the device in the corresponding mode. Once a specific mode has been selected it can’t be changed later anymore.

barebox:/ devinfo otg0
  mode: otg ("otg", "host", "peripheral")
barebox:/ otg0.mode=host
musb-hdrc: ConfigData=0xde (UTMI-8, dyn FIFOs, bulk combine, bulk split, HB-ISO Rx, HB-ISO Tx, SoftConn)
musb-hdrc: MHDRC RTL version 2.0
musb-hdrc: setup fifo_mode 4
musb-hdrc: 28/31 max ep, 16384/16384 memory

1.16.4. USB Type-C support

barebox can usually stay oblivious to the type of connector used. Sometimes though, board code and user scripts may want to base their decisions on how a USB-C connector is connected. Type C drivers can thus register with the Type C driver core to export a number of device parameters:

  • $typec0.usb_role = { none, device, host }

  • $typec0.pwr_role = { sink, source }

  • $typec0.accessory = { none, audio, debug }

Currently, only the TUSB320 is supported, but it’s straight-forward to port more drivers from Linux.

1.16.5. USB Gadget autostart Support

Barebox can be configured to start usbgadget automatically by using global variables, instead of creating boot script. This can be useful if autostart policy should be chosen at boot time from other driver or script. To get usbgadget autostart support barebox has to be compiled with CONFIG_USB_GADGET_AUTOSTART enabled. USB Gadget autostart Options


Boolean flag. If set to 1, usbgadget will be started automatically on boot and enable USB OTG mode. (Default 0).


Boolean flag. If set to 1, CDC ACM function will be created. See usbgadget - Create USB Gadget multifunction device -a. (Default 0).


Common function description for all of DFU, fastboot and USB mass storage gadgets. Both Fastboot and DFU partitions also have dedicated override variables for backwards-compatibility:


Function description for DFU. See usbgadget - Create USB Gadget multifunction device -D [desc].


Function description for fastboot. See usbgadget - Create USB Gadget multifunction device -A [desc].


Export barebox update handlers. See usbgadget - Create USB Gadget multifunction device -b. (Default 0).