16. USB support

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.

16.1.1. USB Networking

barebox supports ASIX-compatible devices and the SMSC95xx. After detection, the device shows up as eth0 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:

#!/bin/sh

usb

16.1.2. 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.

16.2. USB device support

16.2.1. 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 dfu - device firmware update command. It is passed a list of partitions to provide to the host. The partition list has the form <file>(<name>)<flags>. file is the path to the device or regular file which shall be updated. name is the name under which the partition shall be provided to the host. For the possible flags see dfu - device firmware update. A typical dfu command could look like this:

dfu "/dev/nand0.barebox.bb(barebox)sr,/dev/nand0.kernel.bb(kernel)r,/dev/nand0.root.bb(root)r"

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.

16.2.2. 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.

16.2.3. 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 :ref:command_usbgadget for a usage description.

The Fastboot gadget supports the following commands:

• fastboot flash
• fastboot getvar
• fastboot boot
• fastboot reboot

NOTE fastboot erase is not yet implemented. This means flashing MTD partitions does not yet work.

The barebox Fastboot gadget supports the following non standard extensions:

• fastboot getvar all Shows a list of all variables
• 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.

16.2.4. 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.

16.3. USB OTG support

barebox does not have 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 device puts the device in the corresponding mode. Once a specific mode has been selected it can’t be changed later anymore.