Systemd unit for configuring USB gadgets

This implements the procedure described in the Kernel's gadget_configfs.txt as a system unit.

Installation

If you're running on a Pi, you will first need to enable the appropriate device overlays:

echo "dtoverlay=dwc2" | sudo tee -a /boot/config.txt
echo "dwc2" | sudo tee -a /etc/modules
echo "libcomposite" | tee -a /etc/modules

• If you plan to use your USB port as both a host and a gadget, then enable OTG mode by adding the following to /boot/config.txt:

  dtoverlay=dwc2,dr_mode=otg
  

• If you will be operating exclusively as a gadget, then instead use:

  dtoverlay=dwc2,dr_mode=peripheral
  

After editing /boot/config.txt, you must reboot for the changes to become active.

To install the systemd unit, run make install in the source directory, which will place the support scripts into /sbin and the systemd unit into /etc/systemd/system.

Configuration

Create one or files in /etc/gadget named <gadget_name>.conf. These files must contain the following configuration keys:

• USB_FUNCTIONS -- List of functions implemented by the gadget.

These files may contain any of the following configuration keys:

• <func> -- List of parameters of the function implemented by the gadget. Replace the dot with an underscore.
• USB_IDVENDOR -- Vendor ID. Defaults to 0x1b6d, the Linux Foundation.
• USB_IDPRODUCT -- Product ID. Defaults to 0x0104, "Multifunction Composite Gadget"
• USB_BCDDEVICE -- Device version. Defaults to 0x0100 (for 1.0.0).
• USB_BCDUSB -- Maximum supported USB version. Defaults to 0x0200 (for USB 2.0).
• USB_SERIALNUMBER -- Device serial number. Defaults to deadbeef0000.
• USB_PRODUCT -- Product name. Defaults to Pi Zero Gadget.
• USB_MANUFACTURER -- Manufacturer name. Defaults to Linux.
• USB_MAXPOWER -- Maximum power required. Defaults to 250.

For example, to create a serial gadget named g0, I would create the file /etc/gadget/g0.conf with the following contents:

USB_FUNCTIONS=acm.usb0
acm_usb0="param1=value1 param2=value2"

Or to create a gadget that offered both a serial interface and an ethernet interface:

USB_FUNCTIONS="acm.usb0 rndis.usb0"
acm_usb0="param1=value1 param2=value2"
rndis_usb0="param1=value1 param2=value2"

To enable the gadget at boot, run:

systemctl enable usb-gadget@g0

To create the gadget immediately:

systemctl start usb-gadget@g0

To remove the gadget after starting it:

systemctl stop usb-gadget@g0