USB gadget on a Raspberry Pi 4 for iPad

gadget.sh

#!/bin/sh

# I wanted to use a Raspberry Pi as a USB gadget for audio and ethernet, but got error messages on the iPad
# when I extended the camera connection kit with a Lightning extender.
#
# I managed to avoid this by making sure that the USB gadget announces itself as self-powered (see configs/c.1/bmAttributes)
# With this, the Pi 4 and a Hifiberry Amp supplies audio out and ethernet.
#
# For audio out, alsaloop or similar must also be running.


# Load libcomposite
modprobe libcomposite

SERIAL=`cat /proc/cpuinfo | grep Serial | cut -d ' ' -f 2`
GADGET_DIR="audio+ethernet"

start() {
    # Create a gadget called $GAGDET_DIR
    cd /sys/kernel/config/usb_gadget/
    mkdir -p ${GADGET_DIR}
    cd ${GADGET_DIR}

    # Configure our gadget details
    echo 0x1d6b > idVendor # Linux Foundation
    echo 0x0104 > idProduct # Multifunction Composite Gadget
    echo 0x0100 > bcdDevice # v1.0.0
    echo 0x0200 > bcdUSB # USB2
    mkdir -p strings/0x409

    echo ${SERIAL} > strings/0x409/serialnumber
    echo "ibp" > strings/0x409/manufacturer
    echo "kuechenradio-usb" > strings/0x409/product
    mkdir -p configs/c.1/strings/0x409
    echo "Config 1: ECM network" > configs/c.1/strings/0x409/configuration
    # 0xc0 is USB_CONFIG_ATT_SELFPOWER
    echo 0xc0 > configs/c.1/bmAttributes
    echo 0 > configs/c.1/MaxPower

    # Ethernet gadget
    mkdir -p functions/ecm.usb0
    # first byte of address must be even
    HOST="32:70:05:18:ff:7a" # "HostPC"
    SELF="32:70:05:18:ff:7b" # "Ethernet Gadget"
    echo $HOST > functions/ecm.usb0/host_addr
    echo $SELF > functions/ecm.usb0/dev_addr
    ln -s functions/ecm.usb0 configs/c.1/

    # UAC2 (audio) gadget
    mkdir -p functions/uac2.usb0
    echo "44100,48000" > functions/uac2.usb0/c_srate
    echo "2" > functions/uac2.usb0/c_ssize
    echo "44100,48000" > functions/uac2.usb0/p_srate
    echo "2" > functions/uac2.usb0/p_ssize
    ln -s functions/uac2.usb0 configs/c.1

    # End functions
    ls /sys/class/udc > UDC

    brctl addbr br0
    brctl addif br0 eth0
    brctl addif br0 usb0

}

stop() {

    ifconfig br0 down

    brctl delif br0 usb0
    brctl delif br0 eth0
    brctl delbr br0

    echo "" > UDC

    cd /sys/kernel/config/usb_gadget/${GADGET_DIR}
    rm configs/c.1/ecm.usb0
    rm configs/c.1/uac2.usb0
    rmdir configs/c.1/strings/0x409
    rmdir configs/c.1/
    rmdir functions/ecm.usb0
    rmdir functions/uac2.usb0
    rmdir strings/0x409
    cd ..
    rmdir ${GADGET_DIR}

}

case $1 in
    start)
	start;;
    stop)
	stop;;
esac