create a USB gadget using configfs

creategadget.sh

#!/bin/sh

# Originally from: https://gitlab.com/postmarketOS/pmaports/-/blob/master/main/postmarketos-mkinitfs/init_functions.sh#L264
# See: https://www.kernel.org/doc/Documentation/usb/gadget_configfs.txt

mkdir -p /config
mount none -t configfs /config

CONFIGFS=/config/usb_gadget

if ! [ -e "$CONFIGFS" ]; then
    echo "  $CONFIGFS does not exist, skipping configfs usb gadget"
    return
fi

# Default values for USB-related deviceinfo variables
usb_idVendor="${deviceinfo_usb_idVendor:-0x18D1}"   # default: Google Inc.
usb_idProduct="${deviceinfo_usb_idProduct:-0xD001}" # default: Nexus 4 (fastboot)
usb_serialnumber="${deviceinfo_usb_serialnumber:-postmarketOS}"
usb_rndis_function="${deviceinfo_usb_rndis_function:-rndis.usb0}"

echo "  Setting up an USB gadget through configfs"
# Create an usb gadet configuration
mkdir $CONFIGFS/g1 || echo "  Couldn't create $CONFIGFS/g1"
echo "$usb_idVendor"  > "$CONFIGFS/g1/idVendor"
echo "$usb_idProduct" > "$CONFIGFS/g1/idProduct"

# Create english (0x409) strings
mkdir $CONFIGFS/g1/strings/0x409 || echo "  Couldn't create $CONFIGFS/g1/strings/0x409"

# shellcheck disable=SC2154
echo "$deviceinfo_manufacturer" > "$CONFIGFS/g1/strings/0x409/manufacturer"
echo "$usb_serialnumber"        > "$CONFIGFS/g1/strings/0x409/serialnumber"
# shellcheck disable=SC2154
echo "$deviceinfo_name"         > "$CONFIGFS/g1/strings/0x409/product"

# Create rndis function. The function can be named differently in downstream kernels.
mkdir $CONFIGFS/g1/functions/"$usb_rndis_function" \
    || echo "  Couldn't create $CONFIGFS/g1/functions/$usb_rndis_function"

# Create configuration instance for the gadget
mkdir $CONFIGFS/g1/configs/c.1 \
    || echo "  Couldn't create $CONFIGFS/g1/configs/c.1"
mkdir $CONFIGFS/g1/configs/c.1/strings/0x409 \
    || echo "  Couldn't create $CONFIGFS/g1/configs/c.1/strings/0x409"
echo "rndis" > $CONFIGFS/g1/configs/c.1/strings/0x409/configuration \
    || echo "  Couldn't write configration name"

# Link the rndis instance to the configuration
ln -s $CONFIGFS/g1/functions/"$usb_rndis_function" $CONFIGFS/g1/configs/c.1 \
    || echo "  Couldn't symlink $usb_rndis_function"

# Check if there's an USB Device Controller
if [ -z "$(ls /class/udc)" ]; then
    echo "  No USB Device Controller available"
    return
fi

# Link the gadget instance to an USB Device Controller. This activates the gadget.
# See also: https://github.com/postmarketOS/pmbootstrap/issues/338
# shellcheck disable=SC2005
echo "$(ls /class/udc)" > $CONFIGFS/g1/UDC || echo "  Couldn't write UDC"