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Linux USB gadget configfs

Using configs we can easily set up usb gadget in user space. You even can make your board as a compound USB device, just by few commands.

Enable and mount configfs

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modprobe libcomposite
mount -t configfs configfs /sys/kernel/config

Create gadget(s)

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mkdir /sys/kernel/config/usb_gadget/g1

Create English strings and write to common attributes

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mkdir -p /sys/kernel/config/usb_gadget/g1/strings/0x409
echo ${SERIALNUMBER} > /sys/kernel/config/usb_gadget/g1/strings/0x409/serialnumber
echo ${MANUFACTURER} > /sys/kernel/config/usb_gadget/g1/strings/0x409/manufacturer
echo ${PRODUCT_NAME} > /sys/kernel/config/usb_gadget/g1/strings/0x409/product

Create gadget configuration(s)

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mkdir -p /sys/kernel/config/usb_gadget/g1/configs/c1
mkdir -p /sys/kernel/config/usb_gadget/g1/configs/c1/strings/0x409
echo "Config 1: ADB" > /sys/kernel/config/usb_gadget/g1/configs/c1/strings/0x409/configuration
echo 250 > /sys/kernel/config/usb_gadget/g1/configs/c1/MaxPower

Create gadget function(s)

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# Here we use ffs adb as an example
mkdir -p /sys/kernel/config/usb_gadget/g1/functions/ffs.adb

Associate gadget functions with configuration(s)

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ln -s /sys/kernel/config/usb_gadget/g1/functions/ffs.adb /sys/kernel/config/usb_gadget/g1/configs/c1

Extra setup for gadget functions

Sometime there is some steps need to be done before enabling gadget(i.e. ffs need to be mounted before active)

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# Create mount point for ffs and start adbd
mkdir -p "${mount_point}"
mount -t functionfs adb "${mount_point}" -o uid=0,gid=0
adbd &

If no extra step is needed, we can directly enable gadget

Enable the gadget

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# Find the available udc
udc=$(ls -1 /sys/class/udc/)
if [ -z $udc ]; then
echo "No UDC driver registered"
exit 1
fi
echo "${udc}" > /sys/kernel/config/usb_gadget/g1/UDC

Compound USB Device

To setup a compound USB device, we just need to create several gadget functions and binding them with configuration(s) before enabling the gadget