Raspberry Pi Zero USB device emulation

Question

I know that the Raspberry Pi Zero supports OTG and USB Peripheral protocols, and there's a lot of cool built in peripherals shown here: https://learn.adafruit.com/turning-your-raspberry-pi-zero-into-a-usb-gadget?view=all#other-modules

The problem is that I need to emulate a USB Peripheral device that does not appear on this list. I have a vendor ID and product ID for the device, and I'm trying to figure out how exactly to go about doing this. Do I need to modify the OTG USB drivers in the Raspbian kernel? Do I have to completely build my own kernel? Or is there a better option I don't even realize?

Thanks in advance!!

Answer 1

Do I need to modify the OTG USB drivers in the Raspbian kernel?

The answer to your first question is "it depends", but if your device doesn't do anything too unusual this could be a No: you need not modify source code for kernel modules nor the kernel.

You're fortunate that Raspbian supports a modern kernel with ConfigFS support. Once you are set up with dtoverlay=dwc2 , you can open up a FunctionFS bulk endpoint as root like so:

modprobe libcomposite
modprobe usb_f_fs
cd /sys/kernel/config/usb_gadget
mkdir -p myperipheral; cd myperipheral
echo 0x1234 > idVendor  # put actual vendor ID here
echo 0xabcd > idProduct # put actual product ID here
mkdir configs/c.1
mkdir configs/c.1/strings/0x409
echo "My Peripheral" > configs/c.1/strings/0x409/configuration
mkdir functions/ffs.my_func_name
ln -s functions/ffs.my_func_name configs/c.1/
mkdir -p /tmp/mount_point
mount my_func_name -t functionfs /tmp/mount_point
# compile ffs-test from source, then copy and run it from /tmp/mount_point
ls /sys/class/udc > UDC

If you need to emulate the other device more closely, it's up to you to set bcdDevice , bcdUSB , serial number, manufacturer, product string, max power, os_desc , and possibly other fields.

AFAIK FunctionFS does not support isochronous endpoints, interrupt transfers, nor out-of-the-ordinary control transfers. If you require this, you may need to start looking into extending existing gadget modules, with source code here .

Update: When I got home to test this, I encountered a severe caveat with Raspbian. It'll initially fail to create ffs.my_func_name because usb_f_fs is not enabled by default. Although you need not modify any kernel modules, you must recompile with an alternate configuration. make menuconfig -> Device Drivers -> USB support -> USB Gadget Support -> USB functions configurable through configfs / Function filesystem (FunctionFS) + some other modules to test. After uploading a new kernel/modules, I tested the above script on Raspbian 8. I would also recommend setting USB Gadget Drivers / Function Filesystem to (M) in case you resort to the simpler g_ffs legacy module in lieu of ConfigFS.