I want to create a USB gadget with a raspberry pi zero 2W. I’m starting with imitating a webcam I already have to see how much of this I can figure out. I’ve used the online documentation and a couple AI bots to get this far quickly, but I’m hung up on a ln command. It’s telling me “ln: failed to create symbolic link ‘configs/c.1/uvc.usb0’: No such file or directory” when trying to create the link. This makes no sense to me though. I’m trying to create the link, of course it doesn’t exist yet. That’s what that command is supposed to do.
I’ve confirmed this problem in alpine linux and raspbian lite.
Below is the little script I have so far just to create the device:
#!/bin/bash
modprobe libcomposite
cd /sys/kernel/config/usb_gadget/
mkdir -p fauxcam
cd fauxcam
echo 0x046d > idVendor # Logitech Vendor ID
echo 0x094b > idProduct # Brio 105 Product ID
echo 0x0200 > bcdUSB
echo 0x9914 > bcdDevice
mkdir -p strings/0x409
echo "111111111111" > strings/0x409/serialnumber
echo "Brio 105" > strings/0x409/product
mkdir -p configs/c.1/strings/0x409
echo "UVC Configuration" > configs/c.1/strings/0x409/configuration
echo 250 > configs/c.1/MaxPower
mkdir -p functions/uvc.usb0
ln -s functions/uvc.usb0 configs/c.1/
echo "usb0" > UDC
IDK how all that works but I will say that the result of this would (in a normal filesystem) create a link named configs/c.1/uvc.usb0 yes, but that link would point to functions/uvc.usb0 relative to its own dir. This doesn’t exist and your symlink would be broken, presumably the special file system there doesn’t like that.
Edit: Apparently that’s not the problem and you’re totally supposed to run an ln command that should logically result in a broken symlink, thanks to kernel driver sysfs abuse.
Normally, you’d be right, but this special filesystem doesn’t care about that. I’ve done similar with different gadgets so many times, always with relativr symlinks, sometimes directly from the root of the special filesystem, sometimes from the gadgets own subdirectory, and it just works.
No, this is ConfigFS, the linux kernel’s special filesystem for configuring kernel stuff, and for some reason that I don’t really know, it doesn’t matter if the symlink is relative or not. The kernel documentation even creates a relative symlink from the same directory as OP did here.
You can check this project : https://github.com/showmewebcam/showmewebcam
It’s uses buildroot and it’s easy to change the sources for your need (I’ve added the wifi support)
Hey, you need to also modprobe usb_f_uvc at the beggining, right after modprobe libcomposite
no change
ln: failed to create symbolic link 'configs/c.1/uvc.usb0': No such file or directory
./fauxcam-gadget.sh: line 19: echo: write error: Device or resource busy
Hmm, not sure then. It seems correct to me. Check out this repo, it has systemd services for all the USB gadgets, you can run uvc very easily with this: https://github.com/BigfootACA/systemd-gadget
I can write specific instructions how to get this working later today, if you’ll need them.
Looking at those units I noticed something… ExecStart=/usr/bin/ln -s ${GADGET}/functions/uvc.0 ${GADGET}/configs/a.1/
It’s an a.1 instead of a c.1. Surely it couldn’t be as simple as just using a different letter, could it?
Aren’t sysfs entries supposed to be created by kernel modules?
