this post was submitted on 19 Apr 2025
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25
creating a USB gadget (lemm.ee)
submitted 1 week ago by [email protected] to c/[email protected]
22 comments fedilink hide all child comments
 

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
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[–] [email protected] 6 points 1 week ago* (last edited 1 week ago) (2 children)

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.

[–] [email protected] 4 points 1 week ago (1 children)

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.

[–] [email protected] 1 points 1 week ago (1 children)

Do you mean you intend to create a broken symlink?

[–] [email protected] 4 points 1 week ago (1 children)

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.

[–] [email protected] 3 points 1 week ago* (last edited 1 week ago) (1 children)

Edit: OK, I looked at the docs, and they sure do make a broken symlink there. I still think it's worth a try to create a non-broken link, maybe the docs are wrong. I would expect they would put a little note there, that yes, you really do want to create a broken symlink (if so, why not a regular file?), but then again its kernel docs and those aren't the most friendly.

I also thought you were OP for some reason, sorry.

Edit2: If you look at the file listing later in the docs, you can see this:

./configs/c.1/ncm.usb0 -> ../../../../usb_gadget/g1/functions/ncm.usb0

Which does look like a real non-broken symlink, so I maintain the docs are wrong and you're not supposed to make a broken symlink.

Original comment, silightly edited:

You misunderstand. I suspect OP cannot create the symlink, because it would be a broken symlink, not because the symlink is relative. Maybe you cannot create broken symlinks in the sysfs for some reason.

I was just trying to explain that a relative symlink is relative to the directory in which it resides. The target to the symlink should point to ../../functions/uvc.usb0 if you want it to point to something that exists. The ln command in OP's listing would result in a broken symlink, since the specified path is not relative to the c.1 directory. It is relative to the working directory, but that's wrong, that's not what ln expects you to put there.

Maybe it needs to be a correct symlink, maybe that will solve the problem.

[–] [email protected] 3 points 1 week ago (1 children)

I've got some experience with Linux USB gadgets, and can confirm that the docs definitely aren't wrong, as I've used them myself to write gadgets using the same symlink commands that the docs mention and that OP used here.

I've got a working USB ethernet gadget and MTP gadget for the Steam Deck that make the same link that should be broken, and here's a repo that implements every USB gadget with almost every gadget making the same symlink, one directory up from where OP does it. I've tested all the gadgets from that repo and they all work, and because they work, I've forked the repo here for future use in the above mentioned Steam Deck plugin.

I can pretty confidently say that it's not a broken symlink. I'm not 100% sure why this doesn't work for OP, but I think it's most likely an issue with not loading the correct driver for the UVC function.

[–] [email protected] 1 points 1 week ago (1 children)

OK I'm convinced, that's apparently how that works. I wonder why it's designed that way.

[–] [email protected] 2 points 1 week ago

Yeah, it's a bit weird that it works like that. I'll be honest, in the last three or four years that I've been tinkering with USB gadgets, I've never noticed that those symlinks you create should be broken until you mentioned it. I just kinda took it for granted that it works (probably because I never had issues with this part of the process).

[–] [email protected] 1 points 1 week ago

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)

[–] [email protected] 2 points 1 week ago* (last edited 1 week ago) (1 children)

Hey, you need to also modprobe usb_f_uvc at the beggining, right after modprobe libcomposite

[–] [email protected] 1 points 1 week ago (1 children)

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
[–] [email protected] 1 points 1 week ago (1 children)

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.

[–] [email protected] 1 points 1 week ago (1 children)

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?

[–] [email protected] 1 points 1 week ago (1 children)

No, unfortunately not :/

That's just the "name" of the configuration, for instance in my USB ethernet gadget I use both c.1 and c.2 as config names, and not a.1.

[–] [email protected] 1 points 1 week ago (1 children)

Once I crack this, I think it could be fun to create a script that could spoof devices plugged into it with the gadget framework. Could save me a lot of trouble creating known-working usb configs down the road. Instead of starting from scratch each time, I would just have to tweak an existing profile.

[–] [email protected] 1 points 1 week ago (1 children)

Hey, if you ever create that, give me a shoutout, I'd love to see it.

[–] [email protected] 1 points 1 week ago (1 children)

Honestly it’s just an idea to suck up time at this point. I do NOT have the skills for this shit, as we can see here. I can correct what ChatGPT gets wrong though and just run with that.

[–] [email protected] 1 points 1 week ago* (last edited 1 week ago) (1 children)

You should definitely try with the systemd-gadgets I linked earlier. It makes all the configuration really easy, you just need to enable the relevant services, so in your case usbgadget-func-uvc.service and gadget-start.service. You also need to copy them beforehand to /etc/systemd/system, including gadget-init.service, and you need to copy gadget to /etc/default/gadget, and the scripts gadget-start.sh and gadget-init.sh to /etc/systemd/scripts. Edit /etc/default/gadget to edit the configs and names of the gadget, and then start gadget-start.service. No need to enable gadget-init.service, it's called as a dependency from other services.

There's an install script in the repo that you can use as well, setup.sh, and a PKGBUILD so you can create an Arch package. After installing with either method, just change /etc/default/gadget, enable the uvc and gadget start services, and then just start the gadget start service.

[–] [email protected] 1 points 1 week ago (1 children)

there is no setup and the instructions for a manual install keep getting me weird complaints from systemd about dependancies. It occurs to me that tackling this is error doesn't make sense because it requires systemd to work and ultimately I want to run this on a diskless alpine linux install which doesnt use systemd.

[–] [email protected] 2 points 1 week ago (1 children)

Sorry, my bad, I forgot that the setup script isn't part of the default repo and is something I added in my own fork. But yeah, if there's no systemd, it's no use.

I can whip up a quick script that should work, I'll test it out on my own hardware, and post it here for you sometime tomorrow.

[–] [email protected] 1 points 1 week ago

woot! let me know what you come up with.

[–] [email protected] 1 points 1 week ago

Aren't sysfs entries supposed to be created by kernel modules?