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creating a USB gadget (lemm.ee)
submitted 1 week ago by muusemuuse@lemm.ee to c/linux@lemmy.ml
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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[-] dafta 4 points 1 week ago

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.

[-] gnuhaut@lemmy.ml 1 points 1 week ago

Do you mean you intend to create a broken symlink?

[-] dafta 4 points 1 week ago

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.

[-] gnuhaut@lemmy.ml 3 points 1 week ago* (last edited 1 week ago)

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.

[-] dafta 3 points 1 week ago

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.

[-] gnuhaut@lemmy.ml 1 points 1 week ago

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

[-] dafta 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).

this post was submitted on 19 Apr 2025
23 points (100.0% liked)

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