dummy_hcd loopback webcam. Impossible?

Question

I've been trying to create a loopback USB Webcam device with the help of dummy_hcd kernel module on my Raspberry PI 2B. Steps I've taken:

I load libcomposite via modprobe libcomposite.

Then I load dummy_hcd via modprobe dummy_hcd. This creates a software-emulated UDC chip. More on that I've found here.

Then I use a script to create and configure the USB webcam device. I've based my script on this and this. Here it is:

#!/bin/bash
GADGET_NAME=virtcam
GADGET_ROOT=/sys/kernel/config/usb_gadget/$GADGET_NAME
mkdir $GADGET_ROOT
cd $GADGET_ROOT
echo 0x1d6b > idVendor
echo 0x0104 > idProduct
echo 0x0100 > bcdDevice
echo 0x0200 > bcdUSB
echo 0xEF > bDeviceClass
echo 0x02 > bDeviceSubClass
echo 0x01 > bDeviceProtocol
mkdir strings/0x409
cd strings/0x409
echo 100000000d2386db > serialnumber
echo "Samsung" > manufacturer
echo "Webcam" > product
cd $GADGET_ROOT
mkdir configs/c.1
mkdir configs/c.1/strings/0x409
cd configs/c.1/strings/0x409
echo "UVC" > configuration
cd $GADGET_ROOT/configs/c.1
echo 500 > MaxPower
cd $GADGET_ROOT
mkdir functions/uvc.usb0
cd functions/uvc.usb0
echo 2048 > streaming_maxpacket
mkdir control/header/h
cd control
ln -s header/h class/fs
ln -s header/h class/ss
cd $GADGET_ROOT/functions/uvc.usb0/streaming/uncompressed
mkdir -p u/720p
cd u/720p
cat <<EOF > dwFrameInterval
666666
1000000
5000000
EOF
cat <<EOF > wWidth
1280
EOF
cat <<EOF > wHeight
720
EOF
cat <<EOF > dwMinBitRate
29491200
EOF
cat <<EOF > dwMaxBitRate
29491200
EOF
cat <<EOF > dwMaxVideoFrameBufferSize
1843200
EOF
mkdir $GADGET_ROOT/functions/uvc.usb0/streaming/header/h
cd $GADGET_ROOT/functions/uvc.usb0/streaming
ln -s uncompressed/u header/h
ln -s header/h class/fs
ln -s header/h class/hs
ln -s header/h class/ss
cd $GADGET_ROOT
ln -s functions/uvc.usb0 configs/c.1/uvc.usb0
# ln -s functions/acm.usb0 configs/c.1/acm.usb0
udevadm settle -t 5 || :

The script completes without errors on both machines I've tested (5.15.16-1-MANJARO and raspberrypi 5.10.94-v7+), but then when I actually do try to connect the created USB device to the dummy UDC, it errors out.

cd $GADGET_ROOT
echo "dummy_udc.0" > UDC
# it returns 'bash: echo: write error: Invalid argument'

It basically means that this device cannot be connected to this UDC chip. When I dmesg -Tw, I get the following:

[Fri Feb  4 00:00:00 2022] configfs-gadget gadget: uvc: uvc_function_bind()
[Fri Feb  4 00:00:00 2022] configfs-gadget gadget: uvc: Unable to allocate streaming EP
[Fri Feb  4 00:00:00 2022] configfs-gadget dummy_udc.0: failed to start virtcam: -22

Error is the same on both devices, so it's not device specific. So I had to hit google with this error. I've found this thread, where a guy says:

I know it's late, but just for anyone else that tries to use g_webcam or the uvc configfs with dummy_hcd ... UVC functions use isochronous transfers, so it won't work.

And, in fact, if we look at the source code, it says in the comments:

Note: The emulation does not include isochronous transfers!

So I'm basically really stuck. It turns out that dummy_hcd doesn't support isochrounous transfers, but webcams seem to use them, so it's either buy a hardware UDC chip (or some device which has it) or find a way around.

Is there really no way to create a fully virtual loopback webcam on Linux? Do webcams really require isochronous transfer to work?

Is there a way to run it someway without writing kernel modifications of dummy_hcd.c? Or should I just buy hardware hoping it works?

Answer 1

I believe you are looking at this from the wrong direction. Although the dummy_hcd module does not provide the functions required to create a USB webcam, there is already a project called v4l2loopback which provides the ability to create a loopback webcam device.

v4l2 is the standard for webcams in the linux world anyway, so this allows you to avoid messing with all of the USB side of things.

The v4l2loopback module will allow dummy video devices to be created, and these can be controlled from userspace. A simple example of this would be playing a local video file as if it were from the webcam, which can be done simply with ffmpeg like this

sudo ffmpeg -i video.mp4 -vf format=yuv420p -f v4l2 /dev/video2

In this example we are playing video.mp4 as if it were from the webcam /dev/video2, and this can be viewed from any other application that reads from the dummy video device.