Ubuntu port of v4l2loopback doesn't behave the same as official port (v0.12.5)

Status changed to 'Confirmed' because the bug affects multiple users.

This bug is still present in v4l2loopback-dkms_0.12.5-1ubuntu3 on impish.

This bug affects me.

This bug affects me.

To those who just wrote "This bug affects me": Please log in and follow this link:
https://bugs.launchpad.net/ubuntu/+source/v4l2loopback/+bug/1946660/+affectsmetoo

This bug affects me.

The first open has:
```
[17047.368576] v4l2-loopback[1992]: opened dev:00000000c093a43d with image:0000000000000000
[17047.368577] /var/lib/dkms/v4l2loopback/0.12.5/build/v4l2loopback.c:1993[v4l2_loopback_open]
[17047.368582] video10: VIDIOC_QUERYCAP: driver=v4l2 loopback, card=OBS Video Source, bus=platform:v4l2loopback-000, version=0x00050f07, capabilities=0x
85200002, device_caps=0x05200002
```

The second open has:
```
[17091.427287] /var/lib/dkms/v4l2loopback/0.12.5/build/v4l2loopback.c:1964[v4l2_loopback_open]
[17091.427292] v4l2-loopback[1992]: opened dev:00000000c093a43d with image:0000000000000000
[17091.427293] /var/lib/dkms/v4l2loopback/0.12.5/build/v4l2loopback.c:1993[v4l2_loopback_open]
[17091.427297] video10: VIDIOC_QUERYCAP: driver=v4l2 loopback, card=OBS Video Source, bus=platform:v4l2loopback-000, version=0x00050f07, capabilities=0x85200000, device_caps=0x05200000
[17091.427304] video10: VIDIOC_G_FMT: error -22: type=vid-out, width=4294966992, height=4294967295, pixelformat=.... little-endian (0x00000000), field=any, bytesperline=1401200640, sizeimage=4288352138, colorspace=-744513136, flags=0xfffffed0, ycbcr_enc=0, quantization=0, xfer_func=0
```

As indicated in the capabilities field returned from the VIDIOC_QUERYCAP call, 0x85200000 means no V4L2_CAP_VIDEO_CAPTURE(0x01) neither V4L2_CAP_VIDEO_OUTPUT(0x02) is set. And since it reports neither, kernel `check_fmt` in "drivers/media/v4l2-core/v4l2-ioctl.c" will evaluate variable is_vid to false, and therefore returns -EINVAL at the next `VIDIOC_G_FMT` call with type=`V4L2_BUF_TYPE_VIDEO_OUTPUT`.

This is a direct result from https://github.com/umlaeute/v4l2loopback/commit/9a77eb6 merged in https://github.com/umlaeute/v4l2loopback/pull/415.

Per inspection result from above comment, the behavior is actually a bug in the upstream v4l2loopback source, function v4l2_loopback_write:

  /* there's at least one writer, so don'stop announcing output capabilities */
  dev->ready_for_output = 0;

Which set `dev->ready_for_output` to zero without correcting its `dev->type`.

Continue discuss on upstream issue https://github.com/umlaeute/v4l2loopback/issues/404. Stay tuned.

The issue persists into 22.04

For Ubuntu users, I've made a PPA containing builds of `v4l2loopback` for 20.04 (Focal), 21.10 (Impish), 22.04 (Jammy), 22.10 (Kinetic) that includes backports of the following patches:

 🩹 https://github.com/umlaeute/v4l2loopback/pull/477
 🩹 https://github.com/umlaeute/v4l2loopback/pull/485

The patch above fixes the ability to start/stop the Virtual Camera in OBS many times. It "works for me"™

The PPA is here:

 📦 https://launchpad.net/~flexiondotorg/+archive/ubuntu/v4l2loopback

Feedback welcome. If this fix works widely, I'll work towards landing in the official packages for Ubuntu.

Martin,

thanks a lot for providing that. I would like to confirm that it works now, but I was on the downgraded original version 0.12.5-1 before and replacing it with your package leaves me with

$ find /lib/modules/ | grep v4l2loopback.ko
/lib/modules/5.15.0-40-generic/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-41-generic/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-40-lowlatency/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-41-lowlatency/kernel/v4l2loopback/v4l2loopback.ko
/lib/modules/5.15.0-41-lowlatency/updates/dkms/v4l2loopback.ko

How can I be sure that the new module is actually loaded (given that I am currently running 5.15.0-41-lowlatency)? You can also see that the new module was built only for the kernel running at install time.

Martin's PPA version works for me on Jammy. See if your srcversion matches mine maybe?

redkrieg@cortex-1:~$ modinfo v4l2loopback
filename: /lib/modules/5.17.15-76051715-generic/extra/v4l2loopback.ko
license: GPL
author: Vasily Levin, IOhannes m zmoelnig <email address hidden>,Stefan Diewald,Anton Novikovet al.
description: V4L2 loopback video device
srcversion: B6031838219D91DE046F379
depends: videodev
retpoline: Y
name: v4l2loopback
vermagic: 5.17.15-76051715-generic SMP preempt mod_unload modversions
parm: debug:debugging level (higher values == more verbose) (int)
parm: max_buffers:how many buffers should be allocated (int)
parm: max_openers:how many users can open loopback device (int)
parm: devices:how many devices should be created (int)
parm: video_nr:video device numbers (-1=auto, 0=/dev/video0, etc.) (array of int)
parm: card_label:card labels for every device (array of charp)
parm: exclusive_caps:whether to announce OUTPUT/CAPTURE capabilities exclusively or not (array of bool)
parm: max_width:maximum frame width (int)
parm: max_height:maximum frame height (int)
redkrieg@cortex-1:~$ ls -hal /lib/modules/5.17.15-76051715-generic/extra/v4l2loopback.ko
-rw-r--r-- 1 root root 91K Jul 15 10:09 /lib/modules/5.17.15-76051715-generic/extra/v4l2loopback.ko

I confirm that the issue is in Kubuntu 22.04 LTS as well.