Snapshot failed to work with v4l2-relayd/v4l2loopback devices

[hugh chao]

Currently mipi camera failed to work on snapshot app on Ubuntu 24.04,
hope this issue can be fixed.

[Launchpad Janitor (janitor)]

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

[Rex Tsai (chihchun)]

Need more debug information.

GNOME Snapshot use pipewiredeviceprovider when it's not inside a jail. The problem can be rephrased as "why pw-dump doesn't give the v4l2loopback created video device. It seems /usr/bin/snapshot.snapshot itself already has some problem to support the general video4linux devices.

[Shih-Yuan Lee (fourdollars)]

After some rough investigations, there are three different issues.

The first issue: the v4l2 video device is not listed from `pw-dump`, but I can make it listed after executing `systemctl --user restart pipewire`. So it could be related to some initialization problem within the pipewire infrastructure.

The second issue is a stream error that makes it unable to play camera stream. https://paste.ubuntu.com/p/MYPXTs3K39/

The third issue is only observed by my own USB camera device so far that even there is no problem to initialize the device on snapshot, but it can not fetch the camera stream to show or to record. I am not sure if we will encounter the third issue after we solved the previous two issues. https://paste.ubuntu.com/p/dcW4f6ykzs/

FYI, https://gitlab.gnome.org/GNOME/snapshot/-/blob/main/TROUBLESHOOTING.md

[Sebastien Bacher (seb128)]

The first issue seems to be similar to bug #2061687
which is reported upstream as https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/3539

The stream error message seems similar to https://gitlab.gnome.org/GNOME/snapshot/-/issues/180 . Could you perhaps report it against gstreamer upstream on https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues ?

Could you also report the issue about your own device on https://gitlab.gnome.org/GNOME/snapshot/-/issues ?

[Shih-Yuan Lee (fourdollars)]

Another webcam device in an old iMac works fine. https://paste.ubuntu.com/p/m3Mm476SVf/

[Shih-Yuan Lee (fourdollars)]

I reported https://gitlab.gnome.org/GNOME/snapshot/-/issues/223 for my own Logitech QuickCam Notebook Pro.

[Shih-Yuan Lee (fourdollars)]

The error is from /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstpipewire.so belonged to pipewire. https://paste.ubuntu.com/p/jNRQ8zbkHq/
Should we probably report the second issue to https://gitlab.freedesktop.org/pipewire/pipewire/-/issues instead?

[Shih-Yuan Lee (fourdollars)]

I can also duplicate the second issue by the Virtual Camera created by v4l2-relayd. https://paste.ubuntu.com/p/xvyNRxSxf5/

So the problem is basically between v4l2-relayd/v4l2loopback and pipewire.

[Sebastien Bacher (seb128)]

Sending to https://gitlab.freedesktop.org/pipewire/pipewire/-/issues probably makes sense yes. Assuming that #1 is identical to the other report mentioned and #3 device specific I'm going to retitle the bug to be about #2 if that's ok with you?

[Sebastien Bacher (seb128)]

is that the same as https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/707 ?

[Shih-Yuan Lee (fourdollars)]

The symptom looks like https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/707.
I agree that we should focus on the major #2 issue.

[You-Sheng Yang (vicamo)]

https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/707 I don't think that's possible, or say it's a different use scenario.

The pipewire loopback is possible, because the virtual sinks/sources it created has to be accessed through libpipewire. The v4l2loopback kernel module is meant to provide kernel devices, and it must be kernel devices to be compatible with kernel V4L2 ioctl interfaces. You can certainly create loopback pipewire devices, but that doesn't mean the problems of obs/zoom/Google Meets/... will be resolved automatically until they adopt pipewire. That's a different thing.

[You-Sheng Yang (vicamo)]

Snapshots reqbuf with V4L2_MEMORY_USERPTR and v4l2loopback supports only V4L2_MEMORY_MMAP. Still investigating...