[Bug 1899633] Re: Repeated kernel warnings/stacktraces with kernel 5.4 and newer

[Brian Murray]

Hello Stefan, or anyone else affected,

Accepted v4l2loopback into focal-proposed. The package will build now
and be available at
https://launchpad.net/ubuntu/+source/v4l2loopback/0.12.3-1ubuntu0.1 in a
few hours, and then in the -proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested, what testing has been
performed on the package and change the tag from verification-needed-
focal to verification-done-focal. If it does not fix the bug for you,
please add a comment stating that, and change the tag to verification-
failed-focal. In either case, without details of your testing we will
not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

** Changed in: v4l2loopback (Ubuntu Focal)
       Status: In Progress => Fix Committed

** Tags added: verification-needed verification-needed-focal

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https://bugs.launchpad.net/bugs/1899633

Title:
  Repeated kernel warnings/stacktraces with kernel 5.4 and newer

Status in v4l2loopback package in Ubuntu:
  Fix Released
Status in v4l2loopback source package in Focal:
  Fix Committed

Bug description:
  Release: 20.04/Focal Fossa

  [SRU Justification]

  Impact: Starting with kernel version 5.4 the v4l2-core started to add
  sanity checking on drivers when querying capabilities. First drivers
  are not supposed to modify an initially set device_caps attribute. And
  second, the returned set of capabilities must be a superset of those
  in device_caps. The v4l2loopback module fails on both checks which
  results in repeated warnings (with additional stack trace) starting
  with:

  - WARNING: CPU: 2 PID: 7234 at drivers/media/v4l2-core/v4l2-ioctl.c:1064 v4l_querycap+0x8f/0xa0 [videodev]
  - WARNING: CPU: 2 PID: 7234 at drivers/media/v4l2-core/v4l2-ioctl.c:1069 v4l_querycap+0x8f/0xa0 [videodev]

  Fix:
  The proposed fix adds compat code which stops modifying device_caps after initial setup for kernel versions 5.4 and later. Also for those versions the initial setup excludes V4L2_CAP_VIDEO_CAPTURE and V4L2_CAP_VIDEO_OUTPUT as those capabilities might be optionally set when calling v4l_querycap() to indicate readyness.

  Testcase:
  #1
     DKMS build the updated version (should be implicitly done on an
     upgrade of v4l2loopback)
     But one can try to rebuild explicitly and check if any warnings are 
     given. This would be extra useful if a pre 5.4 kernel could be used as 
     well (not officially part of focal, therefore optional)

  #2
     Inserting the DKMS module the following way should start showing the 
     above warnings:
       $ sudo modprobe v4l2loopback exclusive_caps=1
     Once the fixed version is used, there should no longer be any warnings.

  Regression Potential:
  The change affects the internal device_caps variable only. If that is unexpectedly used by anything external (the kernel code should be assumed to expect the modified form) the loopback device might become unusable for recording/playback. I have been using the modified driver for a while now as v4l2sink in obs-studio and saw now issues.

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