[Bug 1921474] [NEW] Support client usage notification via V4l2 Event API

[Launchpad Bug Tracker]

You have been subscribed to a public bug by You-Sheng Yang (vicamo):

[SRU Justification]

[Impact]

On a MIPI camera through Intel IPU6 platform that its raw V4L2 loopback
interface is preserved for Intel Camera HAL libraries, a relay daemon +
v4l2loopback is used to allow the usage of legacy V4L2 based apps.

By design, the relayd will open v4l2sink to v4l2loopback OUTPUT deivce,
and it will only open libcamhal based GStreamer source element, and
therefore underlying camera hardware, until received new client
notifications via V4L2 Event API that is introduced in this SRU.

Besides, frame sizes/intervals enumeration is also fixed to meet better
compliance with user apps.

[Test Plan]

v4l2loopback doesn't support V4L2 Event API until recently, so requests
for the usage will always fail:

  struct v4l2_event_subscription sub;
  int fd;

  memset (&sub, 0, sizeof (sub));
  sub.type = ...;
  if (ioctl (fd, VIDIOC_SUBSCRIBE_EVENT, &sub) == 0) // fail

With this fix, it shall support Event API operations, e.g.
VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT, VIDIOC_DQEVENT.

[Where problems could occur]

While a custom type of V4L2 event is now registered for OEM projects,
software expected same ID (numerically equivalent to
V4L2_EVENT_PRIVATE_START) may get confused. While this falls in the
private usage section, it's not supposed to be used in general, and when
it does, it should be under some presumptions, e.g. selected hardware,
so it's unlikely to happen on OEM projects.

For generic Ubuntu, programs may begin to take advantage of this new
capability to update its UI, or to take other actions after receiving
desired events. There might be behavior changes, but should be under the
original design if was done carefully.

[Other Info]

For Focal backports, 0.12.5-1 is equivalent to 0.12.3-1ubuntu0.3 plus
micro version updates, so there should be little risk backport a new
release.

========= original bug report ==========

In addition to kernel/firmware proposed in bug 1921345, several user
space daemons/libraries, as well as fixes/features in v4l2loopback-dkms,
are also required to have seamless support for legacy/existing Linux
kernel V4L2 API based applications to adopt libcamera or Intel libcamhal
provided camera interfaces.

The idea is to add a v4l2 streaming relay daemon (currently developed in
[1] and packaged in [2]) that helps redirecting V4L2 buffer streams into
v4l2loopback output device, and then legacy apps open v4l2loopback
capture device in the ordinary way.

  hw -> libcamera/libcamhal -> v4l2-relayd -> v4l2loopback -> v4l2-based
apps

To achieve this, we'd like v4l2-relayd to only open GStreamer icamerasrc
pipeline (provided by [3],[4]) when there is actual usage request from
v4l2-based apps for privacy and power consumption's concerns. However,
current (0.12.5 and therefore Ubuntu/Debian 0.12.5-1) doesn't have any
available mechanism for this, therefore a V4L2 Event API based proposal
has been sent and accepted by upstream[5][6].

This is needed for Ubuntu OEM projects with MIPI cameras through Intel
IPU6 (Imaging Processing Unit version 6).

[1]: https://gitlab.com/vicamo/v4l2-relayd
[2]: https://code.launchpad.net/~oem-solutions-engineers/v4l2-relayd/+git/packaging
[3]: https://github.com/intel/ipu6-camera-hal
[4]: https://github.com/intel/icamerasrc
[5]: https://github.com/umlaeute/v4l2loopback/pull/345
[6]: https://github.com/umlaeute/v4l2loopback/pull/352

** Affects: hwe-next
     Importance: Undecided
         Status: New

** Affects: v4l2loopback (Ubuntu)
     Importance: High
     Assignee: You-Sheng Yang (vicamo)
         Status: In Progress

** Affects: v4l2loopback (Ubuntu Focal)
     Importance: High
     Assignee: You-Sheng Yang (vicamo)
         Status: In Progress

** Affects: v4l2loopback (Ubuntu Hirsute)
     Importance: High
     Assignee: You-Sheng Yang (vicamo)
         Status: In Progress


** Tags: fossa-mewtwo ihv-intel oem-priority originate-from-1917691 somerville
-- 
Support client usage notification via V4l2 Event API
https://bugs.launchpad.net/bugs/1921474
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