[Bug 753592] New: v4l2: introduce GstV4l2Meta metadata

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=753592

            Bug ID: 753592
           Summary: v4l2: introduce GstV4l2Meta metadata
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: Normal
         Component: gst-plugins-good
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: aurelien.zanelli at parrot.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Currently, we have no way to retrieve the original timestamp of the v4l2buffer
in downstream element due to the addition of the delay between the time at
which buffer has been timestamped and the processing by v4l2src element.

The original v4l2 timestamp could be useful for application or elements which
needs the exact acquisition timestamp to perform some processing which involve
external timestamped values.

For instance here is my current use case:
I have an element which takes raw frames, normally from a v4l2src, and perform
video stabilization using data from an IMU (inertial measurement unit). These
data are timestamped during acquisition, and are queried with timestamp. To
perform accurate video stabilization, I shall use the frame acquisition
timestamp.

So I propose to add V4L2 metadata to buffer pushed by v4l2src containing the
v4l2buffer timestamp. These metadata could be extended as well to contains
other v4l2 specific fields.
This changes is very specific to V4L2 and maybe overkill, I mean adding a meta
just for a timestamp (currently). It may also be very specific to my use case.
However, I didn't find an other way to retrieve this info, nor generic metadata
which can handle this.

Don't hesitate to comment

P.S.: Could be linked with https://bugzilla.gnome.org/show_bug.cgi?id=753562

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