[Bug 779213] buffer: Add GstReferenceTimestampMeta

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=779213

--- Comment #10 from Nicolas Dufresne (stormer) <nicolas at ndufresne.ca> ---
Summary of the V4L2 time features:

struct v4l2_buffer:
  timestamp; used to produce timestamp
  timecode; I believe should be mappable to timecode meta
  flags; Specify what fields of v4l2_buffer are are valid and extra meaning

The timestamp flags are:

***V4L2_BUF_FLAG_TIMESTAMP_MASK;

Allow to extract the timestamp format (an enum inside the bitmask) Valid values
are one of:

  V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN;

Was set on legacy driver with no evident clock id correlation, in theory it
should be real-time or monotonic. (it's the value 0, so basically drivers that
haven been updated will have that by default).

  V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;

All new and maintained driver capture driver have this flag, since timestamp
are derived to the monotonic clock

  V4L2_BUF_FLAG_TIMESTAMP_COPY;

this is used by color converter, decoder, encoder etc. to indicate that the
timestamp has simply been copied from input to output (even if there is
re-ordering happening).

***V4L2_BUF_FLAG_TSTAMP_SRC_MASK

This is another enum inside the bitmask, which indicate the moment the
timestamp is taken. Values are:

  V4L2_BUF_FLAG_TSTAMP_SRC_EOF

The timestamp was taken at the End of Frame. This is the default, it is also
used when the timestamp has been software generated. This happens when the last
pixel has been received over some serial bus.

  V4L2_BUF_FLAG_TSTAMP_SRC_SOE

Timestamp taken at Start of Exposure. Only valid for capture device, will be
set on some UVC devices, no other driver uses it (other then vivid).


References:
https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/buffer.html#struct-v4l2-buffer
https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/buffer.html#buffer-flags

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