QoS and dropped frames from sources

[Josh Doe]

I've got a bunch of source plugins for hardware that can report when
frames are dropped, usually because they're not pulled off the
hardware memory fast enough. I want to report this to the application.

QoS messages seem appropriate, but I notice that QoS is not included
in GstBaseSrc, rather only in GstBaseSink and GstBaseTransform. I can
only find one source that sends QoS messages, v4l2src.

Looking at the docs for QoS, it actually seems QoS is more meant to be
used by a source to possibly throttle throughput, not to notify the
application of dropped frames.

Any thoughts on this? Seems like if I use QoS messages to report
dropped frames from a source it might be a slight misuse of QoS. Of
course I can easily use a custom message, but wanted to check here
first.

Thanks,
-Josh