When a sink or source belongs to a card, I think it would be a good idea to make pa_card the owner of the sink or source. This would mean that pa_card would decide when to create and destroy sinks and sources (they would be created at profile activation and destroyed at profile deactivation). I think this would make a lot of sense conceptually, but I'm not whether the effects on the back end complexity would be positive or negative (I would expect a positive effect).

The unclear part is how to delegate the back end specific parts of the sink initialization (and deinitialization) to the back end, if pa_sink_new() is called from card.c. I don't think we currently have any similar object initialization scheme in PA.

This idea occurred to me while implementing device prototypes. It's annoying that card back ends need to explicitly link the device prototypes with the sink or source whenever the back end creates a sink/source. It's only one line per created sink/source, but still it feels like unnecessary boilerplate code. If pa_card controlled the sink and source creation, the common code in card.c could take care of hooking the sinks and sources to the corresponding device prototype.