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- Why we need Jack kcontrols
- ==========================
- ALSA uses kcontrols to export audio controls(switch, volume, Mux, ...)
- to user space. This means userspace applications like pulseaudio can
- switch off headphones and switch on speakers when no headphones are
- pluged in.
- The old ALSA jack code only created input devices for each registered
- jack. These jack input devices are not readable by userspace devices
- that run as non root.
- The new jack code creates embedded jack kcontrols for each jack that
- can be read by any process.
- This can be combined with UCM to allow userspace to route audio more
- intelligently based on jack insertion or removal events.
- Jack Kcontrol Internals
- =======================
- Each jack will have a kcontrol list, so that we can create a kcontrol
- and attach it to the jack, at jack creation stage. We can also add a
- kcontrol to an existing jack, at anytime when required.
- Those kcontrols will be freed automatically when the Jack is freed.
- How to use jack kcontrols
- =========================
- In order to keep compatibility, snd_jack_new() has been modified by
- adding two params :-
- - @initial_kctl: if true, create a kcontrol and add it to the jack
- list.
- - @phantom_jack: Don't create a input device for phantom jacks.
- HDA jacks can set phantom_jack to true in order to create a phantom
- jack and set initial_kctl to true to create an initial kcontrol with
- the correct id.
- ASoC jacks should set initial_kctl as false. The pin name will be
- assigned as the jack kcontrol name.
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