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authorJie Yang <yang.jie@intel.com>2015-04-27 21:21:02 +0800
committerTakashi Iwai <tiwai@suse.de>2015-04-27 21:39:03 +0200
commit12e180a27f3e066a4ed4a446d428fd117f168beb (patch)
tree59905577f9a0e584b5b0be95d4b471692d888ab5
parent807845e3641d3dc4c41ec84c5e576250c7218abf (diff)
ALSA: Docs: Add documentation for Jack kcontrols
Add documentation describing Jack kcontrols and how to use them with HD-Audio and ASoC. Signed-off-by: Jie Yang <yang.jie@intel.com> Signed-off-by: Takashi Iwai <tiwai@suse.de>
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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.