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