ALSA: Add documentation about HD-audio DP MST

To support DP MST audio, HD Audio hdmi codec driver introduces virtual pin

and dynamic pcm assignment.

Virtual pin is an extension of per_pin. The most difference of DP MST

from legacy is that DP MST introduces device entry. Each pin can contain

several device entries. Each device entry behaves as a pin.

As each pin may contain several device entries and each codec may contain

several pins, if we use one pcm per per_pin, there will be many PCMs.

The new solution is to create a few PCMs and to dynamically bind pcm to

per_pin. Driver uses spec->dyn_pcm_assign flag to indicate whether to use

the new solution.

PCM

===

To be added

Jack

====

Presume:

 - MST must be dyn_pcm_assign, and it is acomp (for Intel scenario);

 - NON-MST may or may not be dyn_pcm_assign, it can be acomp or !acomp;

So there are the following scenarios:

 a. MST (&& dyn_pcm_assign && acomp)

 b. NON-MST && dyn_pcm_assign && acomp

 c. NON-MST && !dyn_pcm_assign && !acomp

Below discussion will ignore MST and NON-MST difference as it doesn't

impact on jack handling too much.

Driver uses struct hdmi_pcm pcm[] array in hdmi_spec and snd_jack is

a member of hdmi_pcm. Each pin has one struct hdmi_pcm * pcm pointer.

For !dyn_pcm_assign, per_pin->pcm will assigned to spec->pcm[n] statically.

For dyn_pcm_assign, per_pin->pcm will assigned to spec->pcm[n]

when monitor is hotplugged.

Build Jack

----------

- dyn_pcm_assign

Will not use hda_jack but use snd_jack in spec->pcm_rec[pcm_idx].jack directly.

- !dyn_pcm_assign

Use hda_jack and assign spec->pcm_rec[pcm_idx].jack = jack->jack statically.

Unsolicited Event Enabling

--------------------------

Enable unsolicited event if !acomp.

Monitor Hotplug Event Handling

------------------------------

- acomp

pin_eld_notify() -> check_presence_and_report() -> hdmi_present_sense() ->

sync_eld_via_acomp().

Use directly snd_jack_report() on spec->pcm_rec[pcm_idx].jack for

both dyn_pcm_assign and !dyn_pcm_assign

- !acomp

Hdmi_unsol_event() -> hdmi_intrinsic_event() -> check_presence_and_report() ->

hdmi_present_sense() -> hdmi_prepsent_sense_via_verbs()

Use directly snd_jack_report() on spec->pcm_rec[pcm_idx].jack for dyn_pcm_assign.

Use hda_jack mechanism to handle jack events.

Others to be added later

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