ASoC: Add some documentation for the ASoC jack API

ASoC jack detection

===================

ALSA has a standard API for representing physical jacks to user space,

the kernel side of which can be seen in include/sound/jack.h.  ASoC

provides a version of this API adding two additional features:

 - It allows more than one jack detection method to work together on one

   user visible jack.  In embedded systems it is common for multiple

   to be present on a single jack but handled by separate bits of

   hardware.

 - Integration with DAPM, allowing DAPM endpoints to be updated

   automatically based on the detected jack status (eg, turning off the

   headphone outputs if no headphones are present).

This is done by splitting the jacks up into three things working

together: the jack itself represented by a struct snd_soc_jack, sets of

snd_soc_jack_pins representing DAPM endpoints to update and blocks of

code providing jack reporting mechanisms.

For example, a system may have a stereo headset jack with two reporting

mechanisms, one for the headphone and one for the microphone.  Some

systems won't be able to use their speaker output while a headphone is

connected and so will want to make sure to update both speaker and

headphone when the headphone jack status changes.

The jack - struct snd_soc_jack

==============================

This represents a physical jack on the system and is what is visible to

user space.  The jack itself is completely passive, it is set up by the

machine driver and updated by jack detection methods.

Jacks are created by the machine driver calling snd_soc_jack_new().

snd_soc_jack_pin

================

These represent a DAPM pin to update depending on some of the status

bits supported by the jack.  Each snd_soc_jack has zero or more of these

which are updated automatically.  They are created by the machine driver

and associated with the jack using snd_soc_jack_add_pins().  The status

of the endpoint may configured to be the opposite of the jack status if

required (eg, enabling a built in microphone if a microphone is not

connected via a jack).

Jack detection methods

======================

Actual jack detection is done by code which is able to monitor some

input to the system and update a jack by calling snd_soc_jack_report(),

specifying a subset of bits to update.  The jack detection code should

be set up by the machine driver, taking configuration for the jack to

update and the set of things to report when the jack is connected.

Often this is done based on the status of a GPIO - a handler for this is

provided by the snd_soc_jack_add_gpio() function.  Other methods are

also available, for example integrated into CODECs.  One example of

CODEC integrated jack detection can be see in the WM8350 driver.

Each jack may have multiple reporting mechanisms, though it will need at

least one to be useful.

Machine drivers

===============

These are all hooked together by the machine driver depending on the

system hardware.  The machine driver will set up the snd_soc_jack and

the list of pins to update then set up one or more jack detection

mechanisms to update that jack based on their current status.