src/kernel/linux/v4.19/Documentation/sound/soc/jack.rst

===================
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.