72 lines
3.0 KiB
Plaintext
72 lines
3.0 KiB
Plaintext
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.
|