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-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().
-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.