Documentation/sound/soc/dpcm.rst

Source file repositories/reference/linux-study-clean/Documentation/sound/soc/dpcm.rst

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Linux kernel
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Documentation/sound/soc/dpcm.rst
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.rst
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Support Tooling And Documentation
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Documentation
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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===========
Dynamic PCM
===========

Description
===========

Dynamic PCM allows an ALSA PCM device to digitally route its PCM audio to
various digital endpoints during the PCM stream runtime. e.g. PCM0 can route
digital audio to I2S DAI0, I2S DAI1 or PDM DAI2. This is useful for on SoC DSP
drivers that expose several ALSA PCMs and can route to multiple DAIs.

The DPCM runtime routing is determined by the ALSA mixer settings in the same
way as the analog signal is routed in an ASoC codec driver. DPCM uses a DAPM
graph representing the DSP internal audio paths and uses the mixer settings to
determine the path used by each ALSA PCM.

DPCM re-uses all the existing component codec, platform and DAI drivers without
any modifications.


Phone Audio System with SoC based DSP
-------------------------------------

Consider the following phone audio subsystem. This will be used in this
document for all examples :-
::

  | Front End PCMs    |  SoC DSP  | Back End DAIs | Audio devices |
  
                      *************
  PCM0 <------------> *           * <----DAI0-----> Codec Headset
                      *           *
  PCM1 <------------> *           * <----DAI1-----> Codec Speakers
                      *   DSP     *
  PCM2 <------------> *           * <----DAI2-----> MODEM
                      *           *
  PCM3 <------------> *           * <----DAI3-----> BT
                      *           *
                      *           * <----DAI4-----> DMIC
                      *           *
                      *           * <----DAI5-----> FM
                      *************

This diagram shows a simple smart phone audio subsystem. It supports Bluetooth,
FM digital radio, Speakers, Headset Jack, digital microphones and cellular
modem. This sound card exposes 4 DSP front end (FE) ALSA PCM devices and
supports 6 back end (BE) DAIs. Each FE PCM can digitally route audio data to any
of the BE DAIs. The FE PCM devices can also route audio to more than 1 BE DAI.



Example - DPCM Switching playback from DAI0 to DAI1
---------------------------------------------------

Audio is being played to the Headset. After a while the user removes the headset
and audio continues playing on the speakers.

Playback on PCM0 to Headset would look like :-
::

                      *************
  PCM0 <============> *           * <====DAI0=====> Codec Headset
                      *           *
  PCM1 <------------> *           * <----DAI1-----> Codec Speakers
                      *   DSP     *
  PCM2 <------------> *           * <----DAI2-----> MODEM
                      *           *
  PCM3 <------------> *           * <----DAI3-----> BT
                      *           *

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