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Audio Drivers Architecture

In Fuchsia there are many ways drivers can be architected as defined by the number of drivers used, how they communicate and their responsibilities. Audio drivers responsibilities are determined by the interface(s) exposed to driver clients, clients could be other drivers or applications users of the drivers facilities.

Definitions

Term Definition
Codec A real or virtual device that encodes/decodes a signal from digital/analog to/from analog/digital including all combinations, e.g. digital to digital. Example codecs include DAC-Amplifiers combos and ADC converters.
Controller or engine The HW part of a system that manages the audio signals, for example an SOC's audio subsystem.
DAI Digital Audio Interface. Interface between audio HW, for instance a TDM or PDM link between controllers and codecs.

Audio interfaces

The main API in use by applications is the Audio Streaming Interface. This API allows applications to capture or render audio. Examples of audio applications using audio drivers via the streaming interface include audio_core and audio-driver-ctl. The former is the core of the audio system (providing software mixing, routing, etc.) and the latter is a utility used for testing and bringup of new platforms.

A driver providing the streaming interface abstracts the HW functionality described in the API, but it does not need to be the driver actually configuring all the HW. A common split in audio HW is to have an audio engine that configures a DAI communicating with an audio HW codec. In this split we can have one driver for the audio engine and one for the codec. The Codec Interface allows having a driver for the codec implementation and another driver configuring the audio engine HW including the DAI and driving the codec(s) configuration. In this configuration the codec(s) are secondary to the controller. For instance the mt8167-tdm-output is a driver for the MediaTek MT8167S audio subsystem (audio engine) also providing the streaming interface for applications and communicating with any codec driver, for example a tas58xx exposing the codec interface as shown below:

                           +-----------------+
                           |   audio_core    |
                           +-----------------+
                                    |
                           Streaming Interface
                                    |
                           +-----------------+
                           |mt8167-tdm-output|
                           +-----------------+
                                    |
                             Codec Interface
                                    |
                           +-----------------+
                           |     tas58xx     |
                           +-----------------+

Another way to architect drivers with the engine/codec split is to have a codec providing the streaming audio interface, and the audio engine providing a DAI interface. For example a aml-g12-tdm-dai driver for the AMLogic g12 audio subsystem (audio engine) providing the DAI interface for other drivers or applications to use, and a codec can drive the engine and provide the streaming interface to applications like audio-driver-ctl.

                           +-----------------+
                           |audio-driver-ctl |
                           +-----------------+
                                    |
                           Streaming Interface
                                    |
                           +-----------------+
                           |  codec-driver   |
                           +-----------------+
                                    |
                              DAI Interface
                                    |
                           +-----------------+
                           | aml-g12-tdm-dai |
                           +-----------------+

We can also have a non-driver component use either DAI or codec interface to access an audio engine or audio HW codec directly. In the figure below we have Bluetooth Audio providing the streaming interface and making use of the DAI interface to configure the AMLogic g12 audio subsystem:

         +-----------------+    Streaming    +-----------------+
         |    BT Stack     +-----------------+   audio_core    |
         +-----------------+    Interface    +-----------------+
                  |
             DAI Interface
                  |
         +-----------------+
         | aml-g12-tdm-dai |
         +-----------------+

It is also possible to have both the DAI and codec interfaces in use, for example tied with another driver providing the streaming interface. An example usage of this architecture for a system with two different codecs physically connected to the same DAI stream.

                           +-----------------+
                           |   audio_core    |
                           +-----------------+
                                    |
                           Streaming Interface
                                    |
                           +-----------------+
                +----------+   glue driver   +----------+
                |          +-----------------+          |
                |                   |                   |
          DAI Interface      Codec Interface     Codec Interface
                |                   |                   |
       +-----------------+ +-----------------+ +-----------------+
       | aml-g12-tdm-dai | |    tas-5720     | |    tas-5720     |
       +-----------------+ +-----------------+ +-----------------+

Finally, it is also possible to just have one driver configuring all HW and exposing the streaming interface. This should be used when there is no logical DAI or codec HW separation:

                           +-----------------+
                           |   audio_core    |
                           +-----------------+
                                    |
                           Streaming Interface
                                    |
                           +-----------------+
                           |USB audio driver |
                           +-----------------+