# Linux Audio - ASoC

[TOC]

## Abstractions and Hierarchies

See [ALSA SoC Layer Overview](https://www.kernel.org/doc/html/latest/sound/soc/overview.html) and [ALSA SoC Layer](https://www.kernel.org/doc/html/latest/sound/soc/index.html) in kernel documentation.

### The "component" drivers

The ALSA SoC framework breaks down what used to be in a single ALSA driver into smaller *component* drivers, each of them are a `struct snd_soc_component_driver`.



In ALSA, components are divided into two categories: *codec* and *CPU DAI*s. The former ones represent a standalone DSP external to the SoC; the later ones are abstraction for the intrinsic functionalities of the SoC itself.

See [ALSA overview](https://wiki.st.com/stm32mpu/wiki/ALSA_overview) for further information.

### DAIs

An *DAI* of a component describes an interface with which a component can connect to other components in ALSA. They are They are provided by instances of `snd_soc_dai_driver`. Every component is associate with a sets of DAI. Components can be connected to each other by connecting the DAIs. The DAIs for a component are provided alone with the component driver when it is registered. For example, in the [`devm_snd_soc_register_component()`](https://elixir.bootlin.com/linux/latest/source/sound/soc/soc-devres.c#L56)

int devm_snd_soc_register_component(struct device *dev,
			 const struct snd_soc_component_driver *cmpnt_drv,
			 struct snd_soc_dai_driver *dai_drv, int num_dai)

The parameters with which the data stream should use when passing through the DAI is decrbed in the [`snd_soc_pcm_stream`](https://elixir.bootlin.com/linux/latest/source/include/sound/soc.h#L620), in particular the `capture` and `playback` member.

### Codec Drivers

### Machine Drivers

A *machine* driver glues the components together by connecting. With each combination of components there in theory should be a different machine driver for that combination, making it a board-specific driver.

## Special Files

For some devices using the ASoC framework (for example, the sound open firmware), the `struct snd_card` is wrapped in the [`struct snd_soc_card`](https://elixir.bootlin.com/linux/latest/source/include/sound/soc.h#L927), and the names are initialized in the [`snd_soc_bind_card()`](https://elixir.bootlin.com/linux/latest/source/sound/soc/soc-core.c#L2149) of the `sound/soc/soc-core.c`:

...
	snd_soc_set_dmi_name(card, NULL);

	soc_setup_card_name(card, card->snd_card->shortname,
			    card->name, NULL);
	soc_setup_card_name(card, card->snd_card->longname,
			    card->long_name, card->name);
	soc_setup_card_name(card, card->snd_card->driver,
			    card->driver_name, card->name);
...

`devm_snd_soc_register_card()` then `snd_soc_register_card()` then `snd_soc_bind_card()`

## Userspace Tools

## References

### [ASoC: Supporting Audio on an Embedded Board - Alexandre Belloni, Bootlin](https://youtu.be/572T8RDHK_A)

{%youtube 572T8RDHK_A%}

### [Generic ALSA SoC Sound Card History (Simple/Graph) - Kuninori Morimoto, Renesas Electronics](https://youtu.be/usRwGpJJkpk)

{%youtube usRwGpJJkpk %}

### [Insight of an Audio Driver Based on ALSA - Chandrasekar Ramakrishnan, Samsung](https://youtu.be/6R8Wjytv-eQ)

Note that the slides are available [here](https://static.sched.com/hosted_files/ossna2022/78/OSS_ELC_2022_INSIGHT_OF_AN_AUDIO_DRIVER_BASED_ON_ALSA.pdf).

{%youtube 6R8Wjytv-eQ %}