The ALSA (Advanced Linux Sound Architecture) audio subsystem is an audio driver architecture for the Linux operating system, which provides support for a variety of audio hardware and audio applications. It is a part of the Linux kernel. It not only provides a set of audio drivers, but also provides a user space library, making it easier for developers to use audio functions in applications.
The driver framework of the ALSA audio subsystem includes the following components:
音频设备驱动程序：这些驱动程序实现了特定的音频设备的接口，包括声卡、MIDI设备、USB音频设备等。每个驱动程序提供了一组ALSA核心的回调函数，用于实现数据的输入和输出。
ALSA核心：ALSA核心是音频子系统的核心部分，它实现了ALSA的API，包括音频设备的打开和关闭、读写、参数设置等等。它还提供了一些底层的设备管理功能，如设备文件的创建、访问权限的管理等。
应用程序接口：ALSA提供了一些API，可以使应用程序更方便地使用ALSA音频子系统。这些API包括访问音频设备、获取音频设备参数、读写音频数据、以及音频数据格式转换等。
控制接口：ALSA提供了一组控制接口，用于控制音量、混音等音频控制功能。
中间件：ALSA还提供了一些中间件，用于处理音频数据的流动和转换，以及音频效果的处理。
To sum up, the driver framework of the ALSA audio subsystem is very complete. It provides a complete set of audio drivers, APIs, control interfaces and middleware, which is convenient for developers to develop and debug audio applications.
example device tree
sound {
compatible = "simple-audio-card";
simple-audio-card,format = "i2s";
simple-audio-card,name = "My ALSA Device";
simple-audio-card,bitclock-master = <&dai>;
simple-audio-card,frame-master = <&dai>;
simple-audio-card,widgets = "Microphone", "Speaker";
simple-audio-card,rates = <48000>;
simple-audio-card,dai-link@0 {
format = "i2s";
bitclock-master = <&dai>;
frame-master = <&dai>;
widgets = "Microphone", "Speaker";
dai-format = <I2S>;
cpu {
sound-dai = <&cpu_dai>;
};
codec {
sound-dai = <&codec_dai>;
};
};
};
Among them, compatible = "simple-audio-card"; indicates that this is a simple audio device node. simple-audio-card,format = "i2s"; means use I2S format for audio transmission. simple-audio-card,name = “My ALSA Device”; is the name of the device. simple-audio-card, bitclock-master and simple-audio-card, frame-master are clock settings. simple-audio-card,widgets represent input and output components on the device. simple-audio-card,rates indicates the sampling rate supported by the device.
simple-audio-card,dai-link@0 is a DAI (Digital Audio Interface) link node. Among them, format = "i2s"; indicates that the node also uses I2S format for audio transmission. dai-format = ; is to specify the format of the digital audio interface. cpu and codec are the DAI interface between CPU and codec. sound-dai = <&cpu_dai>; and sound-dai = <&codec_dai>; are DAI nodes specifying CPU and codec.
**
Driver example
**
#include <linux/module.h>
#include <sound/soc.h>
#include <sound/pcm_params.h>
static const struct snd_soc_dapm_widget simple_card_widgets[] = {
SND_SOC_DAPM_MIC("Microphone", NULL),
SND_SOC_DAPM_SPK("Speaker", NULL),
};
static const struct snd_soc_dapm_route simple_card_routes[] = {
{
"Microphone", NULL, "dai-link-0" },
{
"dai-link-0", NULL, "Speaker" },
};
static struct snd_soc_dai_link simple_card_dai[] = {
{
.name = "Simple-Audio-Card",
.stream_name = "Simple Audio",
.cpu_dai_name = "cpu-dai",
.codec_dai_name = "codec-dai",
.platform_name = "simple-audio-card",
.codecs = SND_SOC_DAILINK_REG(SND_SOC_NOPM, 0),
.ops = &simple_card_ops,
.dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF | SND_SOC_DAIFMT_CBS_CFS,
.init = simple_card_init,
},
};
static struct snd_soc_card simple_card = {
.name = "My ALSA Device",
.dai_link = simple_card_dai,
.num_links = ARRAY_SIZE(simple_card_dai),
.dapm_widgets = simple_card_widgets,
.num_dapm_widgets = ARRAY_SIZE(simple_card_widgets),
.dapm_routes = simple_card_routes,
.num_dapm_routes = ARRAY_SIZE(simple_card_routes),
};
static int simple_card_probe(struct platform_device *pdev)
{
int ret;
struct snd_soc_card *card = &simple_card;
struct device_node *np = pdev->dev.of_node;
if (!np) {
dev_err(&pdev->dev, "device tree node not found\n");
return -EINVAL;
}
ret = snd_soc_of_parse_card_name(card, "model");
if (ret < 0) {
dev_err(&pdev->dev, "Failed to get card name: %d\n", ret);
return ret;
}
ret = snd_soc_of_parse_audio_routing(card, "simple-audio-card,dai-link-0");
if (ret < 0) {
dev_err(&pdev->dev, "Failed to parse audio routing: %d\n", ret);
return ret;
}
ret = snd_soc_register_card(card);
if (ret < 0) {
dev_err(&pdev->dev, "snd_soc_register_card() failed: %d\n", ret);
return ret;
}
platform_set_drvdata(pdev, card);
return 0;
}
static int simple_card_remove(struct platform_device *pdev)
{
struct snd_soc_card *card = platform_get_drvdata(pdev);
snd_soc_unregister_card(card);
return 0;
}
static const struct of_device_id simple_card_of_match[] = {
{
.compatible = "simple-audio-card", },
{
},
};
MODULE_DEVICE_TABLE(of, simple_card_of_match);
static struct platform_driver simple_card_driver = {
.driver = {
.name = "simple-audio-card",
.of_match_table = simple_card_of_match,
},
.probe = simple_card_probe,
.remove = simple_card_remove,
};
module_platform_driver(simple_audio_driver);
MODULE_DESCRIPTION("Simple ALSA SoC Audio driver");
MODULE_AUTHOR("Your Name");
MODULE_LICENSE("GPL");