ASOC的出现是为了让Codec独立于CPU，减少和CPU之间的耦合，这样同一个Codec驱动无需修改就可以适用任何一款平台。还是以下图做参考例子:

在Machine中已经知道，snd_soc_dai_link结构就指明了该Machine所使用的Platform和Codec。在Codec这边通过codec_dai和Platform侧的cpu_dai相互通信，既然相互通信，就需要遵守一定的规则，其中codec_dai和cpu_dai统一抽象为struct snd_soc_dai结构，而将dai的相关操作使用snd_soc_dai_driver抽象。同时也需要对所有的codec设备进行抽象封装，linux使用snd_soc_codec进行所有codec设备的抽象，而将codec的驱动抽象为snd_soc_codec_driver结构。

所有简单来说，Codec侧有四个重要的数据结构:

struct snd_soc_dai，struct snd_soc_dai_driver，struct snd_soc_codec，struct snd_soc_codec_driver。

如何找到codec的代码呢？  答案是通过machine中的snd_soc_dai_link结构:

static struct snd_soc_dai_link s3c24xx_uda134x_dai_link = {.name = "UDA134X",.stream_name = "UDA134X",.codec_name = "uda134x-codec",.codec_dai_name = "uda134x-hifi",.cpu_dai_name = "s3c24xx-iis",.ops = &s3c24xx_uda134x_ops,.platform_name  = "s3c24xx-iis",
};

在内核中搜索codec_name="uda134x-codec"，会在kernel/sound/soc/codec/uda134x.c中发现。

static struct platform_driver uda134x_codec_driver = {.driver = {.name = "uda134x-codec",.owner = THIS_MODULE,},.probe = uda134x_codec_probe,.remove = uda134x_codec_remove,
};

之间查看此platform_driver的probe函数

static int uda134x_codec_probe(struct platform_device *pdev)
{return snd_soc_register_codec(&pdev->dev,&soc_codec_dev_uda134x, &uda134x_dai, 1);
}

此出通过snd_soc_register_codec函数注册了uda134x的codec，同时传入了snd_soc_codec_driver和snd_soc_dai_driver结构。

static struct snd_soc_codec_driver soc_codec_dev_uda134x = {.probe =        uda134x_soc_probe,.remove =       uda134x_soc_remove,.suspend =      uda134x_soc_suspend,.resume =       uda134x_soc_resume,.reg_cache_size = sizeof(uda134x_reg),.reg_word_size = sizeof(u8),.reg_cache_default = uda134x_reg,.reg_cache_step = 1,.read = uda134x_read_reg_cache,.write = uda134x_write,.set_bias_level = uda134x_set_bias_level,.dapm_widgets = uda134x_dapm_widgets,.num_dapm_widgets = ARRAY_SIZE(uda134x_dapm_widgets),.dapm_routes = uda134x_dapm_routes,.num_dapm_routes = ARRAY_SIZE(uda134x_dapm_routes),
};

snd_soc_dai_driver结构:

static const struct snd_soc_dai_ops uda134x_dai_ops = {.startup    = uda134x_startup,.shutdown    = uda134x_shutdown,.hw_params  = uda134x_hw_params,.digital_mute  = uda134x_mute,.set_sysclk = uda134x_set_dai_sysclk,.set_fmt  = uda134x_set_dai_fmt,
};static struct snd_soc_dai_driver uda134x_dai = {.name = "uda134x-hifi",/* playback capabilities */.playback = {.stream_name = "Playback",.channels_min = 1,.channels_max = 2,.rates = UDA134X_RATES,.formats = UDA134X_FORMATS,},/* capture capabilities */.capture = {.stream_name = "Capture",.channels_min = 1,.channels_max = 2,.rates = UDA134X_RATES,.formats = UDA134X_FORMATS,},/* pcm operations */.ops = &uda134x_dai_ops,
};

继续进入snd_soc_register_codec函数分析。

1.  分配一个snd_soc_codec结构，设置component参数。

codec = kzalloc(sizeof(struct snd_soc_codec), GFP_KERNEL);
if (codec == NULL)return -ENOMEM;codec->component.dapm_ptr = &codec->dapm;
codec->component.codec = codec;

2.  调用snd_soc_component_initialize函数进行component部件的初始化，会根据snd_soc_codec_driver中的struct snd_soc_component_driver结构设置snd_soc_codec中的component组件，详细代码不在贴出。

ret = snd_soc_component_initialize(&codec->component,&codec_drv->component_driver, dev);

3.  根据snd_soc_codec_driver的参数，进行一系列的初始化，

if (codec_drv->controls) {codec->component.controls = codec_drv->controls;codec->component.num_controls = codec_drv->num_controls;
}
if (codec_drv->dapm_widgets) {codec->component.dapm_widgets = codec_drv->dapm_widgets;codec->component.num_dapm_widgets = codec_drv->num_dapm_widgets;
}
if (codec_drv->dapm_routes) {codec->component.dapm_routes = codec_drv->dapm_routes;codec->component.num_dapm_routes = codec_drv->num_dapm_routes;
}if (codec_drv->probe)codec->component.probe = snd_soc_codec_drv_probe;
if (codec_drv->remove)codec->component.remove = snd_soc_codec_drv_remove;
if (codec_drv->write)codec->component.write = snd_soc_codec_drv_write;
if (codec_drv->read)codec->component.read = snd_soc_codec_drv_read;
codec->component.ignore_pmdown_time = codec_drv->ignore_pmdown_time;
codec->dapm.idle_bias_off = codec_drv->idle_bias_off;
codec->dapm.suspend_bias_off = codec_drv->suspend_bias_off;
if (codec_drv->seq_notifier)codec->dapm.seq_notifier = codec_drv->seq_notifier;
if (codec_drv->set_bias_level)codec->dapm.set_bias_level = snd_soc_codec_set_bias_level;
codec->dev = dev;
codec->driver = codec_drv;
codec->component.val_bytes = codec_drv->reg_word_size;
mutex_init(&codec->mutex);

4.  根据snd_soc_dai_driver中的参数设置Format

    for (i = 0; i < num_dai; i++) {fixup_codec_formats(&dai_drv[i].playback);fixup_codec_formats(&dai_drv[i].capture);}

5.  调用snd_soc_register_dais接口注册dai，传入参数有dai的驱动，以及dai的参数，因为一个codec不止一个dai接口。

ret = snd_soc_register_dais(&codec->component, dai_drv, num_dai, false);
if (ret < 0) {dev_err(dev, "ASoC: Failed to regster DAIs: %d\n", ret);goto err_cleanup;
}

根据dai的数目，分配snd_soc_dai结构，根据dai的数目设置dai的名字，这是dai的传入参数驱动，然后将此dai加入到component的dai_list中。

for (i = 0; i < count; i++) {dai = kzalloc(sizeof(struct snd_soc_dai), GFP_KERNEL);if (dai == NULL) {ret = -ENOMEM;goto err;}/** Back in the old days when we still had component-less DAIs,* instead of having a static name, component-less DAIs would* inherit the name of the parent device so it is possible to* register multiple instances of the DAI. We still need to keep* the same naming style even though those DAIs are not* component-less anymore.*/if (count == 1 && legacy_dai_naming) {dai->name = fmt_single_name(dev, &dai->id);} else {dai->name = fmt_multiple_name(dev, &dai_drv[i]);if (dai_drv[i].id)dai->id = dai_drv[i].id;elsedai->id = i;}if (dai->name == NULL) {kfree(dai);ret = -ENOMEM;goto err;}dai->component = component;dai->dev = dev;dai->driver = &dai_drv[i];if (!dai->driver->ops)dai->driver->ops = &null_dai_ops;list_add(&dai->list, &component->dai_list);dev_dbg(dev, "ASoC: Registered DAI '%s'\n", dai->name);
}

6.   根据component的dai_list，对所有的dai设置其所属的codec。

list_for_each_entry(dai, &codec->component.dai_list, list)dai->codec = codec;

7.   将所有的组间加入到component_list中，然后将注册的codec存入codec_list中。

mutex_lock(&client_mutex);
snd_soc_component_add_unlocked(&codec->component);
list_add(&codec->list, &codec_list);
mutex_unlock(&client_mutex);

至此，codec的注册就分析完毕。

总结：  通过调用snd_soc_register_codec函数之后，分配的codec最终放入了codec_list链表中，codec下的component组件全部放入component_list链表中，codec下的dai全部存放入code->component.dai_list中。 可以在上节的Machine中看到，machine匹配codec_dai和cpu_dai也是从code->component.dai_list中获取的。

关于codec侧驱动总结:

1.   分配名字为"codec_name"的平台驱动，注册。
2.   定义struct snd_soc_codec_driver结构，设置，初始化。

3.   定义struct snd_soc_dai_driver结构，设置，初始化。

4.   调用snd_soc_register_codec函数注册codec。