概述
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。
Codec代码分析
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)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。
{
return snd_soc_register_codec(&pdev->dev,
&soc_codec_dev_uda134x, &uda134x_dai, 1);
}
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);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。
if (codec == NULL)
return -ENOMEM;
codec->component.dapm_ptr = &codec->dapm;
codec->component.codec = codec;
ret = snd_soc_component_initialize(&codec->component,3. 根据snd_soc_codec_driver的参数，进行一系列的初始化，
&codec_drv->component_driver, dev);
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);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。
if (ret < 0) {
dev_err(dev, "ASoC: Failed to regster DAIs: %d\n", ret);
goto err_cleanup;
}
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;
else
dai->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)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。
dai->codec = codec;
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。
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。