函数snd_soc_register_dais()分析

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函数snd_soc_register_dais()主要作用是向ASoC core注册DAIs(Digital Audio Interface).

一个codec可能支持多种DAIs(Digital Audio Interface)，如AC97，I2S，PCM，最近的项目中，一般只使用I2S。但是，代码中是支持多个DAIs(Digital Audio Interface)，函数的第3个参数”count“就是DAIs(Digital Audio Interface)的数目。

下面一起来看看这个函数是如何运行的，

1. 上面我们提到，代码中是支持多个DAIs(Digital Audio Interface)，函数的第3个参数”count“就是DAIs(Digital Audio Interface)的数目。函数刚开始就有一个for循环来实现这个功能，

for (i = 0; i < count; i++) {

2. 下面是为DAIs(Digital Audio Interface)设备的结构体申请内核空间并初始化，

dai = kzalloc(sizeof(struct snd_soc_dai), GFP_KERNEL);if (dai == NULL) {ret = -ENOMEM;goto err;}

3. 下面是为每一个DAIs(Digital Audio Interface)创建一个名称

/* create DAI component name */dai->name = fmt_multiple_name(dev, &dai_drv[i]);if (dai->name == NULL) {kfree(dai);ret = -EINVAL;goto err;}

4. 下面是对DAIs（Digital Audio Interface）设备结构体的初始化，包括dev/driver/id

dai->dev = dev;dai->driver = &dai_drv[i];if (dai->driver->id)dai->id = dai->driver->id;elsedai->id = i;dai->dapm.dev = dev;if (!dai->driver->ops)dai->driver->ops = &null_dai_ops;

5. 下面首先从列表(list)获取codec设备，并与DAI(Digital Audio Interface)关联起来，之后把DAI(Digital Audio Interface)也存放到列表中。

list_for_each_entry(codec, &codec_list, list) {if (codec->dev == dev) {dev_dbg(dev, "ASoC: Mapped DAI %s to ""CODEC %s\n", dai->name, codec->name);dai->codec = codec;break;}}if (!dai->codec)dai->dapm.idle_bias_off = 1;list_add(&dai->list, &dai_list);

至此，DAI(Digital Audio Interface)与codec也已经关联起来。函数snd_soc_register_dais()到此结束。

下面是snd_soc_dai结构体定义，

/* * Digital Audio Interface runtime data. * * Holds runtime data for a DAI. */struct snd_soc_dai {const char *name;int id;struct device *dev;void *ac97_pdata;/* platform_data for the ac97 codec *//* driver ops */struct snd_soc_dai_driver *driver;/* DAI runtime info */unsigned int capture_active:1;/* stream is in use */unsigned int playback_active:1;/* stream is in use */unsigned int symmetric_rates:1;struct snd_pcm_runtime *runtime;unsigned int active;unsigned char probed:1;struct snd_soc_dapm_widget *playback_widget;struct snd_soc_dapm_widget *capture_widget;struct snd_soc_dapm_context dapm;/* DAI DMA data */void *playback_dma_data;void *capture_dma_data;/* Symmetry data - only valid if symmetry is being enforced */unsigned int rate;/* parent platform/codec */struct snd_soc_platform *platform;struct snd_soc_codec *codec;struct snd_soc_card *card;struct list_head list;struct list_head card_list;};

snd_soc_dai_driver结构体定义如下，

/* * Digital Audio Interface Driver. * * Describes the Digital Audio Interface in terms of its ALSA, DAI and AC97 * operations and capabilities. Codec and platform drivers will register this * structure for every DAI they have. * * This structure covers the clocking, formating and ALSA operations for each * interface. */struct snd_soc_dai_driver {/* DAI description */const char *name;unsigned int id;int ac97_control;unsigned int base;/* DAI driver callbacks */int (*probe)(struct snd_soc_dai *dai);int (*remove)(struct snd_soc_dai *dai);int (*suspend)(struct snd_soc_dai *dai);int (*resume)(struct snd_soc_dai *dai);/* compress dai */bool compress_dai;/* ops */const struct snd_soc_dai_ops *ops;/* DAI capabilities */struct snd_soc_pcm_stream capture;struct snd_soc_pcm_stream playback;unsigned int symmetric_rates:1;/* probe ordering - for components with runtime dependencies */int probe_order;int remove_order;};

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