函数snd_soc_register_dais()分析

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本文详细介绍了snd_soc_register_dais()函数的工作原理及流程，该函数用于向ASoC core注册数字音频接口（DAI）。文章深入解析了DAI的注册过程，包括内存分配、名称创建、设备初始化等步骤。

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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;
		else
			dai->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;
};