struct snd_soc_dai_ops接口介绍

CPU DAI和codec DAI 驱动用结构体 struct snd_soc_dai_driver描述，其字段ops的类型是 struct snd_soc_dai_ops，定义了DAI驱动的操作接口。
代码如下：

struct snd_soc_dai_ops {
	/*
	 * DAI clocking configuration, all optional.
	 * Called by soc_card drivers, normally in their hw_params.
	 */
	int (*set_sysclk)(struct snd_soc_dai *dai,
		int clk_id, unsigned int freq, int dir);
	int (*set_pll)(struct snd_soc_dai *dai, int pll_id, int source,
		unsigned int freq_in, unsigned int freq_out);
	int (*set_clkdiv)(struct snd_soc_dai *dai, int div_id, int div);
	int (*set_bclk_ratio)(struct snd_soc_dai *dai, unsigned int ratio);

	/*
	 * DAI format configuration
	 * Called by soc_card drivers, normally in their hw_params.
	 */
	int (*set_fmt)(struct snd_soc_dai *dai, unsigned int fmt);
	int (*xlate_tdm_slot_mask)(unsigned int slots,
		unsigned int *tx_mask, unsigned int *rx_mask);
	int (*set_tdm_slot)(struct snd_soc_dai *dai,
		unsigned int tx_mask, unsigned int rx_mask,
		int slots, int slot_width);
	int (*set_channel_map)(struct snd_soc_dai *dai,
		unsigned int tx_num, unsigned int *tx_slot,
		unsigned int rx_num, unsigned int *rx_slot);
	int (*set_tristate)(struct snd_soc_dai *dai, int tristate);

	/*
	 * DAI digital mute - optional.
	 * Called by soc-core to minimise any pops.
	 */
	int (*digital_mute)(struct snd_soc_dai *dai, int mute);
	int (*mute_stream)(struct snd_soc_dai *dai, int mute, int stream);

	/*
	 * ALSA PCM audio operations - all optional.
	 * Called by soc-core during audio PCM operations.
	 */
	int (*startup)(struct snd_pcm_substream *,
		struct snd_soc_dai *);
	void (*shutdown)(struct snd_pcm_substream *,
		struct snd_soc_dai *);
	int (*hw_params)(struct snd_pcm_substream *,
		struct snd_pcm_hw_params *, struct snd_soc_dai *);
	int (*hw_free)(struct snd_pcm_substream *,
		struct snd_soc_dai *);
	int (*prepare)(struct snd_pcm_substream *,
		struct snd_soc_dai *);
	/*
	 * NOTE: Commands passed to the trigger function are not necessarily
	 * compatible with the current state of the dai. For example this
	 * sequence of commands is possible: START STOP STOP.
	 * So do not unconditionally use refcounting functions in the trigger
	 * function, e.g. clk_enable/disable.
	 */
	int (*trigger)(struct snd_pcm_substream *, int,
		struct snd_soc_dai *);
	int (*bespoke_trigger)(struct snd_pcm_substream *, int,
		struct snd_soc_dai *);
	/*
	 * For hardware based FIFO caused delay reporting.
	 * Optional.
	 */
	snd_pcm_sframes_t (*delay)(struct snd_pcm_substream *,
		struct snd_soc_dai *);
};

1. DAI 时钟配置函数，通常由snd_card驱动即machine驱动的 hw_params函数调用
   set_sysclk 设置dai的主时钟
   set_pll 设置codec内PLL参数
   set_clkdiv 设置分频系数

2. DAI 格式配置函数，通常由snd_card驱动即machine驱动的 hw_params函数调用
   set_fmt 设置dai的格式，例如主从模式、i2s或dsp模式等
   set_tdm_slot 设置DAI支持tdm时分复用，用来设置时分复用的slot，codec设备在第几个slot上读取数据。
   set_channel_map 声道的时分复用映射设置
   set_tristate 设置dai引脚的状态，当与其他dai并联使用同一引脚时需要使用该回调

3. 标准的snd_soc_ops回调 通常由asoc core在进行PCM操作时调用
   startup
   shutdown
   hw_params
   hw_free
   prepare
   trigger

4. 降低pop音，由asoc-core调用
   digital_mute

soc-dai.h文件里定义的下面函数，通常在machine驱动 struct snd_soc_ops接口的hw_params()函数里 被调用，接收struct snd_soc_dai 实例参数。

参考文档：
https://blog.csdn.net/DroidPhone/article/details/7316061