安卓声卡驱动：5.dai驱动

一 dai驱动简介

dai驱动用于驱动component上的音频接口，dai驱动通常不会单独存在，dai驱动会与component驱动一起注册。
在注册codec时，就要求提供dai驱动，而platform驱动在编写时也会根据需要，在注册platform驱动的同时调用devm_snd_soc_register_component或snd_soc_register_dais来为dai驱动单独注册。
这两个函数的差异就是snd_soc_register_dais需要自己创建一个component，而devm_snd_soc_register_component则会自动为dai创建一个单独的component。

dai driver用于管理数字音频接口，所以dai驱动中除了管理接口的函数，还会有各种音频属性，如采样率，位宽等。
dai按方向可以分为playback和capture两种，一个dai driver可以同时有playback和Capture两种接口。

二 dai驱动重要结构体

dai driver有2个重要的结构体struct snd_soc_dai_driver和dai的操作函数struct snd_soc_dai_ops

struct snd_soc_dai_driver {/* DAI description */const char *name;unsigned int id;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 */int (*compress_new)(struct snd_soc_pcm_runtime *rtd, int num);/* DAI is also used for the control bus */bool bus_control;/* 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;unsigned int symmetric_channels:1;unsigned int symmetric_samplebits:1;/* probe ordering - for components with runtime dependencies */int probe_order;int remove_order;
};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 *);
};

name：dai的名字，在dai_link中就是依赖这个名字来指定dai接口
ops：接口操作函数，如果是I2S接口则是I2S的控制函数，PCM接口则是PCM的控制函数。
capture：描述dai接口的录音能力，指定声道数，采样率，位深等
playback：描述dai接口的播放能力
probe_order：初始化声卡时该component的probe函数调用顺序
remove_order：卸载时的顺序

dai的注册过程

dai注册的函数是snd_soc_register_dais

/*** snd_soc_register_dais - Register a DAI with the ASoC core** @component: The component the DAIs are registered for* @dai_drv: DAI driver to use for the DAIs* @count: Number of DAIs* @legacy_dai_naming: Use the legacy naming scheme and let the DAI inherit the*                     parent's name.*/
static int snd_soc_register_dais(struct snd_soc_component *component,struct snd_soc_dai_driver *dai_drv, size_t count,bool legacy_dai_naming)
{struct device *dev = component->dev;struct snd_soc_dai *dai;unsigned int i;int ret;dev_dbg(dev, "ASoC: dai register %s #%Zu\n", dev_name(dev), count);component->dai_drv = dai_drv;component->num_dai = count;for (i = 0; i < count; i++) {                                                                //可以一次注册多个daidai = kzalloc(sizeof(struct snd_soc_dai), GFP_KERNEL);        //为dai分配内存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 &&                              //格式化名称(dai_drv[i].id == 0 || dai_drv[i].name == NULL)) {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);                //把dai挂到component的dai_list链表上去dev_dbg(dev, "ASoC: Registered DAI '%s'\n", dai->name);}return 0;err:snd_soc_unregister_dais(component);return ret;
}

dai driver的注册需要一个component，这个component可以是单独的component，也可以是和platform/codec共用