alsa结构体流程3

/* SoC Audio Codec device */
struct snd_soc_codec {
    const char *name;                 /* Codec的名字*/
    const char *name_prefix;
    int id;
    struct device *dev;              /* 指向Codec设备的指针 */    
    const struct snd_soc_codec_driver *driver;  /* 指向该codec的驱动的指针 */ 
    struct mutex mutex;
    struct snd_soc_card *card; /* 指向Machine驱动的card实例 */
    struct list_head list;
    struct list_head card_list;
    int num_dai;   /* 该Codec数字接口的个数，目前越来越多的Codec带有多个I2S或者是PCM接口 */
    enum snd_soc_compress_type compress_type;
    size_t reg_size;    /* reg_cache_size * reg_word_size */
    int (*volatile_register)(struct snd_soc_codec *, unsigned int);
    int (*readable_register)(struct snd_soc_codec *, unsigned int);

    /* runtime */
    struct snd_ac97 *ac97;  /* for ad-hoc ac97 devices */
    unsigned int active;
    unsigned int cache_bypass:1; /* Suppress access to the cache */
    unsigned int suspended:1; /* Codec is in suspend PM state */
    unsigned int probed:1; /* Codec has been probed */
    unsigned int ac97_registered:1; /* Codec has been AC97 registered */
    unsigned int ac97_created:1; /* Codec has been created by SoC */
    unsigned int sysfs_registered:1; /* codec has been sysfs registered */
    unsigned int cache_init:1; /* codec cache has been initialized */
    u32 cache_only;  /* Suppress writes to hardware */
    u32 cache_sync; /* Cache needs to be synced to hardware */

    /* codec IO */
    void *control_data; /* codec control (i2c/3wire) data 该指针指向的结构用于对codec的控制，通常和read，write字段联合使用 */
    hw_write_t hw_write;
    unsigned int (*hw_read)(struct snd_soc_codec *, unsigned int);
    unsigned int (*read)(struct snd_soc_codec *, unsigned int);  /* 读取Codec寄存器的函数 */ 
    int (*write)(struct snd_soc_codec *, unsigned int, unsigned int);  /* 写入Codec寄存器的函数 */  
    void *reg_cache;
    const void *reg_def_copy;
    const struct snd_soc_cache_ops *cache_ops;
    struct mutex cache_rw_mutex;

    /* dapm */
    struct snd_soc_dapm_context dapm;  /* 用于DAPM控件 */
#ifdef CONFIG_DEBUG_FS
    struct dentry *debugfs_codec_root;
    struct dentry *debugfs_reg;
    struct dentry *debugfs_dapm;
#endif
};

/* codec driver */
struct snd_soc_codec_driver {

	/* driver ops */
	int (*probe)(struct snd_soc_codec *);
	int (*remove)(struct snd_soc_codec *);
	int (*suspend)(struct snd_soc_codec *,
			pm_message_t state);
	int (*resume)(struct snd_soc_codec *);

	/* Default DAPM setup, added after probe() is run */
	const struct snd_soc_dapm_widget *dapm_widgets;
	int num_dapm_widgets;
	const struct snd_soc_dapm_route *dapm_routes;
	int num_dapm_routes;

	/* codec wide operations */
	int (*set_sysclk)(struct snd_soc_codec *codec,
			  int clk_id, unsigned int freq, int dir);
	int (*set_pll)(struct snd_soc_codec *codec, int pll_id, int source,
		unsigned int freq_in, unsigned int freq_out);

	/* codec IO */
	unsigned int (*read)(struct snd_soc_codec *, unsigned int);
	int (*write)(struct snd_soc_codec *, unsigned int, unsigned int);
	int (*display_register)(struct snd_soc_codec *, char *,
				size_t, unsigned int);
	int (*volatile_register)(struct snd_soc_codec *, unsigned int);
	int (*readable_register)(struct snd_soc_codec *, unsigned int);
	short reg_cache_size;
	short reg_cache_step;
	short reg_word_size;
	const void *reg_cache_default;
	short reg_access_size;
	const struct snd_soc_reg_access *reg_access_default;
	enum snd_soc_compress_type compress_type;

	/* codec bias level */
	int (*set_bias_level)(struct snd_soc_codec *,
			      enum snd_soc_bias_level level);

	void (*seq_notifier)(struct snd_soc_dapm_context *,
			     enum snd_soc_dapm_type, int);
};

codec驱动把自己注册为一个platform driver，那对应的platform device在哪里定义？

答案是在以下代码文件中：/drivers/mfd/wm8994-core.c。

WM8994本身具备多种功能，除了codec外，它还有作为LDO和GPIO使用，这几种功能共享一些IO和中断资源，linux为这种设备提供了一套标准的实现方法：mfd设备。其基本思想是为这些功能的公共部分实现一个父设备，以便共享某些系统资源和功能，然后每个子功能实现为它的子设备，这样既共享了资源和代码，又能实现合理的设备层次结构，主要利用到的API就是：mfd_add_devices()，mfd_remove_devices()，mfd_cell_enable()，mfd_cell_disable()，mfd_clone_cell()。

from:

Linux ALSA声卡驱动之八：ASoC架构中的Platform