ALSA driver---register card

通过snd_soc_register_card来注册card，即注册整个machine driver. 

此函数接收一个参数 snd_soc_card：

/* SoC card */
struct snd_soc_card {
    const char *name;
    const char *long_name;
    const char *driver_name;
    struct device *dev;
    struct snd_card *snd_card;
    struct module *owner;

    struct mutex mutex;
    struct mutex dapm_mutex;

    bool instantiated;

    int (*probe)(struct snd_soc_card *card);
    int (*late_probe)(struct snd_soc_card *card);
    int (*remove)(struct snd_soc_card *card);

    /* the pre and post PM functions are used to do any PM work before and
     * after the codec and DAI's do any PM work. */
    int (*suspend_pre)(struct snd_soc_card *card);
    int (*suspend_post)(struct snd_soc_card *card);
    int (*resume_pre)(struct snd_soc_card *card);
    int (*resume_post)(struct snd_soc_card *card);

    /* callbacks */
    int (*set_bias_level)(struct snd_soc_card *,
                  struct snd_soc_dapm_context *dapm,
                  enum snd_soc_bias_level level);
    int (*set_bias_level_post)(struct snd_soc_card *,
                   struct snd_soc_dapm_context *dapm,
                   enum snd_soc_bias_level level);

    int (*add_dai_link)(struct snd_soc_card *,
                struct snd_soc_dai_link *link);
    void (*remove_dai_link)(struct snd_soc_card *,
                struct snd_soc_dai_link *link);

    long pmdown_time;

    /* CPU <--> Codec DAI links  */
    struct snd_soc_dai_link *dai_link;  /* predefined links only */
    int num_links;  /* predefined links only */
    struct list_head dai_link_list; /* all links */
    int num_dai_links;

    struct list_head rtd_list;
    int num_rtd;

    /* optional codec specific configuration */
    struct snd_soc_codec_conf *codec_conf;
    int num_configs;

    /*
     * optional auxiliary devices such as amplifiers or codecs with DAI
     * link unused
     */
    struct snd_soc_aux_dev *aux_dev;
    int num_aux_devs;
    struct list_head aux_comp_list;

    const struct snd_kcontrol_new *controls;
    int num_controls;

    /*
     * Card-specific routes and widgets.
     * Note: of_dapm_xxx for Device Tree; Otherwise for driver build-in.
     */
    const struct snd_soc_dapm_widget *dapm_widgets;
    int num_dapm_widgets;
    const struct snd_soc_dapm_route *dapm_routes;
    int num_dapm_routes;
    const struct snd_soc_dapm_widget *of_dapm_widgets;
    int num_of_dapm_widgets;
    const struct snd_soc_dapm_route *of_dapm_routes;
    int num_of_dapm_routes;
    bool fully_routed;

    struct work_struct deferred_resume_work;

    /* lists of probed devices belonging to this card */
    struct list_head codec_dev_list;

    struct list_head widgets;
    struct list_head paths;
    struct list_head dapm_list;
    struct list_head dapm_dirty;

    /* attached dynamic objects */
    struct list_head dobj_list;

    /* Generic DAPM context for the card */
    struct snd_soc_dapm_context dapm;
    struct snd_soc_dapm_stats dapm_stats;
    struct snd_soc_dapm_update *update;

#ifdef CONFIG_DEBUG_FS
    struct dentry *debugfs_card_root;
    struct dentry *debugfs_pop_time;
#endif
    u32 pop_time;

    void *drvdata;
};

snd_soc_dai_link中，指定了Platform、Codec、codec_dai、cpu_dai的名字，稍后Machine驱动将会利用这些名字去匹配已经在系统中注册的platform，codec，dai，这些注册的部件都是在另外相应的Platform驱动和Codec驱动的代码文件中定义的，这样看来，Machine驱动的设备初始化代码无非就是选择合适Platform和Codec以及dai，用他们填充以上几个数据结构，然后注册Platform设备即可。

/**
 * snd_soc_register_card - Register a card with the ASoC core
 *
 * @card: Card to register
 *
 */
int snd_soc_register_card(struct snd_soc_card *card)
{
    int i, ret;
    struct snd_soc_pcm_runtime *rtd;

    if (!card->name || !card->dev)
        return -EINVAL;

    for (i = 0; i < card->num_links; i++) {
        struct snd_soc_dai_link *link = &card->dai_link[i];

        ret = soc_init_dai_link(card, link);
        if (ret) {
            dev_err(card->dev, "ASoC: failed to init link %s\n",
                link->name);
            return ret;
        }
    }

    dev_set_drvdata(card->dev, card);

    snd_soc_initialize_card_lists(card);

    INIT_LIST_HEAD(&card->dai_link_list);
    card->num_dai_links = 0;

    INIT_LIST_HEAD(&card->rtd_list);
    card->num_rtd = 0;

    INIT_LIST_HEAD(&card->dapm_dirty);
    INIT_LIST_HEAD(&card->dobj_list);
    card->instantiated = 0;
    mutex_init(&card->mutex);
    mutex_init(&card->dapm_mutex);

    ret = snd_soc_instantiate_card(card);
    if (ret != 0)
        return ret;

    /* deactivate pins to sleep state */
    list_for_each_entry(rtd, &card->rtd_list, list)  {
        struct snd_soc_dai *cpu_dai = rtd->cpu_dai;
        int j;

        for (j = 0; j < rtd->num_codecs; j++) {
            struct snd_soc_dai *codec_dai = rtd->codec_dais[j];
            if (!codec_dai->active)
                pinctrl_pm_select_sleep_state(codec_dai->dev);
        }

        if (!cpu_dai->active)
            pinctrl_pm_select_sleep_state(cpu_dai->dev);
    }

    return ret;
}