Sound card registration
A sound card is represented in the kernel as an instance of struct snd_soc_card, defined as follows:
struct snd_soc_card {
    const char *name;
    struct module *owner;
    [...]
    /* 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 *,
                            ...
