音訊相關術語

• PCM(Pulse Code Modulation)
  脈衝編碼調製，對連續變化的模擬訊號進行抽樣、量化和編碼，在驅動中一般音訊流裝置都稱為pcm裝置

• I2S
  I2S是對PCM格式的資料進行規範化，可以說是PCM的子集，I2S只有左右兩通道資料

• TDM(Time Division Multiplexing)
  時分複用，可以用單根線傳送多通道資料

• midi(Musical Instrument Digital Interface)
  MIDI是編曲界最廣泛的音樂標準格式，可稱為“計算機能理解的樂譜”。它用音符的數字控制訊號來記錄音樂。一首完整的MIDI音樂只有幾十KB大，而能包含數十條音樂軌道。幾乎所有的現代音樂都是用MIDI加上音色庫來製作合成的。MIDI 傳輸的不是聲音訊號， 而是音符、控制引數等指令, 它指示MIDI 裝置要做什麼，怎麼做， 如演奏哪個音符、多大音量等。

• timer(音序器)
  音序器，又稱聲音序列發生器，可將所有MIDI通道中的演奏資訊同時自動播放演奏

驅動架構圖

OSS(Open Sound System)，linux以前的驅動架構，現在已廢棄不用

linux目前的音訊框架為ALSA(Advanced Linux Sound Architecture)，框架如下圖所示，分三層，其中ASoc適用於目前的嵌入式裝置，在ALSA-core基礎上又分出一個小架構

其中alsa-core層的主要結構如下，一個音效卡用snd_card來表示，在一個音效卡上可能集成了好幾個子裝置，每個子裝置負責不用的功能，分為pcm/control/midi/timer等

音效卡結構體

一個音效卡下是由多個裝置的，總音效卡用結構體snd_card來表示，各個子裝置用結構體snd_device來抽象，當然對於具體的子裝置，還會有相應的結構體來表示，這個會在子裝置的文章中詳細介紹

snd_card

(include/sound/core.h)

struct snd_card {
    int number;         /* number of soundcard (index to
                                snd_cards) */

    char id[16];            /* id string of this card */
 

    char driver[16];        /* driver name */
    char shortname[32];     /* short name of this soundcard */
    char longname[80];      /* name of this soundcard */
    char mixername[80];     /* mixer name */
    char components[128];       /* card components delimited with
                                space */
    struct module *module;      /* top-level module */

    void *private_data;     /* private data for soundcard */
    void (*private_free) (struct snd_card *card); /* callback for freeing of
                                private data */
    struct list_head devices;   /* devices */---------snd_device加入此連結串列

    unsigned int last_numid;    /* last used numeric ID */
    struct rw_semaphore controls_rwsem; /* controls list lock */
    rwlock_t ctl_files_rwlock;  /* ctl_files list lock */
    int controls_count;     /* count of all controls */
    int user_ctl_count;     /* count of all user controls */
    struct list_head controls;  /* all controls for this card */---控制裝置snd_device加入此連結串列
    struct list_head ctl_files; /* active control files */
    struct mutex user_ctl_lock; /* protects user controls against
                       concurrent access */

    struct snd_info_entry *proc_root;   /* root for soundcard specific files */
    struct snd_info_entry *proc_id; /* the card id */
    struct proc_dir_entry *proc_root_link;  /* number link to real id */

    struct list_head files_list;    /* all files associated to this card */
    struct snd_shutdown_f_ops *s_f_ops; /* file operations in the shutdown
                                state */
    spinlock_t files_lock;      /* lock the files for this card */
    int shutdown;           /* this card is going down */
    int free_on_last_close;     /* free in context of file_release */
    wait_queue_head_t shutdown_sleep;
    atomic_t refcount;      /* refcount for disconnection */
    struct device *dev;     /* device assigned to this card */
    struct device *card_dev;    /* cardX object for sysfs */

#ifdef CONFIG_PM
    unsigned int power_state;   /* power state */
    struct mutex power_lock;    /* power lock */
    wait_queue_head_t power_sleep;
#endif

#if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)
    struct snd_mixer_oss *mixer_oss;
    int mixer_oss_change_count;
#endif
};

snd_device

(include/sound/core.h)

struct snd_device {
    struct list_head list;      /* list of registered devices */---加入音效卡的連結串列節點
    struct snd_card *card;      /* card which holds this device */-對應的音效卡
    snd_device_state_t state;   /* state of the device */
    snd_device_type_t type;     /* device type */
    void *device_data;      /* device structure */------------對於具體子裝置的私有資料
    struct snd_device_ops *ops; /* operations */------------------對子設備註冊的操作
};

音效卡註冊

snd_card_register

(sound/core/init.c)

int snd_card_register(struct snd_card *card)
{
    int err;

    if (snd_BUG_ON(!card))
        return -EINVAL;

    if (!card->card_dev) {
        card->card_dev = device_create(sound_class, card->dev,
                           MKDEV(0, 0), card,
                           "card%i", card->number);---註冊音效卡
        if (IS_ERR(card->card_dev))
            card->card_dev = NULL;
    }

    if ((err = snd_device_register_all(card)) < 0)-----註冊音效卡下的所有子裝置
        return err;
    mutex_lock(&snd_card_mutex);
    if (snd_cards[card->number]) {
        /* already registered */
        mutex_unlock(&snd_card_mutex);
        return 0;
    }
    if (*card->id) {
        /* make a unique id name from the given string */
        char tmpid[sizeof(card->id)];
        memcpy(tmpid, card->id, sizeof(card->id));
        snd_card_set_id_no_lock(card, tmpid, tmpid);
    } else {
        /* create an id from either shortname or longname */
        const char *src;
        src = *card->shortname ? card->shortname : card->longname;
        snd_card_set_id_no_lock(card, src,
                    retrieve_id_from_card_name(src));
    }
    snd_cards[card->number] = card;
    mutex_unlock(&snd_card_mutex);
    init_info_for_card(card);
#if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)
    if (snd_mixer_oss_notify_callback)
        snd_mixer_oss_notify_callback(card, SND_MIXER_OSS_NOTIFY_REGISTER);
#endif
    if (card->card_dev) {
        err = device_create_file(card->card_dev, &card_id_attrs);
        if (err < 0)
            return err;
        err = device_create_file(card->card_dev, &card_number_attrs);
        if (err < 0)
            return err;
    }

    return 0;
}

snd_device_register_all

int snd_device_register_all(struct snd_card *card)
{
    struct snd_device *dev;
    int err;

    if (snd_BUG_ON(!card))
        return -ENXIO;
    list_for_each_entry(dev, &card->devices, list) {----遍歷音效卡的裝置連結串列
        if (dev->state == SNDRV_DEV_BUILD && dev->ops->dev_register) {
            if ((err = dev->ops->dev_register(dev)) < 0)
                return err;-----------------呼叫每個設備註冊函式，進行註冊
            dev->state = SNDRV_DEV_REGISTERED;
        }
    }
    return 0;
}

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