Linux ALSA声卡驱动之二：声卡的创建

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1. struct snd_card

 

1.1. snd_card是什么

snd_card可以说是整个ALSA音频驱动最顶层的一个结构，整个声卡的软件逻辑结构开始于该结构，几乎所有与声音相关的逻辑设备都是在snd_card的管理之下，声卡驱动的第一个动作通常就是创建一个snd_card结构体。正因为如此，本节中，我们也从 struct cnd_card开始吧。

 

1.2. snd_card的定义

snd_card的定义位于改头文件中：include/sound/core.h

/* main structure for soundcard */

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 */

    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 */
    struct list_head ctl_files; /* active control files */

    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;
    struct device *dev;     /* device assigned to this card */
#ifndef CONFIG_SYSFS_DEPRECATED
    struct device *card_dev;    /* cardX object for sysfs */
#endif

#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
};

• struct list_head devices     记录该声卡下所有逻辑设备的链表
• struct list_head controls    记录该声卡下所有的控制单元的链表
• void *private_data            声卡的私有数据，可以在创建声卡时通过参数指定数据的大小

2. 声卡的建立流程

 

2.1.1. 第一步，创建snd_card的一个实例

 

struct snd_card *card;
int err;
....
err = snd_card_create(index, id, THIS_MODULE, 0, &card);

• index           一个整数值，该声卡的编号
• id                字符串，声卡的标识符
• 第四个参数    该参数决定在创建snd_card实例时，需要同时额外分配的私有数据的大小，该数据的指针最终会赋值给snd_card的private_data数据成员
• card             返回所创建的snd_card实例的指针

 

2.1.2. 第二步，创建声卡的芯片专用数据

声卡的专用数据主要用于存放该声卡的一些资源信息，例如中断资源、io资源、dma资源等。可以有两种创建方法：

• 通过上一步中snd_card_create()中的第四个参数，让snd_card_create自己创建

// struct mychip 用于保存专用数据
err = snd_card_create(index, id, THIS_MODULE,
                sizeof(struct mychip), &card);
// 从private_data中取出
struct mychip *chip = card->private_data;

• 自己创建：

struct mychip {
    struct snd_card *card;
    ....
};
struct snd_card *card;
struct mychip *chip;
err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
// 专用数据记录snd_card实例
chip->card = card;
.....
chip = kzalloc(sizeof(*chip), GFP_KERNEL);

然后，把芯片的专有数据注册为声卡的一个低阶设备：

static int snd_mychip_dev_free(struct snd_device *device)
{
    return snd_mychip_free(device->device_data);
}

static struct snd_device_ops ops = {
    .dev_free = snd_mychip_dev_free,
};
....
snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);

 

2.1.3. 第三步，设置Driver的ID和名字

strcpy(card->driver, "My Chip");
strcpy(card->shortname, "My Own Chip 123");
sprintf(card->longname, "%s at 0x%lx irq %i",
            card->shortname, chip->ioport, chip->irq);

snd_card的driver字段保存着芯片的ID字符串，user空间的alsa-lib会使用到该字符串，所以必须要保证该ID的唯一性。shortname字段更多地用于打印信息，longname字段则会出现在/proc/asound/cards中。

 

2.1.4. 第四步，创建声卡的功能部件（逻辑设备），例如PCM，Mixer，MIDI等

这时候可以创建声卡的各种功能部件了，还记得开头的snd_card结构体的devices字段吗？每一种部件的创建最终会调用snd_device_new()来生成一个snd_device实例，并把该实例链接到snd_card的devices链表中。

通常，alsa-driver的已经提供了一些常用的部件的创建函数，而不必直接调用snd_device_new()，比如：

    PCM  ----        snd_pcm_new()

    RAWMIDI --    snd_rawmidi_new()

    CONTROL --   snd_ctl_create()

    TIMER   --       snd_timer_new()

    INFO    --        snd_card_proc_new()

    JACK    --        snd_jack_new()

 

2.1.5. 第五步，注册声卡

 

err = snd_card_register(card);
if (err < 0) {
    snd_card_free(card);
    return err;
}

 

 

2.2. 一个实际的例子

我把/sound/arm/pxa2xx-ac97.c的部分代码贴上来：

static int __devinit pxa2xx_ac97_probe(struct platform_device *dev)
{
    struct snd_card *card;
    struct snd_ac97_bus *ac97_bus;
    struct snd_ac97_template ac97_template;
    int ret;
    pxa2xx_audio_ops_t *pdata = dev->dev.platform_data;

    if (dev->id >= 0) {
        dev_err(&dev->dev, "PXA2xx has only one AC97 port./n");
        ret = -ENXIO;
        goto err_dev;
    }
////(1)////
    ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
                  THIS_MODULE, 0, &card);
    if (ret < 0)
        goto err;

    card->dev = &dev->dev;
////(3)////
    strncpy(card->driver, dev->dev.driver->name, sizeof(card->driver));

////(4)////
    ret = pxa2xx_pcm_new(card, &pxa2xx_ac97_pcm_client, &pxa2xx_ac97_pcm);
    if (ret)
        goto err;
////(2)////
    ret = pxa2xx_ac97_hw_probe(dev);
    if (ret)
        goto err;

////(4)////
    ret = snd_ac97_bus(card, 0, &pxa2xx_ac97_ops, NULL, &ac97_bus);
    if (ret)
        goto err_remove;
    memset(&ac97_template, 0, sizeof(ac97_template));
    ret = snd_ac97_mixer(ac97_bus, &ac97_template, &pxa2xx_ac97_ac97);
    if (ret)
        goto err_remove;
////(3)////
    snprintf(card->shortname, sizeof(card->shortname),
         "%s", snd_ac97_get_short_name(pxa2xx_ac97_ac97));
    snprintf(card->longname, sizeof(card->longname),
         "%s (%s)", dev->dev.driver->name, card->mixername);

    if (pdata && pdata->codec_pdata[0])
        snd_ac97_dev_add_pdata(ac97_bus->codec[0], pdata->codec_pdata[0]);
    snd_card_set_dev(card, &dev->dev);
////(5)////
    ret = snd_card_register(card);
    if (ret == 0) {
        platform_set_drvdata(dev, card);
        return 0;
    }

err_remove:
    pxa2xx_ac97_hw_remove(dev);
err:
    if (card)
        snd_card_free(card);
err_dev:
    return ret;
}

static int __devexit pxa2xx_ac97_remove(struct platform_device *dev)
{
    struct snd_card *card = platform_get_drvdata(dev);

    if (card) {
        snd_card_free(card);
        platform_set_drvdata(dev, NULL);
        pxa2xx_ac97_hw_remove(dev);
    }

    return 0;
}

static struct platform_driver pxa2xx_ac97_driver = {
    .probe      = pxa2xx_ac97_probe,
    .remove     = __devexit_p(pxa2xx_ac97_remove),
    .driver     = {
        .name   = "pxa2xx-ac97",
        .owner  = THIS_MODULE,
#ifdef CONFIG_PM
        .pm = &pxa2xx_ac97_pm_ops,
#endif
    },
};

static int __init pxa2xx_ac97_init(void)
{
    return platform_driver_register(&pxa2xx_ac97_driver);
}

static void __exit pxa2xx_ac97_exit(void)
{
    platform_driver_unregister(&pxa2xx_ac97_driver);
}

module_init(pxa2xx_ac97_init);
module_exit(pxa2xx_ac97_exit);

MODULE_AUTHOR("Nicolas Pitre");
MODULE_DESCRIPTION("AC97 driver for the Intel PXA2xx chip");

 

 

驱动程序通常由probe回调函数开始，对一下2.1中的步骤，是否有相似之处？

 

经过以上的创建步骤之后，声卡的逻辑结构如下图所示：

                                             图 2.2.1  声卡的软件逻辑结构

 

下面的章节里我们分别讨论一下snd_card_create()和snd_card_register()这两个函数。

 

3. snd_card_create()

 snd_card_create()在/sound/core/init.c中定义。

/**
 *  snd_card_create - create and initialize a soundcard structure
 *  @idx: card index (address) [0 ... (SNDRV_CARDS-1)]
 *  @xid: card identification (ASCII string)
 *  @module: top level module for locking
 *  @extra_size: allocate this extra size after the main soundcard structure
 *  @card_ret: the pointer to store the created card instance
 *
 *  Creates and initializes a soundcard structure.
 *
 *  The function allocates snd_card instance via kzalloc with the given
 *  space for the driver to use freely.  The allocated struct is stored
 *  in the given card_ret pointer.
 *
 *  Returns zero if successful or a negative error code.
 */
int snd_card_create(int idx, const char *xid,
            struct module *module, int extra_size,
            struct snd_card **card_ret)

首先，根据extra_size参数的大小分配内存，该内存区可以作为芯片的专有数据使用（见前面的介绍）：

card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
if (!card)
    return -ENOMEM;

拷贝声卡的ID字符串：

if (xid)
    strlcpy(card->id, xid, sizeof(card->id));

如果传入的声卡编号为-1，自动分配一个索引编号：

if (idx < 0) {
    for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
        /* idx == -1 == 0xffff means: take any free slot */
        if (~snd_cards_lock & idx & 1<<idx2) {
            if (module_slot_match(module, idx2)) {
                idx = idx2;
                break;
            }
        }
}
if (idx < 0) {
    for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
        /* idx == -1 == 0xffff means: take any free slot */
        if (~snd_cards_lock & idx & 1<<idx2) {
            if (!slots[idx2] || !*slots[idx2]) {
                idx = idx2;
                break;
            }
        }
}

初始化snd_card结构中必要的字段：

    card->number = idx;
    card->module = module;
    INIT_LIST_HEAD(&card->devices);
    init_rwsem(&card->controls_rwsem);
    rwlock_init(&card->ctl_files_rwlock);
    INIT_LIST_HEAD(&card->controls);
    INIT_LIST_HEAD(&card->ctl_files);
    spin_lock_init(&card->files_lock);
    INIT_LIST_HEAD(&card->files_list);
    init_waitqueue_head(&card->shutdown_sleep);
#ifdef CONFIG_PM
    mutex_init(&card->power_lock);
    init_waitqueue_head(&card->power_sleep);
#endif

建立逻辑设备：Control

/* the control interface cannot be accessed from the user space until */
/* snd_cards_bitmask and snd_cards are set with snd_card_register */
err = snd_ctl_create(card);

建立proc文件中的info节点：通常就是/proc/asound/card0

err = snd_info_card_create(card);

把第一步分配的内存指针放入private_data字段中：

if (extra_size > 0)
    card->private_data = (char *)card + sizeof(struct snd_card);

4. snd_card_register()

  snd_card_create()在/sound/core/init.c中定义。

/**
 *  snd_card_register - register the soundcard
 *  @card: soundcard structure
 *
 *  This function registers all the devices assigned to the soundcard.
 *  Until calling this, the ALSA control interface is blocked from the
 *  external accesses.  Thus, you should call this function at the end
 *  of the initialization of the card.
 *
 *  Returns zero otherwise a negative error code if the registrain failed.
 */
int snd_card_register(struct snd_card *card)

首先，创建sysfs下的设备：

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

其中，sound_class是在/sound/sound_core.c中创建的：

static char *sound_devnode(struct device *dev, mode_t *mode)
{
    if (MAJOR(dev->devt) == SOUND_MAJOR)
        return NULL;
    return kasprintf(GFP_KERNEL, "snd/%s", dev_name(dev));
}
static int __init init_soundcore(void)
{
    int rc;

    rc = init_oss_soundcore();
    if (rc)
        return rc;

    sound_class = class_create(THIS_MODULE, "sound");
    if (IS_ERR(sound_class)) {
        cleanup_oss_soundcore();
        return PTR_ERR(sound_class);
    }

    sound_class->devnode = sound_devnode;

    return 0;
}

 

由此可见，声卡的class将会出现在文件系统的/sys/class/sound/下面，并且，sound_devnode()也决定了相应的设备节点也将会出现在/dev/snd/下面。

接下来的步骤，通过snd_device_register_all()注册所有挂在该声卡下的逻辑设备，snd_device_register_all()实际上是通过snd_card的devices链表，遍历所有的snd_device，并且调用snd_device的ops->dev_register()来实现各自设备的注册的。

if ((err = snd_device_register_all(card)) < 0)
    return err;

最后就是建立一些相应的proc和sysfs下的文件或属性节点，代码就不贴了。

至此，整个声卡完成了建立过程。