1.创建声卡

snd_card是对声卡硬件抽象出来的结构体，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。

我们可以通过调用snd_card_new来创建一个snd_card结构体。

struct snd_card *card;
int err;
err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);

/**
* snd_card_new - create and initialize a soundcard structure
* @parent: the parent device object
* @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
*/
int snd_card_new(struct device *parent, int idx, const char *xid, struct module *module, int extra_size, struct snd_card **card_ret)

这里extra_size是为card->private_data分配的内存大小。通常private_data保存chip-specifc data.

2.创建声卡的芯片专用数据

chip-specific data包含I/O port address, its resource pointer, or the irq number等信息。

有两种方法可以来分配chip specific data.

1）.通过snd_card_new实现：

struct mychip {struct snd_card *card;....};

err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,sizeof(struct mychip), &card);

struct mychip *chip = card->private_data;

chip->card = card;

在snd_card_new中分配chip。并将chip和card关联起来。card结构中的private_data就是chip,chip中又包含card。

2）.通过snd_device_new来将chip作为一个低阶device注册到card上。

在snd_device_new中指定extra_size为0.

  struct snd_card *card;struct mychip *chip;err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,0, &card);.....chip = kzalloc(sizeof(*chip), GFP_KERNEL);

  chip->card = card;

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

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

/**
* snd_device_new - create an ALSA device component
* @card: the card instance
* @type: the device type, SNDRV_DEV_XXX
* @device_data: the data pointer of this device
* @ops: the operator table
*/
int snd_device_new(struct snd_card *card, enum snd_device_type type,
void *device_data, struct snd_device_ops *ops)

snd_device_new不为芯片专用数据device_data分配空间，因此在调用之前，必须为芯片专用分配空间，在ops的dev_free中定义析构函数对芯片专用数据进行析构。dev_free会在调用snd_card_free时自动调用。对于用户自定义的 device， type可以使用SNDRV_DEV_LOWLEVEL。

snd_mychip_dev_free() 是用来free前面kzmalloc的空间。

 3.设置声卡驱动名字和声卡名字。

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);

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