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Linux ALSA声卡驱动之二：声卡的创建

分类： LINUX

2012-08-18 10:58:05

声明：本博内容均由http://blog.csdn.net/droidphone原创，转载请注明出处，谢谢！
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

[c-sharp] view plaincopyprint?
  1. /* main structure for soundcard */
  3. struct snd_card {
  4. int number; /* number of soundcard (index to
  5. snd_cards) */
  7. char id[16]; /* id string of this card */
  8. char driver[16]; /* driver name */
  9. char shortname[32]; /* short name of this soundcard */
  10. char longname[80]; /* name of this soundcard */
  11. char mixername[80]; /* mixer name */
  12. char components[128]; /* card components delimited with
  13. space */
  14. struct module *module; /* top-level module */
  16. void *private_data; /* private data for soundcard */
  17. void (*private_free) (struct snd_card *card); /* callback for freeing of
  18. private data */
  19. struct list_head devices; /* devices */
  21. unsigned int last_numid; /* last used numeric ID */
  22. struct rw_semaphore controls_rwsem; /* controls list lock */
  23. rwlock_t ctl_files_rwlock; /* ctl_files list lock */
  24. int controls_count; /* count of all controls */
  25. int user_ctl_count; /* count of all user controls */
  26. struct list_head controls; /* all controls for this card */
  27. struct list_head ctl_files; /* active control files */
  29. struct snd_info_entry *proc_root; /* root for soundcard specific files */
  30. struct snd_info_entry *proc_id; /* the card id */
  31. struct proc_dir_entry *proc_root_link; /* number link to real id */
  33. struct list_head files_list; /* all files associated to this card */
  34. struct snd_shutdown_f_ops *s_f_ops; /* file operations in the shutdown
  35. state */
  36. spinlock_t files_lock; /* lock the files for this card */
  37. int shutdown; /* this card is going down */
  38. int free_on_last_close; /* free in context of file_release */
  39. wait_queue_head_t shutdown_sleep;
  40. struct device *dev; /* device assigned to this card */
  41. #ifndef CONFIG_SYSFS_DEPRECATED
  42. struct device *card_dev; /* cardX object for sysfs */
  43. #endif
  45. #ifdef CONFIG_PM
  46. unsigned int power_state; /* power state */
  47. struct mutex power_lock; /* power lock */
  48. wait_queue_head_t power_sleep;
  49. #endif
  51. #if defined(CONFIG_SND_MIXER_OSS) || defined(CONFIG_SND_MIXER_OSS_MODULE)
  52. struct snd_mixer_oss *mixer_oss;
  53. int mixer_oss_change_count;
  54. #endif
  55. };
/* 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

[c-sharp] view plaincopyprint?
  1. struct snd_card *card;
  2. int err;
  3. ....
  4. err = snd_card_create(index, id, THIS_MODULE, 0, &card);
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

[c-sharp] view plaincopyprint?
  1. // struct mychip 用于保存专用数据
  2. err = snd_card_create(index, id, THIS_MODULE,
  3. sizeof(struct mychip), &card);
  4. // 从private_data中取出
  5. struct mychip *chip = card->private_data;
// 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：

[c-sharp] view plaincopyprint?
  1. struct mychip {
  2. struct snd_card *card;
  3. ....
  4. };
  5. struct snd_card *card;
  6. struct mychip *chip;
  7. err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
  8. // 专用数据记录snd_card实例
  9. chip->card = card;
  10. .....
  11. chip = kzalloc(sizeof(*chip), GFP_KERNEL);
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：

[c-sharp] view plaincopyprint?
  1. static int snd_mychip_dev_free(struct snd_device *device)
  2. {
  3. return snd_mychip_free(device->device_data);
  4. }
  6. static struct snd_device_ops ops = {
  7. .dev_free = snd_mychip_dev_free,
  8. };
  9. ....
  10. 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); } 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

[c-sharp] view plaincopyprint?
  1. strcpy(card->driver, "My Chip");
  2. strcpy(card->shortname, "My Own Chip 123");
  3. sprintf(card->longname, "%s at 0x%lx irq %i",
  4. card->shortname, chip->ioport, chip->irq);
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

[c-sharp] view plaincopyprint?
  1. err = snd_card_register(card);
  2. if (err < 0) {
  3. snd_card_free(card);
  4. return err;
  5. }
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：

[cpp] view plaincopyprint?
  1. static int __devinit pxa2xx_ac97_probe(struct platform_device *dev)
  2. {
  3. struct snd_card *card;
  4. struct snd_ac97_bus *ac97_bus;
  5. struct snd_ac97_template ac97_template;
  6. int ret;
  7. pxa2xx_audio_ops_t *pdata = dev->dev.platform_data;
  9. if (dev->id >= 0) {
  10. dev_err(&dev->dev, "PXA2xx has only one AC97 port./n");
  11. ret = -ENXIO;
  12. goto err_dev;
  13. }
  14. ////(1)////
  15. ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
  16. THIS_MODULE, 0, &card);
  17. if (ret < 0)
  18. goto err;
  20. card->dev = &dev->dev;
  21. ////(3)////
  22. strncpy(card->driver, dev->dev.driver->name, sizeof(card->driver));
  24. ////(4)////
  25. ret = pxa2xx_pcm_new(card, &pxa2xx_ac97_pcm_client, &pxa2xx_ac97_pcm);
  26. if (ret)
  27. goto err;
  28. ////(2)////
  29. ret = pxa2xx_ac97_hw_probe(dev);
  30. if (ret)
  31. goto err;
  33. ////(4)////
  34. ret = snd_ac97_bus(card, 0, &pxa2xx_ac97_ops, NULL, &ac97_bus);
  35. if (ret)
  36. goto err_remove;
  37. memset(&ac97_template, 0, sizeof(ac97_template));
  38. ret = snd_ac97_mixer(ac97_bus, &ac97_template, &pxa2xx_ac97_ac97);
  39. if (ret)
  40. goto err_remove;
  41. ////(3)////
  42. snprintf(card->shortname, sizeof(card->shortname),
  43. "%s", snd_ac97_get_short_name(pxa2xx_ac97_ac97));
  44. snprintf(card->longname, sizeof(card->longname),
  45. "%s (%s)", dev->dev.driver->name, card->mixername);
  47. if (pdata && pdata->codec_pdata[0])
  48. snd_ac97_dev_add_pdata(ac97_bus->codec[0], pdata->codec_pdata[0]);
  49. snd_card_set_dev(card, &dev->dev);
  50. ////(5)////
  51. ret = snd_card_register(card);
  52. if (ret == 0) {
  53. platform_set_drvdata(dev, card);
  54. return 0;
  55. }
  57. err_remove:
  58. pxa2xx_ac97_hw_remove(dev);
  59. err:
  60. if (card)
  61. snd_card_free(card);
  62. err_dev:
  63. return ret;
  64. }
  66. static int __devexit pxa2xx_ac97_remove(struct platform_device *dev)
  67. {
  68. struct snd_card *card = platform_get_drvdata(dev);
  70. if (card) {
  71. snd_card_free(card);
  72. platform_set_drvdata(dev, NULL);
  73. pxa2xx_ac97_hw_remove(dev);
  74. }
  76. return 0;
  77. }
  79. static struct platform_driver pxa2xx_ac97_driver = {
  80. .probe = pxa2xx_ac97_probe,
  81. .remove = __devexit_p(pxa2xx_ac97_remove),
  82. .driver = {
  83. .name = "pxa2xx-ac97",
  84. .owner = THIS_MODULE,
  85. #ifdef CONFIG_PM
  86. .pm = &pxa2xx_ac97_pm_ops,
  87. #endif
  88. },
  89. };
  91. static int __init pxa2xx_ac97_init(void)
  92. {
  93. return platform_driver_register(&pxa2xx_ac97_driver);
  94. }
  96. static void __exit pxa2xx_ac97_exit(void)
  97. {
  98. platform_driver_unregister(&pxa2xx_ac97_driver);
  99. }
  101. module_init(pxa2xx_ac97_init);
  102. module_exit(pxa2xx_ac97_exit);
  104. MODULE_AUTHOR("Nicolas Pitre");
  105. MODULE_DESCRIPTION("AC97 driver for the Intel PXA2xx chip");
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。

[cpp] view plaincopyprint?
  1. /**
  2. * snd_card_create - create and initialize a soundcard structure
  3. * @idx: card index (address) [0 ... (SNDRV_CARDS-1)]
  4. * @xid: card identification (ASCII string)
  5. * @module: top level module for locking
  6. * @extra_size: allocate this extra size after the main soundcard structure
  7. * @card_ret: the pointer to store the created card instance
  8. *
  9. * Creates and initializes a soundcard structure.
  10. *
  11. * The function allocates snd_card instance via kzalloc with the given
  12. * space for the driver to use freely. The allocated struct is stored
  13. * in the given card_ret pointer.
  14. *
  15. * Returns zero if successful or a negative error code.
  16. */
  17. int snd_card_create(int idx, const char *xid,
  18. struct module *module, int extra_size,
  19. struct snd_card **card_ret)
/** * 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）：

[c-sharp] view plaincopyprint?
  1. card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
  2. if (!card)
  3. return -ENOMEM;
card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL); if (!card) return -ENOMEM;

拷贝声卡的ID字符串：

[c-sharp] view plaincopyprint?
  1. if (xid)
  2. strlcpy(card->id, xid, sizeof(card->id));
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：

[c-sharp] view plaincopyprint?
  1. if (idx < 0) {
  2. for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
  3. /* idx == -1 == 0xffff means: take any free slot */
  4. if (~snd_cards_lock & idx & 1<
  5. if (module_slot_match(module, idx2)) {
  6. idx = idx2;
  7. break;
  8. }
  9. }
  10. }
  11. if (idx < 0) {
  12. for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
  13. /* idx == -1 == 0xffff means: take any free slot */
  14. if (~snd_cards_lock & idx & 1<
  15. if (!slots[idx2] || !*slots[idx2]) {
  16. idx = idx2;
  17. break;
  18. }
  19. }
  20. }
if (idx < 0) { for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++) /* idx == -1 == 0xffff means: take any free slot */ if (~snd_cards_lock & idx & 1<

初始化snd_card结构中必要的字段：

[c-sharp] view plaincopyprint?
  1. card->number = idx;
  2. card->module = module;
  3. INIT_LIST_HEAD(&card->devices);
  4. init_rwsem(&card->controls_rwsem);
  5. rwlock_init(&card->ctl_files_rwlock);
  6. INIT_LIST_HEAD(&card->controls);
  7. INIT_LIST_HEAD(&card->ctl_files);
  8. spin_lock_init(&card->files_lock);
  9. INIT_LIST_HEAD(&card->files_list);
  10. init_waitqueue_head(&card->shutdown_sleep);
  11. #ifdef CONFIG_PM
  12. mutex_init(&card->power_lock);
  13. init_waitqueue_head(&card->power_sleep);
  14. #endif
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

[c-sharp] view plaincopyprint?
  1. /* the control interface cannot be accessed from the user space until */
  2. /* snd_cards_bitmask and snd_cards are set with snd_card_register */
  3. err = snd_ctl_create(card);
/* 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

[c-sharp] view plaincopyprint?
  1. err = snd_info_card_create(card);
err = snd_info_card_create(card);

把第一步分配的内存指针放入private_data字段中：

[c-sharp] view plaincopyprint?
  1. if (extra_size > 0)
  2. card->private_data = (char *)card + sizeof(struct snd_card);
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。

[c-sharp] view plaincopyprint?
  1. /**
  2. * snd_card_register - register the soundcard
  3. * @card: soundcard structure
  4. *
  5. * This function registers all the devices assigned to the soundcard.
  6. * Until calling this, the ALSA control interface is blocked from the
  7. * external accesses. Thus, you should call this function at the end
  8. * of the initialization of the card.
  9. *
  10. * Returns zero otherwise a negative error code if the registrain failed.
  11. */
  12. int snd_card_register(struct snd_card *card)
/** * 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：

[c-sharp] view plaincopyprint?
  1. if (!card->card_dev) {
  2. card->card_dev = device_create(sound_class, card->dev,
  3. MKDEV(0, 0), card,
  4. "card%i", card->number);
  5. if (IS_ERR(card->card_dev))
  6. card->card_dev = NULL;
  7. }
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：

[c-sharp] view plaincopyprint?
  1. static char *sound_devnode(struct device *dev, mode_t *mode)
  2. {
  3. if (MAJOR(dev->devt) == SOUND_MAJOR)
  4. return NULL;
  5. return kasprintf(GFP_KERNEL, "snd/%s", dev_name(dev));
  6. }
  7. static int __init init_soundcore(void)
  8. {
  9. int rc;
  11. rc = init_oss_soundcore();
  12. if (rc)
  13. return rc;
  15. sound_class = class_create(THIS_MODULE, "sound");
  16. if (IS_ERR(sound_class)) {
  17. cleanup_oss_soundcore();
  18. return PTR_ERR(sound_class);
  19. }
  21. sound_class->devnode = sound_devnode;
  23. return 0;
  24. }
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。

[c-sharp] view plaincopyprint?
  1. if ((err = snd_device_register_all(card)) < 0)
  2. return err;
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。

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