冷烟花
哎呀哎呀哎呀我的妈~蜗牛背着那重重的壳呀,一步一步地往上爬~

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

 1 /* main structure for soundcard */
 2 
 3 struct snd_card {
 4     int number;            /* number of soundcard (index to
 5                                 snd_cards) */
 6 
 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 */
15 
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 */
20 
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 */
28 
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 */
32 
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
44 
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
50 
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 };
  • 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 创建snd_card的一个实例

1 struct snd_card *card;
2 int err;
3 ....
4 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

2.2 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
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;
  • 自己创建:
 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);

然后,把芯片的专有数据注册为声卡的一个低阶设备:

 1 static int snd_mychip_dev_free(struct snd_device *device)
 2 {
 3     return snd_mychip_free(device->device_data);
 4 }
 5 
 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);

2.3 设置Driver的ID和名字

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);

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.

2.4 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(),如下:

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()

2.5 注册声卡

1 err = snd_card_register(card);
2 if (err < 0) {
3     snd_card_free(card);
4     return err;
5 }

2.6 一个实际的例子

我把/sound/arm/pxa2xx-ac97.c的部分代码贴上来:

  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;
  8 
  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;
 19 
 20     card->dev = &dev->dev;
 21 ////(3)////
 22     strncpy(card->driver, dev->dev.driver->name, sizeof(card->driver));
 23 
 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;
 32 
 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);
 46 
 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     }
 56 
 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 }
 65 
 66 static int __devexit pxa2xx_ac97_remove(struct platform_device *dev)
 67 {
 68     struct snd_card *card = platform_get_drvdata(dev);
 69 
 70     if (card) {
 71         snd_card_free(card);
 72         platform_set_drvdata(dev, NULL);
 73         pxa2xx_ac97_hw_remove(dev);
 74     }
 75 
 76     return 0;
 77 }
 78 
 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 };
 90 
 91 static int __init pxa2xx_ac97_init(void)
 92 {
 93     return platform_driver_register(&pxa2xx_ac97_driver);
 94 }
 95 
 96 static void __exit pxa2xx_ac97_exit(void)
 97 {
 98     platform_driver_unregister(&pxa2xx_ac97_driver);
 99 }
100 
101 module_init(pxa2xx_ac97_init);
102 module_exit(pxa2xx_ac97_exit);
103 
104 MODULE_AUTHOR("Nicolas Pitre");
105 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 2.1  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.

 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)

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):

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

拷贝声卡的ID字符串::

1     if (xid)
2         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:

 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<<idx2) {
 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<<idx2) {
15                 if (!slots[idx2] || !*slots[idx2]) {
16                     idx = idx2;
17                     break;
18                 }
19             }
20     }

初始化snd_card结构中必要的字段:

 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

建立逻辑设备:Control 

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);

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

1     err = snd_info_card_create(card);

把第一步分配的内存指针放入private_data字段中:

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

4. snd_card_register()

snd_card_register()在/sound/core/init.c中定义.

 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)

首先,创建sysfs下的设备:

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     }

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

 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;
10 
11     rc = init_oss_soundcore();
12     if (rc)
13         return rc;
14 
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     }
20 
21     sound_class->devnode = sound_devnode;
22 
23     return 0;
24 }

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.

1     if ((err = snd_device_register_all(card)) < 0)
2         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.

本文转自:http://blog.csdn.net/droidphone/article/details/6289712

posted on 2013-06-07 11:58  冷烟花  阅读(3898)  评论(0编辑  收藏  举报