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

01./* main structure for soundcard */  
02.  
03.struct snd_card {  
04.    int number;         /* number of soundcard (index to 
05.                                snd_cards) */  
06.  
07.    char id[16];            /* id string of this card */  
08.    char driver[16];        /* driver name */  
09.    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

 

2.1.1. 第一步，创建snd_card的一个实例

01.struct snd_card *card;  
02.int err;  
03.....  
04.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：

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

自己创建：

01.struct mychip {  
02.    struct snd_card *card;  
03.    ....  
04.};  
05.struct snd_card *card;  
06.struct mychip *chip;  
07.err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);  
08.// 专用数据记录snd_card实例   
09.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：

01.static int snd_mychip_dev_free(struct snd_device *device)  
02.{  
03.    return snd_mychip_free(device->device_data);  
04.}  
05.  
06.static struct snd_device_ops ops = {  
07.    .dev_free = snd_mychip_dev_free,  
08.};  
09.....  
10.snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);  

2.1.3. 第三步，设置Driver的ID和名字

01.strcpy(card->driver, "My Chip");  
02.strcpy(card->shortname, "My Own Chip 123");  
03.sprintf(card->longname, "%s at 0x%lx irq %i",  
04.            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

 

01.err = snd_card_register(card);  
02.if (err < 0) {  
03.    snd_card_free(card);  
04.    return err;  
05.}  

.2. 一个实际的例子

我把/sound/arm/pxa2xx-ac97.c的部分代码贴上来：

01.static int __devinit pxa2xx_ac97_probe(struct platform_device *dev)  
02.{  
03.    struct snd_card *card;  
04.    struct snd_ac97_bus *ac97_bus;  
05.    struct snd_ac97_template ac97_template;  
06.    int ret;  
07.    pxa2xx_audio_ops_t *pdata = dev->dev.platform_data;  
08.  
09.    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 2.2.1  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。

01./** 
02. *  snd_card_create - create and initialize a soundcard structure 
03. *  @idx: card index (address) [0 ... (SNDRV_CARDS-1)] 
04. *  @xid: card identification (ASCII string) 
05. *  @module: top level module for locking 
06. *  @extra_size: allocate this extra size after the main soundcard structure 
07. *  @card_ret: the pointer to store the created card instance 
08. * 
09. *  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）：

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

拷贝声卡的ID字符串：

01.if (xid)  
02.    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：

01.if (idx < 0) {  
02.    for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)  
03.        /* idx == -1 == 0xffff means: take any free slot */  
04.        if (~snd_cards_lock & idx & 1<<idx2) {  
05.            if (module_slot_match(module, idx2)) {  
06.                idx = idx2;  
07.                break;  
08.            }  
09.        }  
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：

 card->number = idx;  
02.    card->module = module;  
03.    INIT_LIST_HEAD(&card->devices);  
04.    init_rwsem(&card->controls_rwsem);  
05.    rwlock_init(&card->ctl_files_rwlock);  
06.    INIT_LIST_HEAD(&card->controls);  
07.    INIT_LIST_HEAD(&card->ctl_files);  
08.    spin_lock_init(&card->files_lock);  
09.    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);  

建立逻辑设备：Control

[c-sharp] view plaincopyprint?
01./* the control interface cannot be accessed from the user space until */  
02./* snd_cards_bitmask and snd_cards are set with snd_card_register */  
03.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：

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

4. snd_card_register()

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

01./** 
02. *  snd_card_register - register the soundcard 
03. *  @card: soundcard structure 
04. * 
05. *  This function registers all the devices assigned to the soundcard. 
06. *  Until calling this, the ALSA control interface is blocked from the 
07. *  external accesses.  Thus, you should call this function at the end 
08. *  of the initialization of the card. 
09. * 
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：

01.if (!card->card_dev) {  
02.    card->card_dev = device_create(sound_class, card->dev,  
03.                       MKDEV(0, 0), card,  
04.                       "card%i", card->number);  
05.    if (IS_ERR(card->card_dev))  
06.        card->card_dev = NULL;  
07.}  

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

01.static char *sound_devnode(struct device *dev, mode_t *mode)  
02.{  
03.    if (MAJOR(dev->devt) == SOUND_MAJOR)  
04.        return NULL;  
05.    return kasprintf(GFP_KERNEL, "snd/%s", dev_name(dev));  
06.}  
07.static int __init init_soundcore(void)  
08.{  
09.    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。

01.if ((err = snd_device_register_all(card)) < 0)  
02.    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。