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 tosnd_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 withspace */struct module *module;/* top-level module */void *private_data;/* private data for soundcard */void (*private_free) (struct snd_card *card); /* callback for freeing ofprivate 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 shutdownstate */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_DEPRECATEDstruct device *card_dev;/* cardX object for sysfs */#endif#ifdef CONFIG_PMunsigned 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_PMmutex_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_register()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。

转自：http://write.blog.csdn.net/postedit/8186117