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
