1. struct snd_card
1.1 snd_card是什么
snd_card可以說是整個ALSA音頻驅動最頂層的一個結構,整個聲卡的軟件邏輯結構開始於該結構,幾乎所有與聲音相關的邏輯設備都是在snd_card的管理之下,聲卡驅動的第一個動作通常就是創建一個snd_card結構體.正因為如此,本節中,我們也從 struct cnd_card開始吧.
1.2 snd_card的定義
snd_card的定義位於改頭文件中: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 記錄該聲卡下所有邏輯設備的鏈表
- struct list_head controls 記錄該聲卡下所有的控制單元的鏈表
- void *private_data 聲卡的私有數據,可以在創建聲卡時通過參數指定數據的大小
2. 聲卡的建立流程
2.1 創建snd_card的一個實例
1 struct snd_card *card; 2 int err; 3 .... 4 err = snd_card_create(index, id, THIS_MODULE, 0, &card);
- index 一個整數值,該聲卡的編號
- id 字符串,聲卡的標識符
- 第四個參數 該參數決定在創建snd_card實例時,需要同時額外分配的私有數據的大小,該數據的指針最終會賦值給snd_card的private_data數據成員
- card 返回所創建的snd_card實例的指針
2.2 創建聲卡的芯片專用數據
聲卡的專用數據主要用於存放該聲卡的一些資源信息,例如中斷資源、io資源、dma資源等.可以有兩種創建方法:
- 通過上一步中snd_card_create()中的第四個參數,讓snd_card_create自己創建
1 // struct mychip 用於保存專用數據 2 err = snd_card_create(index, id, THIS_MODULE, 3 sizeof(struct mychip), &card); 4 // 從private_data中取出 5 struct mychip *chip = card->private_data;
- 自己創建:
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 // 專用數據記錄snd_card實例 9 chip->card = card; 10 ..... 11 chip = kzalloc(sizeof(*chip), GFP_KERNEL);
然后,把芯片的專有數據注冊為聲卡的一個低階設備:
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 設置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空間的alsa-lib會使用到該字符串,所以必須要保證該ID的唯一性.shortname字段更多地用於打印信息,longname字段則會出現在/proc/asound/cards中.
2.4 創建聲卡的功能部件(邏輯設備),例如PCM,Mixer,MIDI等
這時候可以創建聲卡的各種功能部件了,還記得開頭的snd_card結構體的devices字段嗎?每一種部件的創建最終會調用snd_device_new()來生成一個snd_device實例,並把該實例鏈接到snd_card的devices鏈表中.
通常,alsa-driver的已經提供了一些常用的部件的創建函數,而不必直接調用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 注冊聲卡
1 err = snd_card_register(card); 2 if (err < 0) { 3 snd_card_free(card); 4 return err; 5 }
2.6 一個實際的例子
我把/sound/arm/pxa2xx-ac97.c的部分代碼貼上來:
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");
驅動程序通常由probe回調函數開始,對一下2.1中的步驟,是否有相似之處?
經過以上的創建步驟之后,聲卡的邏輯結構如下圖所示:
圖 2.1 聲卡的軟件邏輯結構
下面的章節里我們分別討論一下snd_card_create()和snd_card_register()這兩個函數.
3. snd_card_create()
snd_card_create()在/sound/core/init.c中定義.
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,根據extra_size參數的大小分配內存,該內存區可以作為芯片的專有數據使用(見前面的介紹):
1 card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL); 2 if (!card) 3 return -ENOMEM;
拷貝聲卡的ID字符串：:
1 if (xid) 2 strlcpy(card->id, xid, sizeof(card->id));
如果傳入的聲卡編號為-1,自動分配一個索引編號:
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結構中必要的字段:
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
建立邏輯設備: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節點:通常就是/proc/asound/card0
1 err = snd_info_card_create(card);
把第一步分配的內存指針放入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中定義.
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,創建sysfs下的設備:
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中創建的:
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 }
由此可見,聲卡的class將會出現在文件系統的/sys/class/sound/下面,並且,sound_devnode()也決定了相應的設備節點也將會出現在/dev/snd/下面.
接下來的步驟,通過snd_device_register_all()注冊所有掛在該聲卡下的邏輯設備,snd_device_register_all()實際上是通過snd_card的devices鏈表,遍歷所有的snd_device,並且調用snd_device的ops->dev_register()來實現各自設備的注冊的.
1 if ((err = snd_device_register_all(card)) < 0) 2 return err;
最后就是建立一些相應的proc和sysfs下的文件或屬性節點,代碼就不貼了.
至此,整個聲卡完成了建立過程.
本文轉自:http://blog.csdn.net/droidphone/article/details/6289712