1 pcm設備介紹
PCM是英文Pulse-code modulation的縮寫,中文譯名是脈沖編碼調制.我們知道在現實生活中,人耳聽到的聲音是模擬信號,PCM就是要把聲音從模擬轉換成數字信號的一種技術,他的原理簡單地說就是利用一個固定的頻率對模擬信號進行采樣,采樣后的信號在波形上看就像一串連續的幅值不一的脈沖,把這些脈沖的幅值按一定的精度進行量化,這些量化后的數值被連續地輸出、傳輸、處理或記錄到存儲介質中,所有這些組成了數字音頻的產生過程。
PCM信號的兩個重要指標是采樣頻率和量化精度,目前,CD音頻的采樣頻率通常為44100Hz,量化精度是16bit.通常,播放音樂時,應用程序從存儲介質中讀取音頻數據(MP3、WMA、AAC......),經過解碼后,最終送到音頻驅動程序中的就是PCM數據,反過來,在錄音時,音頻驅動不停地把采樣所得的PCM數據送回給應用程序,由應用程序完成壓縮、存儲等任務.所以,音頻驅動的兩大核心任務就是：
playback 如何把用戶空間的應用程序發過來的PCM數據,轉化為人耳可以辨別的模擬音頻
capture 把mic拾取到得模擬信號,經過采樣、量化,轉換為PCM信號送回給用戶空間的應用程序
2 alsa drive中的pcm中間層
ALSA已經為我們實現了功能強勁的PCM中間層,自己的驅動中只要實現一些底層的需要訪問硬件的函數即可。要訪問PCM的中間層代碼,你首先要包含頭文件<sound/pcm.h>,另外,如果需要訪問一些與 hw_param相關的函數,可能也要包含<sound/pcm_params.h>。
每個聲卡最多可以包含4個pcm的實例,每個pcm實例對應一個pcm設備文件.pcm實例數量的這種限制源於linux設備號所占用的位大小,如果以后使用64位的設備號,我們將可以創建更多的pcm實例.不過大多數情況下,在嵌入式設備中,一個pcm實例已經足夠了。一個pcm實例由一個playback stream和一個capture stream組成,這兩個stream又分別有一個或多個substreams組成。
但是在嵌入式系統中,通常不會像上圖2中這么復雜,大多數情況下是一個聲卡,一個pcm實例,pcm下面有一個playback和capture stream,playback和capture下面各自有一個substream.
3 pcm的數據結構
定義位於：include\sound\pcm.h
3.1 struct snd_pcm
是掛在snd_card下面的一個snd_device。
1 struct snd_pcm { 2 struct snd_card *card; 3 struct list_head list; 4 int device; /* device number */ 5 unsigned int info_flags; 6 unsigned short dev_class; 7 unsigned short dev_subclass; 8 char id[64]; 9 char name[80]; 10 struct snd_pcm_str streams[2];//該數組中的兩個元素指向兩個snd_pcm_str結構,分別代表playback stream和capture stream 11 struct mutex open_mutex; 12 wait_queue_head_t open_wait; 13 void *private_data; 14 void (*private_free) (struct snd_pcm *pcm); 15 struct device *dev; /* actual hw device this belongs to */ 16 bool internal; /* pcm is for internal use only */ 17 #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE) 18 struct snd_pcm_oss oss; 19 #endif 20 }
3.2 struct snd_pcm_str
1 struct snd_pcm_str { 2 int stream; /* stream (direction) */ 3 struct snd_pcm *pcm; 4 /* -- substreams -- */ 5 unsigned int substream_count; 6 unsigned int substream_opened; 7 struct snd_pcm_substream *substream;//指向snd_pcm_substream結構 8 #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE) 9 /* -- OSS things -- */ 10 struct snd_pcm_oss_stream oss; 11 #endif 12 #ifdef CONFIG_SND_VERBOSE_PROCFS 13 struct snd_info_entry *proc_root; 14 struct snd_info_entry *proc_info_entry; 15 #ifdef CONFIG_SND_PCM_XRUN_DEBUG 16 unsigned int xrun_debug; /* 0 = disabled, 1 = verbose, 2 = stacktrace */ 17 struct snd_info_entry *proc_xrun_debug_entry; 18 #endif 19 #endif 20 struct snd_kcontrol *chmap_kctl; /* channel-mapping controls */ 21 }
3.3 struct snd_pcm_substream
是pcm中間層的核心,絕大部分任務都是在substream中處理,尤其是他的ops(snd_pcm_ops)字段,許多user空間的應用程序通過alsa-lib對驅動程序的請求都是由該結構中的函數處理.它的runtime字段則指向snd_pcm_runtime結構,snd_pcm_runtime記錄這substream的一些重要的軟件和硬件運行環境和參數。
1 struct snd_pcm_substream { 2 struct snd_pcm *pcm; 3 struct snd_pcm_str *pstr; 4 void *private_data; /* copied from pcm->private_data */ 5 int number; 6 char name[32]; /* substream name */ 7 int stream; /* stream (direction) */ 8 struct pm_qos_request latency_pm_qos_req; /* pm_qos request */ 9 size_t buffer_bytes_max; /* limit ring buffer size */ 10 struct snd_dma_buffer dma_buffer; 11 unsigned int dma_buf_id; 12 size_t dma_max; 13 /* -- hardware operations -- */ 14 struct snd_pcm_ops *ops;//user空間的應用程序通過alsa-lib對驅動程序的請求都是由該結構中的函數處理 15 /* -- runtime information -- */ 16 struct snd_pcm_runtime *runtime;//記錄這substream的一些重要的軟件和硬件運行環境和參數 17 /* -- timer section -- */ 18 struct snd_timer *timer; /* timer */ 19 unsigned timer_running: 1; /* time is running */ 20 /* -- next substream -- */ 21 struct snd_pcm_substream *next; 22 /* -- linked substreams -- */ 23 struct list_head link_list; /* linked list member */ 24 struct snd_pcm_group self_group; /* fake group for non linked substream (with substream lock inside) */ 25 struct snd_pcm_group *group; /* pointer to current group */ 26 /* -- assigned files -- */ 27 void *file; 28 int ref_count; 29 atomic_t mmap_count; 30 unsigned int f_flags; 31 void (*pcm_release)(struct snd_pcm_substream *); 32 struct pid *pid; 33 #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE) 34 /* -- OSS things -- */ 35 struct snd_pcm_oss_substream oss; 36 #endif 37 #ifdef CONFIG_SND_VERBOSE_PROCFS 38 struct snd_info_entry *proc_root; 39 struct snd_info_entry *proc_info_entry; 40 struct snd_info_entry *proc_hw_params_entry; 41 struct snd_info_entry *proc_sw_params_entry; 42 struct snd_info_entry *proc_status_entry; 43 struct snd_info_entry *proc_prealloc_entry; 44 struct snd_info_entry *proc_prealloc_max_entry; 45 #endif 46 /* misc flags */ 47 unsigned int hw_opened: 1; 48 }
3.4 struct snd_pcm_ops
pcm設備的設備操作結構體
1 struct snd_pcm_ops { 2 int (*open)(struct snd_pcm_substream *substream); 3 int (*close)(struct snd_pcm_substream *substream); 4 int (*ioctl)(struct snd_pcm_substream * substream, 5 unsigned int cmd, void *arg); 6 int (*hw_params)(struct snd_pcm_substream *substream, 7 struct snd_pcm_hw_params *params); 8 int (*hw_free)(struct snd_pcm_substream *substream); 9 int (*prepare)(struct snd_pcm_substream *substream); 10 int (*trigger)(struct snd_pcm_substream *substream, int cmd); 11 snd_pcm_uframes_t (*pointer)(struct snd_pcm_substream *substream); 12 int (*wall_clock)(struct snd_pcm_substream *substream, 13 struct timespec *audio_ts); 14 int (*copy)(struct snd_pcm_substream *substream, int channel, 15 snd_pcm_uframes_t pos, 16 void __user *buf, snd_pcm_uframes_t count); 17 int (*silence)(struct snd_pcm_substream *substream, int channel, 18 snd_pcm_uframes_t pos, snd_pcm_uframes_t count); 19 struct page *(*page)(struct snd_pcm_substream *substream, 20 unsigned long offset); 21 int (*mmap)(struct snd_pcm_substream *substream, struct vm_area_struct *vma); 22 int (*ack)(struct snd_pcm_substream *substream); 23 }
3.5 struct snd_pcm_file
1 struct snd_pcm_file { 2 struct snd_pcm_substream *substream; 3 int no_compat_mmap; 4 };
3.6 struct snd_pcm_runtime
記錄這substream的一些重要的軟件和硬件運行環境和參數
1 struct snd_pcm_runtime { 2 /* -- Status -- */ 3 struct snd_pcm_substream *trigger_master; 4 struct timespec trigger_tstamp; /* trigger timestamp */ 5 int overrange; 6 snd_pcm_uframes_t avail_max; 7 snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */ 8 snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time */ 9 unsigned long hw_ptr_jiffies; /* Time when hw_ptr is updated */ 10 unsigned long hw_ptr_buffer_jiffies; /* buffer time in jiffies */ 11 snd_pcm_sframes_t delay; /* extra delay; typically FIFO size */ 12 u64 hw_ptr_wrap; /* offset for hw_ptr due to boundary wrap-around */ 13 14 /* -- HW params -- */ 15 snd_pcm_access_t access; /* access mode */ 16 snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */ 17 snd_pcm_subformat_t subformat; /* subformat */ 18 unsigned int rate; /* rate in Hz */ 19 unsigned int channels; /* channels */ 20 snd_pcm_uframes_t period_size; /* period size */ 21 unsigned int periods; /* periods */ 22 snd_pcm_uframes_t buffer_size; /* buffer size */ 23 snd_pcm_uframes_t min_align; /* Min alignment for the format */ 24 size_t byte_align; 25 unsigned int frame_bits; 26 unsigned int sample_bits; 27 unsigned int info; 28 unsigned int rate_num; 29 unsigned int rate_den; 30 unsigned int no_period_wakeup: 1; 31 32 /* -- SW params -- */ 33 int tstamp_mode; /* mmap timestamp is updated */ 34 unsigned int period_step; 35 snd_pcm_uframes_t start_threshold; 36 snd_pcm_uframes_t stop_threshold; 37 snd_pcm_uframes_t silence_threshold; /* Silence filling happens when 38 noise is nearest than this */ 39 snd_pcm_uframes_t silence_size; /* Silence filling size */ 40 snd_pcm_uframes_t boundary; /* pointers wrap point */ 41 42 snd_pcm_uframes_t silence_start; /* starting pointer to silence area */ 43 snd_pcm_uframes_t silence_filled; /* size filled with silence */ 44 45 union snd_pcm_sync_id sync; /* hardware synchronization ID */ 46 47 /* -- mmap -- */ 48 struct snd_pcm_mmap_status *status; 49 struct snd_pcm_mmap_control *control; 50 51 /* -- locking / scheduling -- */ 52 snd_pcm_uframes_t twake; /* do transfer (!poll) wakeup if non-zero */ 53 wait_queue_head_t sleep; /* poll sleep */ 54 wait_queue_head_t tsleep; /* transfer sleep */ 55 struct fasync_struct *fasync; 56 57 /* -- private section -- */ 58 void *private_data; 59 void (*private_free)(struct snd_pcm_runtime *runtime); 60 61 /* -- hardware description -- */ 62 struct snd_pcm_hardware hw; 63 struct snd_pcm_hw_constraints hw_constraints; 64 65 /* -- interrupt callbacks -- */ 66 void (*transfer_ack_begin)(struct snd_pcm_substream *substream); 67 void (*transfer_ack_end)(struct snd_pcm_substream *substream); 68 69 /* -- timer -- */ 70 unsigned int timer_resolution; /* timer resolution */ 71 int tstamp_type; /* timestamp type */ 72 73 /* -- DMA -- */ 74 unsigned char *dma_area; /* DMA area */ 75 dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */ 76 size_t dma_bytes; /* size of DMA area */ 77 78 struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */ 79 80 #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE) 81 /* -- OSS things -- */ 82 struct snd_pcm_oss_runtime oss; 83 #endif 84 85 #ifdef CONFIG_SND_PCM_XRUN_DEBUG 86 struct snd_pcm_hwptr_log *hwptr_log; 87 #endif 88 }
3.7 struct snd_pcm_hardware
1 /* 2 * Hardware (lowlevel) section 3 */ 4 5 struct snd_pcm_hardware { 6 unsigned int info; /* SNDRV_PCM_INFO_* */ 7 u64 formats; /* SNDRV_PCM_FMTBIT_* */ 8 unsigned int rates; /* SNDRV_PCM_RATE_* */ 9 unsigned int rate_min; /* min rate */ 10 unsigned int rate_max; /* max rate */ 11 unsigned int channels_min; /* min channels */ 12 unsigned int channels_max; /* max channels */ 13 size_t buffer_bytes_max; /* max buffer size */ 14 size_t period_bytes_min; /* min period size */ 15 size_t period_bytes_max; /* max period size */ 16 unsigned int periods_min; /* min # of periods */ 17 unsigned int periods_max; /* max # of periods */ 18 size_t fifo_size; /* fifo size in bytes */ 19 }
3.8 以上pcm各數據結構的關系如下：
4 創建pcm設備
4.1 創建pcm
函數snd_pcm_new，定義位於：sound\core\pcm.c
1 /** 2 * snd_pcm_new - create a new PCM instance 3 * @card: the card instance 4 * @id: the id string 5 * @device: the device index (zero based) 參數device 表示目前創建的是該聲卡下的第幾個pcm,第一個pcm設備從0開始. 6 * @playback_count: the number of substreams for playback 參數playback_count 表示該pcm將會有幾個playback substream. 7 * @capture_count: the number of substreams for capture 參數capture_count 表示該pcm將會有幾個capture substream. 8 * @rpcm: the pointer to store the new pcm instance 9 * 10 * Creates a new PCM instance. 11 * 12 * The pcm operators have to be set afterwards to the new instance 13 * via snd_pcm_set_ops(). 14 * 15 * Return: Zero if successful, or a negative error code on failure. 16 */ 17 int snd_pcm_new(struct snd_card *card, const char *id, int device, 18 int playback_count, int capture_count, struct snd_pcm **rpcm) 19 { 20 return _snd_pcm_new(card, id, device, playback_count, capture_count, 21 false, rpcm); 22 }
調用函數 _snd_pcm_new
調用該api創建一個pcm，然后會做以下事情：
（1）如果有,建立playback stream,相應的substream也同時建立
（2）如果有,建立capture stream,相應的substream也同時建立
（3）調用snd_device_new()把該pcm掛到聲卡中,參數ops中的dev_register字段指向了函數snd_pcm_dev_register,這個回調函數會在聲卡的注冊階段被調用.
1 static int _snd_pcm_new(struct snd_card *card, const char *id, int device, 2 int playback_count, int capture_count, bool internal, 3 struct snd_pcm **rpcm) 4 { 5 struct snd_pcm *pcm;//創建一個pcm 6 int err; 7 static struct snd_device_ops ops = { 8 .dev_free = snd_pcm_dev_free, 9 .dev_register = snd_pcm_dev_register, 10 .dev_disconnect = snd_pcm_dev_disconnect, 11 }; 12 13 if (snd_BUG_ON(!card)) 14 return -ENXIO; 15 if (rpcm) 16 *rpcm = NULL; 17 pcm = kzalloc(sizeof(*pcm), GFP_KERNEL); 18 if (pcm == NULL) { 19 snd_printk(KERN_ERR "Cannot allocate PCM\n"); 20 return -ENOMEM; 21 } 22 pcm->card = card; 23 pcm->device = device; 24 pcm->internal = internal; 25 if (id) 26 strlcpy(pcm->id, id, sizeof(pcm->id)); 27 if ((err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_PLAYBACK, playback_count)) < 0) {//創建playback stream 播放 28 snd_pcm_free(pcm); 29 return err; 30 } 31 if ((err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_CAPTURE, capture_count)) < 0) {//創建capture stream 錄音 32 snd_pcm_free(pcm); 33 return err; 34 } 35 mutex_init(&pcm->open_mutex); 36 init_waitqueue_head(&pcm->open_wait); 37 if ((err = snd_device_new(card, SNDRV_DEV_PCM, pcm, &ops)) < 0) {//把該pcm掛到聲卡中,參數ops中的dev_register字段指向了函數snd_pcm_dev_register,這個回調函數會在聲卡的注冊階段被調用 38 snd_pcm_free(pcm); 39 return err; 40 } 41 if (rpcm) 42 *rpcm = pcm; 43 return 0; 44 }
4.2 snd_device_new函數
定義位於：linux-4.9.73\sound\core\device.c
作用：（1）拿到聲卡邏輯設備的設備操作結構體
（2）將該設備邏輯設備加入聲卡結構體devices鏈表中(該聲卡下邏輯設備比如control、pcm等),dev->list加入card->devies，在注冊時根據此鏈表找到對應邏輯設備，調用其注冊函數。
1 int snd_device_new(struct snd_card *card, enum snd_device_type type, 2 void *device_data, struct snd_device_ops *ops) 3 { 4 struct snd_device *dev; 5 struct list_head *p; 6 7 if (snd_BUG_ON(!card || !device_data || !ops)) 8 return -ENXIO; 9 dev = kzalloc(sizeof(*dev), GFP_KERNEL); 10 if (!dev) 11 return -ENOMEM; 12 INIT_LIST_HEAD(&dev->list); 13 dev->card = card; 14 dev->type = type; 15 dev->state = SNDRV_DEV_BUILD; 16 dev->device_data = device_data; 17 dev->ops = ops;//拿到聲卡邏輯設備的設備操作結構體 18 19 /* insert the entry in an incrementally sorted list */ 20 list_for_each_prev(p, &card->devices) {//將該設備邏輯設備加入聲卡結構體devices鏈表中(該聲卡下邏輯設備比如control、pcm等),dev->list加入card->devies 21 struct snd_device *pdev = list_entry(p, struct snd_device, list);//獲取聲卡邏輯設備結構體 22 if ((unsigned int)pdev->type <= (unsigned int)type)//判斷該邏輯設備的類型，常用的control或者pcm 23 break; 24 } 25 26 list_add(&dev->list, p);//在鏈表p的前面加入鏈表dev->list 27 return 0; 28 }
設備操作結構體定義，位於_snd_pcm_new函數中，最重要的是注冊函數
static struct snd_device_ops ops = {
.dev_free = snd_pcm_dev_free,
.dev_register = snd_pcm_dev_register,
.dev_disconnect = snd_pcm_dev_disconnect,
}
4.3 函數snd_pcm_set_ops
定義位於：sound\core\pcm_lib.c
設置操作該pcm的控制/操作接口函數,參數中的snd_pcm_ops結構中的函數通常就是我們驅動要實現的函數。
1 /** 2 * snd_pcm_set_ops - set the PCM operators 3 * @pcm: the pcm instance 4 * @direction: stream direction, SNDRV_PCM_STREAM_XXX 5 * @ops: the operator table 6 * 7 * Sets the given PCM operators to the pcm instance. 8 */ 9 void snd_pcm_set_ops(struct snd_pcm *pcm, int direction, struct snd_pcm_ops *ops) 10 { 11 struct snd_pcm_str *stream = &pcm->streams[direction]; 12 struct snd_pcm_substream *substream; 13 14 for (substream = stream->substream; substream != NULL; substream = substream->next) 15 substream->ops = ops; 16 }
5 注冊pcm設備
在上面創建的pcm設備信息基礎上，注冊pcm設備。
先調用函數snd_card_register注冊聲卡，然后該函數中會調用pcm注冊函數snd_pcm_dev_register注冊pcm設備。下面分別講解，先看pcm注冊函數。
5.1 pcm設備注冊函數
snd_pcm_dev_register，定義位於sound\core\pcm.c。
1 static int snd_pcm_dev_register(struct snd_device *device) 2 { 3 int cidx, err; 4 struct snd_pcm_substream *substream; 5 struct snd_pcm_notify *notify; 6 struct snd_pcm *pcm; 7 8 if (snd_BUG_ON(!device || !device->device_data)) 9 return -ENXIO; 10 pcm = device->device_data; 11 if (pcm->internal) 12 return 0; 13 14 mutex_lock(®ister_mutex); 15 err = snd_pcm_add(pcm); 16 if (err) 17 goto unlock; 18 for (cidx = 0; cidx < 2; cidx++) { 19 int devtype = -1; 20 if (pcm->streams[cidx].substream == NULL) 21 continue; 22 switch (cidx) { 23 case SNDRV_PCM_STREAM_PLAYBACK: 24 devtype = SNDRV_DEVICE_TYPE_PCM_PLAYBACK; 25 break; 26 case SNDRV_PCM_STREAM_CAPTURE: 27 devtype = SNDRV_DEVICE_TYPE_PCM_CAPTURE; 28 break; 29 } 30 /* register pcm */ 31 err = snd_register_device(devtype, pcm->card, pcm->device, 32 &snd_pcm_f_ops[cidx], pcm,//指定pcm設備的文件操作結構體 33 &pcm->streams[cidx].dev); 34 if (err < 0) { 35 list_del_init(&pcm->list); 36 goto unlock; 37 } 38 39 for (substream = pcm->streams[cidx].substream; substream; substream = substream->next) 40 snd_pcm_timer_init(substream); 41 } 42 43 list_for_each_entry(notify, &snd_pcm_notify_list, list) 44 notify->n_register(pcm); 45 46 unlock: 47 mutex_unlock(®ister_mutex); 48 return err; 49 }
該pcm設備注冊函數snd_pcm_dev_register會加入設備操作結構體snd_device_ops 作為回調函數。
然后調用函數snd_register_device完成最終的注冊.
5.1.1 函數snd_register_device
1 /**
2 * snd_register_device - Register the ALSA device file for the card
3 * @type: the device type, SNDRV_DEVICE_TYPE_XXX
4 * @card: the card instance
5 * @dev: the device index
6 * @f_ops: the file operations
7 * @private_data: user pointer for f_ops->open()
8 * @device: the device to register
9 *
10 * Registers an ALSA device file for the given card.
11 * The operators have to be set in reg parameter.
12 *
13 * Return: Zero if successful, or a negative error code on failure.
14 */
15 int snd_register_device(int type, struct snd_card *card, int dev,
16 const struct file_operations *f_ops, 17 void *private_data, struct device *device) 18 { 19 int minor; 20 int err = 0; 21 struct snd_minor *preg;//建立次設備 22 23 if (snd_BUG_ON(!device)) 24 return -EINVAL; 25 /*給次設備賦值*/ 26 preg = kmalloc(sizeof *preg, GFP_KERNEL); 27 if (preg == NULL) 28 return -ENOMEM; 29 preg->type = type; 30 preg->card = card ? card->number : -1; 31 preg->device = dev; 32 preg->f_ops = f_ops;//獲取文件操作結構體 33 preg->private_data = private_data; 34 preg->card_ptr = card; 35 mutex_lock(&sound_mutex); 36 minor = snd_find_free_minor(type, card, dev); 37 if (minor < 0) { 38 err = minor; 39 goto error; 40 } 41 42 preg->dev = device; 43 device->devt = MKDEV(major, minor); 44 err = device_add(device);//創建設備節點 45 if (err < 0) 46 goto error; 47 48 snd_minors[minor] = preg;//賦值給全局的次設備信息結構體 49 error: 50 mutex_unlock(&sound_mutex); 51 if (err < 0) 52 kfree(preg); 53 return err; 54 }
主要作用：
（1）獲取聲卡次設備snd_minor ，並將其賦值給全局聲卡次設備變量snd_minor中
（2）獲取次設備pcm的文件操作結構體，供用戶層使用的api回調函數。snd_pcm_f_ops作為snd_register_device的參數被傳入,並被記錄在snd_minors[minor]中的字段f_ops中.最后創建設備節點。創建節點之后我們就能在/dev目錄下查看到相應的設備文件。
5.1.2 pcm設備的文件操作結構體
回調函數，供用戶層使用的api。分為playback和capature，即播放設備和錄音設備。
1 /* 2 * Register section 3 */ 4 5 const struct file_operations snd_pcm_f_ops[2] = { 6 { 7 .owner = THIS_MODULE, 8 .write = snd_pcm_write, 9 .write_iter = snd_pcm_writev, 10 .open = snd_pcm_playback_open, 11 .release = snd_pcm_release, 12 .llseek = no_llseek, 13 .poll = snd_pcm_playback_poll, 14 .unlocked_ioctl = snd_pcm_playback_ioctl, 15 .compat_ioctl = snd_pcm_ioctl_compat, 16 .mmap = snd_pcm_mmap, 17 .fasync = snd_pcm_fasync, 18 .get_unmapped_area = snd_pcm_get_unmapped_area, 19 }, 20 { 21 .owner = THIS_MODULE, 22 .read = snd_pcm_read, 23 .read_iter = snd_pcm_readv, 24 .open = snd_pcm_capture_open, 25 .release = snd_pcm_release, 26 .llseek = no_llseek, 27 .poll = snd_pcm_capture_poll, 28 .unlocked_ioctl = snd_pcm_capture_ioctl, 29 .compat_ioctl = snd_pcm_ioctl_compat, 30 .mmap = snd_pcm_mmap, 31 .fasync = snd_pcm_fasync, 32 .get_unmapped_area = snd_pcm_get_unmapped_area, 33 } 34 }
然后看聲卡注冊函數。
5.2 聲卡注冊函數
snd_card_register 詳見：linux - alsa詳解1概括。dev/snd/pcmCxxDxxp、pcmCxxDxxc，設備文件節點的建立。注意和上面pcm設備注冊函數的區別，聲卡注冊函數最終會調用pcm注冊函數注冊pcm設備，或者調用control設備注冊函數注冊控制設備。
snd_card_register 函數調用函數snd_device_register_all
5.3 snd_device_register_all
1 /* 2 * register all the devices on the card. 3 * called from init.c 4 */ 5 int snd_device_register_all(struct snd_card *card) 6 { 7 struct snd_device *dev; 8 int err; 9 10 if (snd_BUG_ON(!card)) 11 return -ENXIO; 12 list_for_each_entry(dev, &card->devices, list) {//遍歷聲卡結構體中的設備鏈表，逐個注冊聲卡下掛載的邏輯設備 13 err = __snd_device_register(dev); 14 if (err < 0) 15 return err; 16 } 17 return 0; 18 }
5.4 函數__snd_device_register
1 static int __snd_device_register(struct snd_device *dev) 2 { 3 if (dev->state == SNDRV_DEV_BUILD) { 4 if (dev->ops->dev_register) { 5 int err = dev->ops->dev_register(dev);//調用聲卡邏輯設備的注冊函數，即5.1中pcm設備注冊函數snd_pcm_dev_register 6 if (err < 0) 7 return err; 8 } 9 dev->state = SNDRV_DEV_REGISTERED; 10 } 11 return 0; 12 }
注冊聲卡,在這個階段會遍歷聲卡下的所有邏輯設備,並且調用各設備的注冊回調函數,對於pcm,就是第二步提到的snd_pcm_dev_register函數,該回調函數建立了和用戶空間應用程序(alsa-lib)通信所用的設備文件節點:/dev/snd/pcmCxxDxxp和/dev/snd/pcmCxxDxxc
注：一共有兩個關鍵的結構體：
（1）設備操作結構體snd_device_ops ，在聲卡創建時注冊回調函數，主要是設備注冊回調函數snd_pcm_dev_register，在聲卡注冊時會調用其注冊pcm設備
（2）文件操作結構體struct file_operations snd_pcm_f_ops，在pcm設備注冊函數snd_pcm_dev_register中，加入全局變量snd_minors數組中，就是告訴聲卡，供用戶調用的接口。用戶層操作時會獲取到數組snd_minors的下標，就可以找到相應的次設備是control或者pcm，操作對用文件操作結構體中的函數。
以上api的調用關系：
以上代碼我們可以看出,對於一個pcm設備,可以生成兩個設備文件,一個用於playback,一個用於capture,代碼中也確定了他們的命名規則:
（1）playback -- pcmCxDxp,通常系統中只有一各聲卡和一個pcm,它就是pcmC0D0p
（2）capture -- pcmCxDxc,通常系統中只有一各聲卡和一個pcm,它就是pcmC0D0c
6 pcm設備的open
以從應用程序到驅動層pcm為例一步一步講解。
6.1 聲卡字符設備的注冊
在sound/core/sound.c中有alsa_sound_init()函數,定義如下:
1 static int __init alsa_sound_init(void) 2 { 3 snd_major = major; 4 snd_ecards_limit = cards_limit; 5 if (register_chrdev(major, "alsa", &snd_fops)) { 6 snd_printk(KERN_ERR "unable to register native major device number %d\n", major); 7 return -EIO; 8 } 9 if (snd_info_init() < 0) { 10 unregister_chrdev(major, "alsa"); 11 return -ENOMEM; 12 } 13 snd_info_minor_register(); 14 #ifndef MODULE 15 printk(KERN_INFO "Advanced Linux Sound Architecture Driver Initialized.\n"); 16 #endif 17 return 0; 18 }
register_chrdev中的參數major與之前創建pcm設備是device_create時的major是同一個,這樣的結果是,當應用程序open設備文件/dev/snd/pcmCxDxp時,會進入snd_fops的open回調函數。
6.2 打開pcm設備
從上一節中我們得知,open一個pcm設備時,將會調用snd_fops的open回調函數,我們先看看snd_fops的定義：
位於：
1 static const struct file_operations snd_fops = 2 { 3 .owner = THIS_MODULE, 4 .open = snd_open, 5 .llseek = noop_llseek, 6 };
跟入snd_open函數,它首先從inode中取出此設備號,然后以次設備號為索引,從snd_minors全局數組中取出當初注冊pcm設備時填充的snd_minor結構(參看linux-alsa詳解1基本知識第4節的內容),然后從snd_minor結構中取出pcm設備的f_ops,並且把file->f_op替換為pcm設備的f_ops,緊接着直接調用pcm設備的f_ops->open(),然后返回.因為file->f_op已經被替換,以后,應用程序的所有read/write/ioctl調用都會進入pcm設備自己的回調函數中,也就是5.1.2節中提到的snd_pcm_f_ops結構中定義的回調。
1 static int snd_open(struct inode *inode, struct file *file) 2 { 3 unsigned int minor = iminor(inode); 4 struct snd_minor *mptr = NULL; 5 const struct file_operations *old_fops; 6 int err = 0; 7 8 if (minor >= ARRAY_SIZE(snd_minors)) 9 return -ENODEV; 10 mutex_lock(&sound_mutex); 11 mptr = snd_minors[minor]; 12 if (mptr == NULL) { 13 mptr = autoload_device(minor); 14 if (!mptr) { 15 mutex_unlock(&sound_mutex); 16 return -ENODEV; 17 } 18 } 19 old_fops = file->f_op; 20 file->f_op = fops_get(mptr->f_ops); 21 if (file->f_op == NULL) { 22 file->f_op = old_fops; 23 err = -ENODEV; 24 } 25 mutex_unlock(&sound_mutex); 26 if (err < 0) 27 return err; 28 29 if (file->f_op->open) { 30 err = file->f_op->open(inode, file); 31 if (err) { 32 fops_put(file->f_op); 33 file->f_op = fops_get(old_fops); 34 } 35 } 36 fops_put(old_fops); 37 return err; 38 }
6.3 總結以上過程
下面的序列圖表示了應用程序如何最終調用到snd_pcm_f_ops結構中的回調函數:
參考博文：
https://www.cnblogs.com/jason-lu/archive/2013/06/07/3123750.html