1.pcm设备

脉冲编码调制(Pulse Code Modulation,PCM)，就是把一个时间连续，取值连续的模拟信号变换成时间离散，取值离散的数字信号后在信道中传输，这是基本原理。

根据此原理，在音频领域的数字音频就用pcm设备来代表，pcm也是一种音频格式，可以自定义通道数，采样率，采样精度；我们经常采用的I2S格式其实属于pcm的一种，不过I2S规定了只有2通道。

音频的采样率(rate)一般采用44.1K，16K，48K等,采样精度(format)一般都是8/16/24/32bit

在ALSA框架中，pcm就是控制音频流的，区别于control

2.PCM设备结构体

这部分重要的结构体主要有：

struct snd_pcm
struct snd_pcm_str
struct snd_pcm_substream

这三者的关系可以用下图来表示：

一个音频设备分播放和录音两个功能，对应到pcm就分PLAYBACK和CAPTURE，分别用结构体snd_pcm_str来表示，一个播放或者录音设备可以集成多个音频流，每个音频流用snd_pcm_substream结构体来表示

这三个结构体的逻辑链接关系如下图：

3.pcm设备注册

pcm设备注册函数为：

int snd_pcm_new(struct snd_card *card, const char *id, int device,int playback_count, int capture_count, struct snd_pcm **rpcm)
{return _snd_pcm_new(card, id, device, playback_count, capture_count,false, rpcm);
}

首先调用_snd_pcm_new来把pcm设备加入到card中，然后card在注册的时候调用pcm的注册函数，把pcm注册到系统中

3.1 创建pcm设备，加入到card中

(sound/core/pcm.c)

static int _snd_pcm_new(struct snd_card *card, const char *id, int device,int playback_count, int capture_count, bool internal,struct snd_pcm **rpcm)
{struct snd_pcm *pcm;int err;static struct snd_device_ops ops = {.dev_free = snd_pcm_dev_free,.dev_register = snd_pcm_dev_register,--------------pcm注册函数（card注册时调用）.dev_disconnect = snd_pcm_dev_disconnect,};if (snd_BUG_ON(!card))return -ENXIO;if (rpcm)*rpcm = NULL;pcm = kzalloc(sizeof(*pcm), GFP_KERNEL);if (pcm == NULL) {snd_printk(KERN_ERR "Cannot allocate PCM\n");return -ENOMEM;}pcm->card = card;pcm->device = device;pcm->internal = internal;if (id)strlcpy(pcm->id, id, sizeof(pcm->id));# snd_pcm_new_stream主要是初始化snd_pcm_substream结构体if ((err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_PLAYBACK, playback_count)) < 0) {snd_pcm_free(pcm);return err;}if ((err = snd_pcm_new_stream(pcm, SNDRV_PCM_STREAM_CAPTURE, capture_count)) < 0) {snd_pcm_free(pcm);return err;}mutex_init(&pcm->open_mutex);init_waitqueue_head(&pcm->open_wait);if ((err = snd_device_new(card, SNDRV_DEV_PCM, pcm, &ops)) < 0) {-----将pcm设备加入到card的devices链表中snd_pcm_free(pcm);return err;}if (rpcm)*rpcm = pcm;return 0;
}

3.2 进行pcm设备的注册

(sound/core/pcm.c)

static int snd_pcm_dev_register(struct snd_device *device)
{int cidx, err;struct snd_pcm_substream *substream;struct snd_pcm_notify *notify;char str[16];struct snd_pcm *pcm;struct device *dev;if (snd_BUG_ON(!device || !device->device_data))return -ENXIO;pcm = device->device_data;mutex_lock(&register_mutex);err = snd_pcm_add(pcm);if (err) {mutex_unlock(&register_mutex);return err;}for (cidx = 0; cidx < 2; cidx++) {int devtype = -1;if (pcm->streams[cidx].substream == NULL || pcm->internal)continue;switch (cidx) {case SNDRV_PCM_STREAM_PLAYBACK:-------------为device命名sprintf(str, "pcmC%iD%ip", pcm->card->number, pcm->device);devtype = SNDRV_DEVICE_TYPE_PCM_PLAYBACK;break;case SNDRV_PCM_STREAM_CAPTURE:sprintf(str, "pcmC%iD%ic", pcm->card->number, pcm->device);devtype = SNDRV_DEVICE_TYPE_PCM_CAPTURE;break;}/* device pointer to use, pcm->dev takes precedence if* it is assigned, otherwise fall back to card's device* if possible */dev = pcm->dev;if (!dev)dev = snd_card_get_device_link(pcm->card);/* register pcm */err = snd_register_device_for_dev(devtype, pcm->card,pcm->device,&snd_pcm_f_ops[cidx],-----pcm设备文件操作函数pcm, str, dev);if (err < 0) {list_del(&pcm->list);mutex_unlock(&register_mutex);return err;}snd_add_device_sysfs_file(devtype, pcm->card, pcm->device,&pcm_attrs);-------pcm设备注册for (substream = pcm->streams[cidx].substream; substream; substream = substream->next)snd_pcm_timer_init(substream);}list_for_each_entry(notify, &snd_pcm_notify_list, list)notify->n_register(pcm);mutex_unlock(&register_mutex);return 0;
}

4.pcm文件操作ops

pcm这块比较难的一点就是这些操作函数了，各种ioctl设置的参数需要对音频技术这块有深入了解，本文就不介绍了，因为core层的东西对于驱动开发来说很少改动（或者基本不改动），遇到问题再解决吧

(sound/core/pcm_native.c)

const struct file_operations snd_pcm_f_ops[2] = {{.owner =        THIS_MODULE,.write =        snd_pcm_write,.aio_write =        snd_pcm_aio_write,.open =         snd_pcm_playback_open,.release =      snd_pcm_release,.llseek =       no_llseek,.poll =         snd_pcm_playback_poll,.unlocked_ioctl =   snd_pcm_playback_ioctl,.compat_ioctl =     snd_pcm_ioctl_compat,.mmap =         snd_pcm_mmap,.fasync =       snd_pcm_fasync,.get_unmapped_area =    snd_pcm_get_unmapped_area,},{.owner =        THIS_MODULE,.read =         snd_pcm_read,.aio_read =     snd_pcm_aio_read,.open =         snd_pcm_capture_open,.release =      snd_pcm_release,.llseek =       no_llseek,.poll =         snd_pcm_capture_poll,.unlocked_ioctl =   snd_pcm_capture_ioctl,.compat_ioctl =     snd_pcm_ioctl_compat,.mmap =         snd_pcm_mmap,.fasync =       snd_pcm_fasync,.get_unmapped_area =    snd_pcm_get_unmapped_area,}
};

4.1 open函数

snd_pcm_capture_open和snd_pcm_playback_open函数最后都会调用snd_pcm_open，这里不详细介绍open的过程了，只介绍下这边引出来的另一个结构体：snd_pcm_runtime

这个结构体只是在运行的时候会动态创建，具体可以参考函数snd_pcm_attach_substream
此结构体主要是设置各种参数，保存运行时的状态等

struct snd_pcm_runtime {/* -- Status -- */struct snd_pcm_substream *trigger_master;struct timespec trigger_tstamp; /* trigger timestamp */int overrange;snd_pcm_uframes_t avail_max;snd_pcm_uframes_t hw_ptr_base;  /* Position at buffer restart */snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time */unsigned long hw_ptr_jiffies;   /* Time when hw_ptr is updated */unsigned long hw_ptr_buffer_jiffies; /* buffer time in jiffies */snd_pcm_sframes_t delay;    /* extra delay; typically FIFO size */u64 hw_ptr_wrap;                /* offset for hw_ptr due to boundary wrap-around *//* -- HW params -- */snd_pcm_access_t access;    /* access mode */snd_pcm_format_t format;    /* SNDRV_PCM_FORMAT_* */snd_pcm_subformat_t subformat;  /* subformat */unsigned int rate;      /* rate in Hz */unsigned int channels;      /* channels */snd_pcm_uframes_t period_size;  /* period size */unsigned int periods;       /* periods */snd_pcm_uframes_t buffer_size;  /* buffer size */snd_pcm_uframes_t min_align;    /* Min alignment for the format */size_t byte_align;unsigned int frame_bits;unsigned int sample_bits;unsigned int info;unsigned int rate_num;unsigned int rate_den;unsigned int no_period_wakeup: 1;/* -- SW params -- */int tstamp_mode;        /* mmap timestamp is updated */unsigned int period_step;snd_pcm_uframes_t start_threshold;snd_pcm_uframes_t stop_threshold;snd_pcm_uframes_t silence_threshold; /* Silence filling happens whennoise is nearest than this */snd_pcm_uframes_t silence_size; /* Silence filling size */snd_pcm_uframes_t boundary; /* pointers wrap point */snd_pcm_uframes_t silence_start; /* starting pointer to silence area */snd_pcm_uframes_t silence_filled; /* size filled with silence */union snd_pcm_sync_id sync; /* hardware synchronization ID *//* -- mmap -- */struct snd_pcm_mmap_status *status;struct snd_pcm_mmap_control *control;/* -- locking / scheduling -- */snd_pcm_uframes_t twake;    /* do transfer (!poll) wakeup if non-zero */wait_queue_head_t sleep;    /* poll sleep */wait_queue_head_t tsleep;   /* transfer sleep */struct fasync_struct *fasync;/* -- private section -- */void *private_data;void (*private_free)(struct snd_pcm_runtime *runtime);/* -- hardware description -- */struct snd_pcm_hardware hw;struct snd_pcm_hw_constraints hw_constraints;/* -- interrupt callbacks -- */void (*transfer_ack_begin)(struct snd_pcm_substream *substream);void (*transfer_ack_end)(struct snd_pcm_substream *substream);/* -- timer -- */unsigned int timer_resolution;  /* timer resolution */int tstamp_type;        /* timestamp type *//* -- DMA -- */           unsigned char *dma_area;    /* DMA area */dma_addr_t dma_addr;        /* physical bus address (not accessible from main CPU) */size_t dma_bytes;       /* size of DMA area */struct snd_dma_buffer *dma_buffer_p;    /* allocated buffer */#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)/* -- OSS things -- */struct snd_pcm_oss_runtime oss;
#endif#ifdef CONFIG_SND_PCM_XRUN_DEBUGstruct snd_pcm_hwptr_log *hwptr_log;
#endif
};

5.change log

date	content	linux
2017.12.8	origin	linux3.10

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