在介绍PCM 之前，我们先给出创建PCM实例的框架。

  #include <sound/pcm.h>
  ....

  /* hardware definition */
  static struct snd_pcm_hardware snd_mychip_playback_hw = {
          .info = (SNDRV_PCM_INFO_MMAP |
                   SNDRV_PCM_INFO_INTERLEAVED |
                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
                   SNDRV_PCM_INFO_MMAP_VALID),
          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
          .rates =            SNDRV_PCM_RATE_8000_48000,
          .rate_min =         8000,
          .rate_max =         48000,
          .channels_min =     2,
          .channels_max =     2,
          .buffer_bytes_max = 32768,
          .period_bytes_min = 4096,
          .period_bytes_max = 32768,
          .periods_min =      1,
          .periods_max =      1024,
  };

  /* hardware definition */
  static struct snd_pcm_hardware snd_mychip_capture_hw = {
          .info = (SNDRV_PCM_INFO_MMAP |
                   SNDRV_PCM_INFO_INTERLEAVED |
                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
                   SNDRV_PCM_INFO_MMAP_VALID),
          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
          .rates =            SNDRV_PCM_RATE_8000_48000,
          .rate_min =         8000,
          .rate_max =         48000,
          .channels_min =     2,
          .channels_max =     2,
          .buffer_bytes_max = 32768,
          .period_bytes_min = 4096,
          .period_bytes_max = 32768,
          .periods_min =      1,
          .periods_max =      1024,
  };

  /* open callback */
  static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          runtime->hw = snd_mychip_playback_hw;
          /* more hardware-initialization will be done here */
          ....
          return 0;
  }

  /* close callback */
  static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          /* the hardware-specific codes will be here */
          ....
          return 0;

  }

  /* open callback */
  static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          runtime->hw = snd_mychip_capture_hw;
          /* more hardware-initialization will be done here */
          ....
          return 0;
  }

  /* close callback */
  static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          /* the hardware-specific codes will be here */
          ....
          return 0;

  }

  /* hw_params callback */
  static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
                               struct snd_pcm_hw_params *hw_params)
  {
          return snd_pcm_lib_malloc_pages(substream,
                                     params_buffer_bytes(hw_params));
  }

  /* hw_free callback */
  static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
  {
          return snd_pcm_lib_free_pages(substream);
  }

  /* prepare callback */
  static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          /* set up the hardware with the current configuration
           * for example...
           */
          mychip_set_sample_format(chip, runtime->format);
          mychip_set_sample_rate(chip, runtime->rate);
          mychip_set_channels(chip, runtime->channels);
          mychip_set_dma_setup(chip, runtime->dma_addr,
                               chip->buffer_size,
                               chip->period_size);
          return 0;
  }

  /* trigger callback */
  static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
                                    int cmd)
  {
          switch (cmd) {
          case SNDRV_PCM_TRIGGER_START:
                  /* do something to start the PCM engine */
                  ....
                  break;
          case SNDRV_PCM_TRIGGER_STOP:
                  /* do something to stop the PCM engine */
                  ....
                  break;
          default:
                  return -EINVAL;
          }
  }

  /* pointer callback */
  static snd_pcm_uframes_t
  snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          unsigned int current_ptr;

          /* get the current hardware pointer */
          current_ptr = mychip_get_hw_pointer(chip);
          return current_ptr;
  }

  /* operators */
  static struct snd_pcm_ops snd_mychip_playback_ops = {
          .open =        snd_mychip_playback_open,
          .close =       snd_mychip_playback_close,
          .ioctl =       snd_pcm_lib_ioctl,
          .hw_params =   snd_mychip_pcm_hw_params,
          .hw_free =     snd_mychip_pcm_hw_free,
          .prepare =     snd_mychip_pcm_prepare,
          .trigger =     snd_mychip_pcm_trigger,
          .pointer =     snd_mychip_pcm_pointer,
  };

  /* operators */
  static struct snd_pcm_ops snd_mychip_capture_ops = {
          .open =        snd_mychip_capture_open,
          .close =       snd_mychip_capture_close,
          .ioctl =       snd_pcm_lib_ioctl,
          .hw_params =   snd_mychip_pcm_hw_params,
          .hw_free =     snd_mychip_pcm_hw_free,
          .prepare =     snd_mychip_pcm_prepare,
          .trigger =     snd_mychip_pcm_trigger,
          .pointer =     snd_mychip_pcm_pointer,
  };

  /*
   *  definitions of capture are omitted here...
   */

  /* create a pcm device */
  static int snd_mychip_new_pcm(struct mychip *chip)
  {
          struct snd_pcm *pcm;
          int err;

          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
          if (err < 0) 
                  return err;
          pcm->private_data = http://www.mamicode.com/chip;"My Chip");
          chip->pcm = pcm;
          /* set operators */
          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
                          &snd_mychip_playback_ops);
          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
                          &snd_mychip_capture_ops);
          /* pre-allocation of buffers */
          /* NOTE: this may fail */
          snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
                                                snd_dma_pci_data(chip->pci),
                                                64*1024, 64*1024);
          return 0;
  }

1.创建pcm实例

ALSA driver为我们提供接口snd_pcm_new来创建PCM实例。但是我们最好是写一个如上述snd_mychip_new_pcm的函数来来对构建pcm实例的过程进行封装。

/**
* snd_pcm_new - create a new PCM instance
* @card: the card instance
* @id: the id string
* @device: the device index (zero based)
* @playback_count: the number of substreams for playback
* @capture_count: the number of substreams for capture
* @rpcm: the pointer to store the new pcm instance
*/
int snd_pcm_new(struct snd_card *card, const char *id, int device, int playback_count, int capture_count, struct snd_pcm **rpcm)

第三个参数表示新创建的PCM实例的index(0,1,2,3).可以在一个card上创建多个PCM 实例。每一个PCM又可以包含多个substream.如果芯片支持多路播放，那么将有多个substream.每次open/close都作用于某个substream.在创建PCM的substream时就指定了number（0~playback_count）.当App在调用alsa lib API:snd_pcm_open时，alsa core通过snd_pcm_attach_substream函数来open一个空闲的substream.

 2.设置PCM的操作函数

创建完PCM函数后，就可设置PCM 的操作函数。

  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
                  &snd_mychip_playback_ops);
  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
                  &snd_mychip_capture_ops);

操作函数即我们写driver时需要实现的功能，以供alsa-core调用。ALSA PCM的操作函数包括：

  static struct snd_pcm_ops snd_mychip_playback_ops = {
          .open =        snd_mychip_pcm_open,
          .close =       snd_mychip_pcm_close,
          .ioctl =       snd_pcm_lib_ioctl,
          .hw_params =   snd_mychip_pcm_hw_params,
          .hw_free =     snd_mychip_pcm_hw_free,
          .prepare =     snd_mychip_pcm_prepare,
          .trigger =     snd_mychip_pcm_trigger,
          .pointer =     snd_mychip_pcm_pointer,
  };

每个函数都包含一个snd_pcm_substream 的指针，指向当前操作的substream.

在上面的例子中，每个操作函数里面都包含如下宏调用：

其中返回的是substream->private_data,sustream的private_data是pcm->private_data的一份拷贝。拷贝动作是在snd_pcm_open时调用的snd_pcm_attach_substream中进行。一般来说pcm的private_data是芯片专用数据，当然我们也可以overwrite以保存别的数据。

2.1 open

当open PCM的一路substream时，open函数就会被调用。

  static int snd_xxx_open(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          runtime->hw = snd_mychip_playback_hw;
          return 0;
  }

在open函数内，至少应该初始化此substream的runtime->hw结构体.snd_mychip_playback_hw是预先定义的硬件描述。

ALSA driver--PCM