Full Code Example
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## Full Code Example

The example code below does not include any hardware access routines but shows only the skeleton, how to build up the PCM interfaces.

Example 5.1. PCM Example Code

      #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 = chip;
              strcpy(pcm->name, "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;
      }

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