Alsa avail

1 第一种size

static inline snd_pcm_uframes_t
snd_pcm_avail(struct snd_pcm_substream *substream)
{
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		return snd_pcm_playback_avail(substream->runtime);
	else
		return snd_pcm_capture_avail(substream->runtime);
}

static inline snd_pcm_uframes_t snd_pcm_playback_avail(struct snd_pcm_runtime *runtime)
{//是指DMA的space size
	snd_pcm_sframes_t avail = runtime->status->hw_ptr + runtime->buffer_size - runtime->control->appl_ptr;
	if (avail < 0)
		avail += runtime->boundary;
	else if ((snd_pcm_uframes_t) avail >= runtime->boundary)
		avail -= runtime->boundary;
	return avail;
}

static inline snd_pcm_uframes_t snd_pcm_capture_avail(struct snd_pcm_runtime *runtime)
{//是指DMA的data size
	snd_pcm_sframes_t avail = runtime->status->hw_ptr - runtime->control->appl_ptr;
	if (avail < 0)
		avail += runtime->boundary;
	return avail;
}

2 第二种size

static inline snd_pcm_uframes_t
snd_pcm_hw_avail(struct snd_pcm_substream *substream)
{
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		return snd_pcm_playback_hw_avail(substream->runtime);
	else
		return snd_pcm_capture_hw_avail(substream->runtime);
}

static inline snd_pcm_sframes_t snd_pcm_playback_hw_avail(struct snd_pcm_runtime *runtime)
{ //播放flow的，是指DMA的data size
	return runtime->buffer_size - snd_pcm_playback_avail(runtime);
}

static inline snd_pcm_sframes_t snd_pcm_capture_hw_avail(struct snd_pcm_runtime *runtime)
{//capture flow的，是指dma的space size
	return runtime->buffer_size - snd_pcm_capture_avail(runtime);
}