__snd_pcm_lib_xfer

/* the common loop for read/write data */
snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
				     void *data, bool interleaved,
				     snd_pcm_uframes_t size, bool in_kernel)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	snd_pcm_uframes_t xfer = 0;
	snd_pcm_uframes_t offset = 0;
	snd_pcm_uframes_t avail;
	pcm_copy_f writer;
	pcm_transfer_f transfer;
	bool nonblock;
	bool is_playback;
	int err;

	err = pcm_sanity_check(substream);
	if (err < 0)
		return err;

	is_playback = substream->stream == SNDRV_PCM_STREAM_PLAYBACK;
	if (interleaved) { interleave 的copy函数 L0R0L1R1
		if (runtime->access != SNDRV_PCM_ACCESS_RW_INTERLEAVED &&
		    runtime->channels > 1)
			return -EINVAL;
		writer = interleaved_copy;
	} else {//non interleave copy函数，L0ROL1R1
		if (runtime->access != SNDRV_PCM_ACCESS_RW_NONINTERLEAVED)
			return -EINVAL;
		writer = noninterleaved_copy;
	}

	if (!data) {
		if (is_playback)
			transfer = fill_silence;//如果没有data，就填0
		else
			return -EINVAL;
	} else if (in_kernel) {
		if (substream->ops->copy_kernel)
			transfer = substream->ops->copy_kernel;
		else
			transfer = is_playback ?
				default_write_copy_kernel : default_read_copy_kernel;
	} else {
		if (substream->ops->copy_user)
			transfer = (pcm_transfer_f)substream->ops->copy_user;//platfrom中定义的copy函数，将data从user space copy到DMA中，然后HW会从DMA中搬数据
		else
			transfer = is_playback ?
				default_write_copy : default_read_copy;
	}

	if (size == 0)
		return 0;

	nonblock = !!(substream->f_flags & O_NONBLOCK);

	snd_pcm_stream_lock_irq(substream);
	err = pcm_accessible_state(runtime);
	if (err < 0)
		goto _end_unlock;

	runtime->twake = runtime->control->avail_min ? : 1;
	if (runtime->status->state == SNDRV_PCM_STATE_RUNNING)//如果已经是播放状态，则更新hw_ptr
		snd_pcm_update_hw_ptr(substream);

	/*
	 * If size < start_threshold, wait indefinitely. Another
	 * thread may start capture
	 */
	if (!is_playback && //capture hw trigger的flow
	    runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
	    size >= runtime->start_threshold) { //capture要都的data size>start_threshold
		err = snd_pcm_start(substream); //启动HW或者trigger HW，开始工作传数据
		if (err < 0)
			goto _end_unlock;
	}

	avail = snd_pcm_avail(substream);//对playback指DMA的space size；对capture是指DMA中的data size

	while (size > 0) {//想要write的data size或者是想要读的data size
		snd_pcm_uframes_t frames, appl_ptr, appl_ofs;
		snd_pcm_uframes_t cont;
		if (!avail) {
			if (!is_playback &&
			    runtime->status->state == SNDRV_PCM_STATE_DRAINING) {
				snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP);
				goto _end_unlock;
			}
			if (nonblock) {
				err = -EAGAIN;
				goto _end_unlock;
			}
			runtime->twake = min_t(snd_pcm_uframes_t, size,
					runtime->control->avail_min ? : 1);
			err = wait_for_avail(substream, &avail);
			if (err < 0)
				goto _end_unlock;
			if (!avail)
				continue; /* draining */
		}
		frames = size > avail ? avail : size;
		appl_ptr = READ_ONCE(runtime->control->appl_ptr);//获取app_ptr
		appl_ofs = appl_ptr % runtime->buffer_size;//app_ptr不能超过buf size
		cont = runtime->buffer_size - appl_ofs;
		if (frames > cont)
			frames = cont;
		if (snd_BUG_ON(!frames)) {
			err = -EINVAL;
			goto _end_unlock;
		}
		snd_pcm_stream_unlock_irq(substream);
		err = writer(substream, appl_ofs, data, offset, frames,//开始写数据到dma
			     transfer);
		snd_pcm_stream_lock_irq(substream);
		if (err < 0)
			goto _end_unlock;
		err = pcm_accessible_state(runtime);
		if (err < 0)
			goto _end_unlock;
		appl_ptr += frames;
		if (appl_ptr >= runtime->boundary)
			appl_ptr -= runtime->boundary;//不能超过boundary
		err = pcm_lib_apply_appl_ptr(substream, appl_ptr);//更新app_ptr
		if (err < 0)
			goto _end_unlock;

		offset += frames;
		size -= frames;
		xfer += frames;
		avail -= frames;
		if (is_playback && //playback，write到dma的data超过start threshold开始trigger HW,开始传数据
		    runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
		    snd_pcm_playback_hw_avail(runtime) >= (snd_pcm_sframes_t)runtime->start_threshold) {
			err = snd_pcm_start(substream);
			if (err < 0)
				goto _end_unlock;
		}
	}
 _end_unlock:
	runtime->twake = 0;
	if (xfer > 0 && err >= 0)
		snd_pcm_update_state(substream, runtime);//更新runtime 的state
	snd_pcm_stream_unlock_irq(substream);
	return xfer > 0 ? (snd_pcm_sframes_t)xfer : err;
}

（1）bounddary的值

static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
			     struct snd_pcm_hw_params *params)
{
	struct snd_pcm_runtime *runtime;
	int err, usecs;
	unsigned int bits;
	snd_pcm_uframes_t frames;

	....

	/* Default sw params */
	runtime->tstamp_mode = SNDRV_PCM_TSTAMP_NONE;
	runtime->period_step = 1;
	runtime->control->avail_min = runtime->period_size;
	runtime->start_threshold = 1;
	runtime->stop_threshold = runtime->buffer_size;
	runtime->silence_threshold = 0;
	runtime->silence_size = 0;
	runtime->boundary = runtime->buffer_size;
	while (runtime->boundary * 2 <= LONG_MAX - runtime->buffer_size)
		runtime->boundary *= 2;//设置long型的最大值，且为buf size的整数倍

（2）更新app_ptr到runtime的变量中

int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
			   snd_pcm_uframes_t appl_ptr)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	snd_pcm_uframes_t old_appl_ptr = runtime->control->appl_ptr;
	int ret;

	if (old_appl_ptr == appl_ptr)
		return 0;

	runtime->control->appl_ptr = appl_ptr;
	if (substream->ops->ack) {
		ret = substream->ops->ack(substream);
		if (ret < 0) {
			runtime->control->appl_ptr = old_appl_ptr;
			return ret;
		}
	}

	trace_applptr(substream, old_appl_ptr, appl_ptr);

	return 0;
}