urb_completion

	frame_bytes = runtime->frame_bits >> 3;
	chunk_length = 4096 / frame_bytes;

        pcm_buffer_size = snd_pcm_lib_buffer_bytes(substream);

        period_elapsed = 0;

	audio->buffer_pos += chunk_length;

	if (audio->buffer_pos >= pcm_buffer_size)
		audio->buffer_pos -= pcm_buffer_size;

	audio->period_pos += chunk_length;
	if (audio->period_pos >= runtime->period_size) {
		audio->period_pos %= runtime->period_size;
		period_elapsed = 1;
	}


	if (period_elapsed)
		snd_pcm_period_elapsed(substream);