启动power_check

static int soc_pcm_prepare(struct snd_pcm_substream *substream)
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		snd_soc_dapm_stream_event(rtd,
					  codec_dai->driver->playback.stream_name,
					  SND_SOC_DAPM_STREAM_START);
	else
		snd_soc_dapm_stream_event(rtd,
					  codec_dai->driver->capture.stream_name,
					  SND_SOC_DAPM_STREAM_START);

int snd_soc_dapm_stream_event(struct snd_soc_pcm_runtime *rtd,
	const char *stream, int event)
	soc_dapm_stream_event(&codec->dapm, stream, event);
		dapm_power_widgets(dapm, event);
			
			list_for_each_entry(w, &card->widgets, list) {
				if (!w->force)
					power = w->power_check(w);	// 执行power_check
				// 放到up_list上
				if (power)
					dapm_seq_insert(w, &up_list, true);
				else
					dapm_seq_insert(w, &down_list, false);
				
				// 对up_list上的widget上电
				dapm_seq_run(dapm, &up_list, event, true);
					list_for_each_entry_safe(w, n, list, power_list) {	// 利用list的地址得到widget地址
						if (!list_empty(&pending))
							dapm_seq_run_coalesced(cur_dapm, &pending);
								reg = list_first_entry(pending, struct snd_soc_dapm_widget,
										       power_list)->reg;
								snd_soc_update_bits(dapm->codec, reg, mask, value);	// 更新reg
					}
			}