+They can be NULL if no special sequence is required. When the resume
+callback is NULL, the driver calls the init callback and resumes the
+registers from the cache. If other handling is needed, you'd need to
+write your own resume callback. There, the amp values can be resumed
+via
+ void snd_hda_codec_resume_amp(struct hda_codec *codec);
+and the other codec registers via
+ void snd_hda_codec_resume_cache(struct hda_codec *codec);
+
+The check_power_status callback is called when the amp value of the
+given widget NID is changed. The codec code can turn on/off the power
+appropriately from this information.