ASoC: snd_soc_dai_ops trigger function description

Add a comment to the trigger function in snd_soc_dai_ops struct about
possible command sequences.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>

include/sound/soc-dai.h

@@ -166,6 +166,13 @@ struct snd_soc_dai_ops {
 		struct snd_soc_dai *);
 	int (*prepare)(struct snd_pcm_substream *,
 		struct snd_soc_dai *);
+	/*
+	 * NOTE: Commands passed to the trigger function are not necessarily
+	 * compatible with the current state of the dai. For example this
+	 * sequence of commands is possible: START STOP STOP.
+	 * So do not unconditionally use refcounting functions in the trigger
+	 * function, e.g. clk_enable/disable.
+	 */
 	int (*trigger)(struct snd_pcm_substream *, int,
 		struct snd_soc_dai *);
 	int (*bespoke_trigger)(struct snd_pcm_substream *, int,