ASoC: SOF: PM: add state machine to comments

Add Audio DSP state machine with comments. Note that the
'D0<-->runtime D0I3' part is not implemented yet.
Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191111223343.19986-2-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/pm.c

@@ -428,6 +428,38 @@ int snd_sof_set_d0_substate(struct snd_sof_dev *sdev,
 }
 EXPORT_SYMBOL(snd_sof_set_d0_substate);
 
+/*
+ * Audio DSP states may transform as below:-
+ *
+ *                                         D0I3 compatible stream
+ *     Runtime    +---------------------+   opened only, timeout
+ *     suspend    |                     +--------------------+
+ *   +------------+       D0(active)    |                    |
+ *   |            |                     <---------------+    |
+ *   |   +-------->                     |               |    |
+ *   |   |Runtime +--^--+---------^--+--+ The last      |    |
+ *   |   |resume     |  |         |  |    opened D0I3   |    |
+ *   |   |           |  |         |  |    compatible    |    |
+ *   |   |     resume|  |         |  |    stream closed |    |
+ *   |   |      from |  | D3      |  |                  |    |
+ *   |   |       D3  |  |suspend  |  | d0i3             |    |
+ *   |   |           |  |         |  |suspend           |    |
+ *   |   |           |  |         |  |                  |    |
+ *   |   |           |  |         |  |                  |    |
+ * +-v---+-----------+--v-------+ |  |           +------+----v----+
+ * |                            | |  +----------->                |
+ * |       D3 (suspended)       | |              |      D0I3      +-----+
+ * |                            | +--------------+                |     |
+ * |                            |  resume from   |                |     |
+ * +-------------------^--------+  d0i3 suspend  +----------------+     |
+ *                     |                                                |
+ *                     |                       D3 suspend               |
+ *                     +------------------------------------------------+
+ *
+ * d0i3_suspend = s0_suspend && D0I3 stream opened,
+ * D3 suspend = !d0i3_suspend,
+ */
+
 int snd_sof_resume(struct device *dev)
 {
 	struct snd_sof_dev *sdev = dev_get_drvdata(dev);