ALSA: compress: document the compress audio state machine

So we had some discussions of the stream states, so I thought it is a
good idea to document the state transitions, so add it documentation
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20200629134737.105993-2-vkoul@kernel.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/designs/compress-offload.rst

@@ -151,6 +151,57 @@ Modifications include:
 - Addition of encoding options when required (derived from OpenMAX IL)
 - Addition of rateControlSupported (missing in OpenMAX AL)
 
+State Machine
+=============
+
+The compressed audio stream state machine is described below ::
+
+                                        +----------+
+                                        |          |
+                                        |   OPEN   |
+                                        |          |
+                                        +----------+
+                                             |
+                                             |
+                                             | compr_set_params()
+                                             |
+                                             v
+         compr_free()                  +----------+
+  +------------------------------------|          |
+  |                                    |   SETUP  |
+  |          +-------------------------|          |<-------------------------+
+  |          |       compr_write()     +----------+                          |
+  |          |                              ^                                |
+  |          |                              | compr_drain_notify()           |
+  |          |                              |        or                      |
+  |          |                              |     compr_stop()               |
+  |          |                              |                                |
+  |          |                         +----------+                          |
+  |          |                         |          |                          |
+  |          |                         |   DRAIN  |                          |
+  |          |                         |          |                          |
+  |          |                         +----------+                          |
+  |          |                              ^                                |
+  |          |                              |                                |
+  |          |                              | compr_drain()                  |
+  |          |                              |                                |
+  |          v                              |                                |
+  |    +----------+                    +----------+                          |
+  |    |          |    compr_start()   |          |        compr_stop()      |
+  |    | PREPARE  |------------------->|  RUNNING |--------------------------+
+  |    |          |                    |          |                          |
+  |    +----------+                    +----------+                          |
+  |          |                            |    ^                             |
+  |          |compr_free()                |    |                             |
+  |          |              compr_pause() |    | compr_resume()              |
+  |          |                            |    |                             |
+  |          v                            v    |                             |
+  |    +----------+                   +----------+                           |
+  |    |          |                   |          |         compr_stop()      |
+  +--->|   FREE   |                   |  PAUSE   |---------------------------+
+       |          |                   |          |
+       +----------+                   +----------+
+
 
 Gapless Playback
 ================