ALSA: compress: document the compress gapless audio state machine

Also documented the galpess transitions. Please note that these are not
really stream states, but show how the stream steps in gapless mode
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20200629134737.105993-3-vkoul@kernel.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/designs/compress-offload.rst

@@ -250,6 +250,38 @@ Sequence flow for gapless would be:
 
 (note: order for partial_drain and write for next track can be reversed as well)
 
+Gapless Playback SM
+===================
+
+For Gapless, we move from running state to partial drain and back, along
+with setting of meta_data and signalling for next track ::
+
+
+                                        +----------+
+                compr_drain_notify()    |          |
+              +------------------------>|  RUNNING |
+              |                         |          |
+              |                         +----------+
+              |                              |
+              |                              |
+              |                              | compr_next_track()
+              |                              |
+              |                              V
+              |                         +----------+
+              |                         |          |
+              |                         |NEXT_TRACK|
+              |                         |          |
+              |                         +----------+
+              |                              |
+              |                              |
+              |                              | compr_partial_drain()
+              |                              |
+              |                              V
+              |                         +----------+
+              |                         |          |
+              +------------------------ | PARTIAL_ |
+                                        |  DRAIN   |
+                                        +----------+
 
 Not supported
 =============