A BE connected to more than one FE, e.g. in a mixer case, can go
through the following transitions.

play FE1    -> BE state is START
pause FE1   -> BE state is PAUSED
play FE2    -> BE state is START
stop FE2    -> BE state is STOP (see note [1] below)
release FE1 -> BE state is START
stop FE1    -> BE state is STOP

play FE1    -> BE state is START
pause FE1   -> BE state is PAUSED
play FE2    -> BE state is START
release FE1 -> BE state is START
stop FE2    -> BE state is START
stop FE1    -> BE state is STOP

play FE1    -> BE state is START
play FE2    -> BE state is START (no change)
pause FE1   -> BE state is START (no change)
pause FE2   -> BE state is PAUSED
release FE1 -> BE state is START
release FE2 -> BE state is START (no change)
stop FE1    -> BE state is START (no change)
stop FE2    -> BE state is STOP

The existing code for PAUSE_RELEASE only allows for the case where the
BE is paused, which clearly would not work in the sequences above.

Extend the allowed states to restart the BE when PAUSE_RELEASE is
received, and increase the refcount if the BE is already in START.

[1] the existing logic does not move the BE state back to PAUSED when
the FE2 is stopped. This patch does not change the logic; it would be
painful to keep a history of changes on the FE side, the state machine
is already rather complicated with transitions based on the last BE
state and the trigger type.
Reported-by: Bard Liao <bard.liao@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20211207173745.15850-7-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>