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If the value/data associated with a control changes in SOF it will send a
notification (SOF_IPC_GLB_COMP_MSG with SOF_IPC_COMP_GET_VALUE/DATA).

We have support for binary volatile control type, but we might have
features where enum/switch/volume changes. Re-implementing everything as
volatile as well would be not much of a gain for several reasons:
- volatile controls would do an IPC all the time, regardless if there is a
  need or not.
- We still don't have notification which forces userspace to continuously
  poll.

When such notification arrives we use snd_ctl_notify_one() to signal
userspace about the change.

The kernel is prepared for two types of notification:
- the notification carries the new data for the control (num_elems != 0)
The new value/data is copied to the control's local data

- blank message about a change
The new flag for the scontrol (comp_data_dirty) is set and when next
time user space reads the value via the kcontrol's get callback we will
refresh the control's local data from the firmware.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Tested-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
Link: https://lore.kernel.org/r/20210903114018.2962-1-peter.ujfalusi@linux.intel.com

Signed-off-by: Mark Brown <broonie@kernel.org>