Currently, each SSAM event notifier is directly tied to one group of
events. This makes sense as registering a notifier will automatically
take care of enabling the corresponding event group and normally drivers
only need notifications for a very limited number of events, associated
with different callbacks for each group.

However, there are rare cases, especially for debugging, when we want to
get notifications for a whole event target category instead of just a
single group of events in that category. Registering multiple notifiers,
i.e. one per group, may be infeasible due to two issues: a) we might not
know every event enable/disable specification as some events are
auto-enabled by the EC and b) forwarding this to the same callback will
lead to duplicate events as we might not know the full event
specification to perform the appropriate filtering.

This commit introduces observer-notifiers, which are notifiers that are
not tied to a specific event group and do not attempt to manage any
events. In other words, they can be registered without enabling any
event group or incrementing the corresponding reference count and just
act as silent observers, listening to all currently/previously enabled
events based on their match-specification.

Essentially, this allows us to register one single notifier for a full
event target category, meaning that we can process all events of that
target category in a single callback without duplication. Specifically,
this will be used in the cdev debug interface to forward events to
user-space via a device file from which the events can be read.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210604134755.535590-2-luzmaximilian@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>