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In the spec, CSA is defined roughly as follows:
 - TBTT x:     beacon with CSA, count=n   (old channel)
 - TBTT x+1:   beacon with CSA, count=n-1 (old channel)
 - TBTT x+n-1: beacon with CSA, count=1   (old channel)
 "A Channel Switch Count field set to 1 indicates that the switch
  occurs immediately before the next TBTT.
 - TBTT x+n:   beacon without CSA         (new channel)

When we detect it, we currently schedule the CSA event to
be at 10 TUs before TBTT x+n-1, for a beacon interval, to
give us quiet time.

When this event *starts*, we currently notify mac80211
that the channel switch happened, which causes us to add
a session protection event to listen for the first beacon
(and enable TX etc. when that arrives).

We don't even ask for a notification when this event ends
so the code that handles that is effectively dead code.

The session protection duration is 3 beacon intervals,
scheduled at 10 TU before TBTT x+n-1. It will thus end
just before TBTT x+n+2.

Unfortunately, if the AP doesn't transmit or we miss just
the first two beacons on the new channel, then this will
cause us to disconnect. Or even just one, if the AP isn't
quite aligned with the TBTT after the switch.

However, listening to the _end_ of the time event isn't
what we want either, because we want all the new PHY and
other config that needs to come from mac80211 to start
early, so we have a head-start for the new channel, since
we're not going to use the old one anyway for this time.

So since we don't really have anything better to do at
this time, and this is relatively rare, just make the
session protection use 5x the beacon interval instead of
just 3x, so it's more likely we catch a beacon even if
the AP neglected to send it, or we just miss it.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230830112059.a74176bac37c.I029a2ebcd1b5012327c728ffa1d33fac19cfdf4b@changeid

Signed-off-by: Johannes Berg <johannes.berg@intel.com>