mac80211.h says: Beacon filter support is advertised with the
IEEE80211_VIF_BEACON_FILTER interface capability. The driver needs to
enable beacon filter support whenever power save is enabled, that is
IEEE80211_CONF_PS is set. When power save is enabled, the stack will
not check for beacon loss and the driver needs to notify about loss
of beacons with ieee80211_beacon_loss().

Some controllers may want to dynamically enable the beacon filter
capabilities on power save entry (CONF_PS) and disable it on exit.
This is the case for the wcn36xx driver which only supports beacon
filtering in PS mode (no CONNECTION_MONITOR support).

When the mac80211 beacon monitor timer expires, the beacon filter
flag must be checked again in case it as been changed in between
(e.g. vif moved to PS mode).
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Link: https://lore.kernel.org/r/1592471863-31402-1-git-send-email-loic.poulain@linaro.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>