mac80211: only set CSA beacon when at least one beacon must be transmitted

A beacon should never have a Channel Switch Announcement information
element with a count of 0, because a count of 1 means switch just
before the next beacon.  So, if a count of 0 was valid in a beacon, it
would have been transmitted in the next channel already, which is
useless.  A CSA count equal to zero is only meaningful in action
frames or probe_responses.

Fix the ieee80211_csa_is_complete() and ieee80211_update_csa()
functions accordingly.

With a CSA count of 0, we won't transmit any CSA beacons, because the
switch will happen before the next TBTT.  To avoid extra work and
potential confusion in the drivers, complete the CSA immediately,
instead of waiting for the driver to call ieee80211_csa_finish().

To keep things simpler, we also switch immediately when the CSA count
is 1, while in theory we should delay the switch until just before the
next TBTT.

Additionally, move the ieee80211_csa_finish() function to cfg.c,
where it makes more sense.

Tested-by: Simon Wunderlich <sw@simonwunderlich.de>
Acked-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index f4ab2fb4d50c..df1004be7ba5 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -2750,11 +2750,13 @@ enum ieee80211_roc_type {
  * @channel_switch_beacon: Starts a channel switch to a new channel.
  *	Beacons are modified to include CSA or ECSA IEs before calling this
  *	function. The corresponding count fields in these IEs must be
- *	decremented, and when they reach zero the driver must call
+ *	decremented, and when they reach 1 the driver must call
  *	ieee80211_csa_finish(). Drivers which use ieee80211_beacon_get()
  *	get the csa counter decremented by mac80211, but must check if it is
- *	zero using ieee80211_csa_is_complete() after the beacon has been
+ *	1 using ieee80211_csa_is_complete() after the beacon has been
  *	transmitted and then call ieee80211_csa_finish().
+ *	If the CSA count starts as zero or 1, this function will not be called,
+ *	since there won't be any time to beacon before the switch anyway.
  *
  * @join_ibss: Join an IBSS (on an IBSS interface); this is called after all
  *	information in bss_conf is set up and the beacon can be retrieved. A
@@ -3452,13 +3454,13 @@ static inline struct sk_buff *ieee80211_beacon_get(struct ieee80211_hw *hw,
  * @vif: &struct ieee80211_vif pointer from the add_interface callback.
  *
  * After a channel switch announcement was scheduled and the counter in this
- * announcement hit zero, this function must be called by the driver to
+ * announcement hits 1, this function must be called by the driver to
  * notify mac80211 that the channel can be changed.
  */
 void ieee80211_csa_finish(struct ieee80211_vif *vif);
 
 /**
- * ieee80211_csa_is_complete - find out if counters reached zero
+ * ieee80211_csa_is_complete - find out if counters reached 1
  * @vif: &struct ieee80211_vif pointer from the add_interface callback.
  *
  * This function returns whether the channel switch counters reached zero.