[RFC v2 1/2] mac80211: don't transmit beacon with CSA count 0

Luciano Coelho
2013-11-05T20:55:00Z

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.

Cc: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
---
In v2, updated the documentation to reflect the change.
---
include/net/mac80211.h | 10 ++++++----
net/mac80211/tx.c | 10 +++++++---
2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index f386c48..2d0c6ab 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -2676,11 +2676,13 @@ 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
@@ -3379,13 +3381,13 @@ 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.
diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 9993fcb..1e0d40f 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -2376,8 +2376,12 @@ static void ieee80211_update_csa(struct ieee80211_sub_if_data *sdata,
if (WARN_ON(counter_offset_beacon >= beacon_data_len))
return;

- /* warn if the driver did not check for/react to csa completeness */
- if (WARN_ON(beacon_data[counter_offset_beacon] == 0))
+ /* Warn if the driver did not check for/react to csa
+ * completeness. A beacon with CSA counter set to 0 should
+ * never occur, because a counter of 1 means switch just
+ * before the next beacon.
+ */
+ if (WARN_ON(beacon_data[counter_offset_beacon] == 1))
return;

beacon_data[counter_offset_beacon]--;
@@ -2434,7 +2438,7 @@ bool ieee80211_csa_is_complete(struct ieee80211_vif *vif)
if (WARN_ON(counter_beacon > beacon_data_len))
goto out;

- if (beacon_data[counter_beacon] == 0)
+ if (beacon_data[counter_beacon] == 1)
ret = true;
out:
rcu_read_unlock();
--
1.8.4.rc3