mac80211: document TX powersave filter requirements

This documents what's required to implement that TX powersave
filter properly wrt. handling hardware queues.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 467eed71be22..cd4eb20f98f5 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -239,7 +239,14 @@ struct ieee80211_bss_conf {
  * @IEEE80211_TX_CTL_AMPDU: this frame should be sent as part of an A-MPDU
  * @IEEE80211_TX_CTL_INJECTED: Frame was injected, internal to mac80211.
  * @IEEE80211_TX_STAT_TX_FILTERED: The frame was not transmitted
- *      because the destination STA was in powersave mode.
+ *      because the destination STA was in powersave mode. Note that to
+ *      avoid race conditions, the filter must be set by the hardware or
+ *      firmware upon receiving a frame that indicates that the station
+ *      went to sleep (must be done on device to filter frames already on
+ *      the queue) and may only be unset after mac80211 gives the OK for
+ *      that by setting the IEEE80211_TX_CTL_CLEAR_PS_FILT (see above),
+ *      since only then is it guaranteed that no more frames are in the
+ *      hardware queue.
  * @IEEE80211_TX_STAT_ACK: Frame was acknowledged
  * @IEEE80211_TX_STAT_AMPDU: The frame was aggregated, so status
  *      is for the whole aggregation.