mac80211: document client powersave

With the addition of uAPSD and driver buffering
the powersave handling has gotten quite complex.
Add a section to the documentation to explain it
for anyone wanting to implement it.

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

1 files changed, 89 insertions, 0 deletions

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 3df32a0..bc79930 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -1539,6 +1539,95 @@ ieee80211_get_alt_retry_rate(const struct ieee80211_hw *hw,
  */
 
 /**
+ * DOC: AP support for powersaving clients
+ *
+ * In order to implement AP and P2P GO modes, mac80211 has support for
+ * client powersaving, both "legacy" PS (PS-Poll/null data) and uAPSD.
+ * There currently is no support for sAPSD.
+ *
+ * There is one assumption that mac80211 makes, namely that a client
+ * will not poll with PS-Poll and trigger with uAPSD at the same time.
+ * Both are supported, and both can be used by the same client, but
+ * they can't be used concurrently by the same client. This simplifies
+ * the driver code.
+ *
+ * The first thing to keep in mind is that there is a flag for complete
+ * driver implementation: %IEEE80211_HW_AP_LINK_PS. If this flag is set,
+ * mac80211 expects the driver to handle most of the state machine for
+ * powersaving clients and will ignore the PM bit in incoming frames.
+ * Drivers then use ieee80211_sta_ps_transition() to inform mac80211 of
+ * stations' powersave transitions. In this mode, mac80211 also doesn't
+ * handle PS-Poll/uAPSD.
+ *
+ * In the mode without %IEEE80211_HW_AP_LINK_PS, mac80211 will check the
+ * PM bit in incoming frames for client powersave transitions. When a
+ * station goes to sleep, we will stop transmitting to it. There is,
+ * however, a race condition: a station might go to sleep while there is
+ * data buffered on hardware queues. If the device has support for this
+ * it will reject frames, and the driver should give the frames back to
+ * mac80211 with the %IEEE80211_TX_STAT_TX_FILTERED flag set which will
+ * cause mac80211 to retry the frame when the station wakes up. The
+ * driver is also notified of powersave transitions by calling its
+ * @sta_notify callback.
+ *
+ * When the station is asleep, it has three choices: it can wake up,
+ * it can PS-Poll, or it can possibly start a uAPSD service period.
+ * Waking up is implemented by simply transmitting all buffered (and
+ * filtered) frames to the station. This is the easiest case. When
+ * the station sends a PS-Poll or a uAPSD trigger frame, mac80211
+ * will inform the driver of this with the @allow_buffered_frames
+ * callback; this callback is optional. mac80211 will then transmit
+ * the frames as usual and set the %IEEE80211_TX_CTL_POLL_RESPONSE
+ * on each frame. The last frame in the service period (or the only
+ * response to a PS-Poll) also has %IEEE80211_TX_STATUS_EOSP set to
+ * indicate that it ends the service period; as this frame must have
+ * TX status report it also sets %IEEE80211_TX_CTL_REQ_TX_STATUS.
+ * When TX status is reported for this frame, the service period is
+ * marked has having ended and a new one can be started by the peer.
+ *
+ * Another race condition can happen on some devices like iwlwifi
+ * when there are frames queued for the station and it wakes up
+ * or polls; the frames that are already queued could end up being
+ * transmitted first instead, causing reordering and/or wrong
+ * processing of the EOSP. The cause is that allowing frames to be
+ * transmitted to a certain station is out-of-band communication to
+ * the device. To allow this problem to be solved, the driver can
+ * call ieee80211_sta_block_awake() if frames are buffered when it
+ * is notified that the station went to sleep. When all these frames
+ * have been filtered (see above), it must call the function again
+ * to indicate that the station is no longer blocked.
+ *
+ * If the driver buffers frames in the driver for aggregation in any
+ * way, it must use the ieee80211_sta_set_buffered() call when it is
+ * notified of the station going to sleep to inform mac80211 of any
+ * TIDs that have frames buffered. Note that when a station wakes up
+ * this information is reset (hence the requirement to call it when
+ * informed of the station going to sleep). Then, when a service
+ * period starts for any reason, @release_buffered_frames is called
+ * with the number of frames to be released and which TIDs they are
+ * to come from. In this case, the driver is responsible for setting
+ * the EOSP (for uAPSD) and MORE_DATA bits in the released frames,
+ * to help the @more_data paramter is passed to tell the driver if
+ * there is more data on other TIDs -- the TIDs to release frames
+ * from are ignored since mac80211 doesn't know how many frames the
+ * buffers for those TIDs contain.
+ *
+ * If the driver also implement GO mode, where absence periods may
+ * shorten service periods (or abort PS-Poll responses), it must
+ * filter those response frames except in the case of frames that
+ * are buffered in the driver -- those must remain buffered to avoid
+ * reordering. Because it is possible that no frames are released
+ * in this case, the driver must call ieee80211_sta_eosp_irqsafe()
+ * to indicate to mac80211 that the service period ended anyway.
+ *
+ * Finally, if frames from multiple TIDs are released from mac80211
+ * but the driver might reorder them, it must clear & set the flags
+ * appropriately (only the last frame may have %IEEE80211_TX_STATUS_EOSP)
+ * and also take care of the EOSP and MORE_DATA bits in the frame.
+ * The driver may also use ieee80211_sta_eosp_irqsafe() in this case.
+ */
+
+/**
  * enum ieee80211_filter_flags - hardware filter flags
  *
  * These flags determine what the filter in hardware should be