Table of Contents
Some parts of the if_conf should be discussed here instead
Insert notes about VLAN interfaces with hw crypto here or in the hw crypto chapter.
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
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
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
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
callback; this callback is optional. mac80211 will then transmit
the frames as usual and set the
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
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.
Additionally, non-bufferable MMPDUs can also be transmitted by
mac80211 with the
IEEE80211_TX_CTL_NO_PS_BUFFER set in them.
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
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 parameter 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
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
and also take care of the EOSP and MORE_DATA bits in the frame.
The driver may also use
ieee80211_sta_eosp in this case.
Note that if the driver ever buffers frames other than QoS-data frames, it must take care to never send a non-QoS-data frame as the last frame in a service period, adding a QoS-nulldata frame after a non-QoS-data frame if needed.