Chapter 9. Powersave support

mac80211 has support for various powersave implementations.

First, it can support hardware that handles all powersaving by itself, such hardware should simply set the IEEE80211_HW_SUPPORTS_PS hardware flag. In that case, it will be told about the desired powersave mode with the IEEE80211_CONF_PS flag depending on the association status. The hardware must take care of sending nullfunc frames when necessary, i.e. when entering and leaving powersave mode. The hardware is required to look at the AID in beacons and signal to the AP that it woke up when it finds traffic directed to it.

IEEE80211_CONF_PS flag enabled means that the powersave mode defined in IEEE 802.11-2007 section 11.2 is enabled. This is not to be confused with hardware wakeup and sleep states. Driver is responsible for waking up the hardware before issuing commands to the hardware and putting it back to sleep at appropriate times.

When PS is enabled, hardware needs to wakeup for beacons and receive the buffered multicast/broadcast frames after the beacon. Also it must be possible to send frames and receive the acknowledment frame.

Other hardware designs cannot send nullfunc frames by themselves and also need software support for parsing the TIM bitmap. This is also supported by mac80211 by combining the IEEE80211_HW_SUPPORTS_PS and IEEE80211_HW_PS_NULLFUNC_STACK flags. The hardware is of course still required to pass up beacons. The hardware is still required to handle waking up for multicast traffic; if it cannot the driver must handle that as best as it can, mac80211 is too slow to do that.

Dynamic powersave is an extension to normal powersave in which the hardware stays awake for a user-specified period of time after sending a frame so that reply frames need not be buffered and therefore delayed to the next wakeup. It's compromise of getting good enough latency when there's data traffic and still saving significantly power in idle periods.

Dynamic powersave is simply supported by mac80211 enabling and disabling PS based on traffic. Driver needs to only set IEEE80211_HW_SUPPORTS_PS flag and mac80211 will handle everything automatically. Additionally, hardware having support for the dynamic PS feature may set the IEEE80211_HW_SUPPORTS_DYNAMIC_PS flag to indicate that it can support dynamic PS mode itself. The driver needs to look at the dynamic_ps_timeout hardware configuration value and use it that value whenever IEEE80211_CONF_PS is set. In this case mac80211 will disable dynamic PS feature in stack and will just keep IEEE80211_CONF_PS enabled whenever user has enabled powersave.

Driver informs U-APSD client support by enabling IEEE80211_VIF_SUPPORTS_UAPSD flag. The mode is configured through the uapsd parameter in conf_tx operation. Hardware needs to send the QoS Nullfunc frames and stay awake until the service period has ended. To utilize U-APSD, dynamic powersave is disabled for voip AC and all frames from that AC are transmitted with powersave enabled.

Note: U-APSD client mode is not yet supported with IEEE80211_HW_PS_NULLFUNC_STACK.