Re: [PATCH] mac80211: Enable power save after receiving NULL packet ACK

[Bassem Dawood <bassem@morsemicro.com>]

On Thu, Apr 08, 2021 at 03:26:06PM +0200, Johannes Berg wrote:
> On Sat, 2021-02-27 at 16:58 +1100, Bassem Dawood wrote:
> > Trigger dynamic_ps_timer to re-evaluate power saving once a null function
> > packet (with PM = 1) is ACKed.
> 
> Can you please say what this fixes/why the change is needed?
> 
> johannes
> 

Sure.

For drivers that declare PS_NULLFUNC_STACK, when sleeping, mac80211 would send a nullfunc (with PS bit set). It won't notify the driver yet (waiting for the NULLFUNC ACK).


a/net/mac80211/mlme.c: void ieee80211_dynamic_ps_enable_work(struct work_struct *work) 

	if (ieee80211_hw_check(&local->hw, PS_NULLFUNC_STACK) &&
	    !(ifmgd->flags & IEEE80211_STA_NULLFUNC_ACKED)) {
		if (drv_tx_frames_pending(local)) {
			mod_timer(&local->dynamic_ps_timer, jiffies +
				  msecs_to_jiffies(
				  local->hw.conf.dynamic_ps_timeout));
		} else {
			ieee80211_send_nullfunc(local, sdata, true);
			/* Flush to get the tx status of nullfunc frame */
			ieee80211_flush_queues(local, sdata, false);
		}
	}

	if (!(ieee80211_hw_check(&local->hw, REPORTS_TX_ACK_STATUS) &&
	      ieee80211_hw_check(&local->hw, PS_NULLFUNC_STACK)) ||
	    (ifmgd->flags & IEEE80211_STA_NULLFUNC_ACKED)) {
		ifmgd->flags &= ~IEEE80211_STA_NULLFUNC_ACKED;
		local->hw.conf.flags |= IEEE80211_CONF_PS;
		ieee80211_hw_config(local, IEEE80211_CONF_CHANGE_PS);
	}

On reception of the ACK, mac80211 will set the  IEEE80211_STA_NULLFUNC_ACKED flag, but won't re-evaluate dynamic_ps_enable_work. So drivers won't get notified to sleep (until ieee80211_dynamic_ps_enable_work is re-evaluated through any other path).

a/net/mac80211/status.c:static void __ieee80211_tx_status(struct ieee80211_hw *hw,
				  struct ieee80211_tx_status *status,
				  int rates_idx, int retry_count) 

	if (ieee80211_is_any_nullfunc(fc) &&
	    ieee80211_has_pm(fc) &&
	    ieee80211_hw_check(&local->hw, REPORTS_TX_ACK_STATUS) &&
	    !(info->flags & IEEE80211_TX_CTL_INJECTED) &&
	    local->ps_sdata && !(local->scanning)) {
		if (info->flags & IEEE80211_TX_STAT_ACK) {
			local->ps_sdata->u.mgd.flags |=
					IEEE80211_STA_NULLFUNC_ACKED;
		} else
			mod_timer(&local->dynamic_ps_timer, jiffies +
					msecs_to_jiffies(10));
	}

This change makes sure  ieee80211_dynamic_ps_enable_work is evaluated once a NULLFUNC is ACKED for a more conssitent notification of the driver.

Regards,
Bassem