Index: if_ath_tdma.c
===================================================================
--- if_ath_tdma.c	(revision 250777)
+++ if_ath_tdma.c	(working copy)
@@ -277,10 +277,20 @@
 		rix = ath_tx_findrix(sc, tp->ucastrate);
 	else
 		rix = ath_tx_findrix(sc, tp->mcastrate);
-	/* XXX short preamble assumed */
-	sc->sc_tdmaguard = ath_hal_computetxtime(ah, sc->sc_currates,
-		ifp->if_mtu + IEEE80211_MAXOVERHEAD, rix, AH_TRUE);
 
+	/*
+	 * If the chip supports enforcing TxOP on transmission,
+	 * we can just delete the guard window.  It isn't at all required.
+	 */
+	if (sc->sc_hasenforcetxop) {
+		sc->sc_tdmaguard = 0;
+	} else {
+		/* XXX short preamble assumed */
+		/* XXX non-11n rate assumed */
+		sc->sc_tdmaguard = ath_hal_computetxtime(ah, sc->sc_currates,
+			ifp->if_mtu + IEEE80211_MAXOVERHEAD, rix, AH_TRUE);
+	}
+
 	ath_hal_intrset(ah, 0);
 
 	ath_beaconq_config(sc);			/* setup h/w beacon q */
@@ -392,8 +402,35 @@
 	 * the packet just received.
 	 */
 	rix = rt->rateCodeToIndex[rs->rs_rate];
-	txtime = ath_hal_computetxtime(ah, rt, rs->rs_datalen, rix,
-	    rt->info[rix].shortPreamble);
+
+	/*
+	 * To calculate the packet duration for legacy rates, we
+	 * only need the rix and preamble.
+	 *
+	 * For 11n non-aggregate frames, we also need the channel
+	 * width and short/long guard interval.
+	 *
+	 * For 11n aggregate frames, the required hacks are a little
+	 * more subtle.  You need to figure out the frame duration
+	 * for each frame, including the delimiters.  However, when
+	 * a frame isn't received successfully, we won't hear it
+	 * (unless you enable reception of CRC errored frames), so
+	 * your duration calculation is going to be off.
+	 *
+	 * However, we can assume that the beacon frames won't be
+	 * transmitted as aggregate frames, so we should be okay.
+	 * Just add a check to ensure that we aren't handed something
+	 * bad.
+	 *
+	 * For ath_hal_pkt_txtime() - for 11n rates, shortPreamble is
+	 * actually short guard interval. For legacy rates,
+	 * it's short preamble.
+	 */
+	txtime = ath_hal_pkt_txtime(ah, rt, rs->rs_datalen,
+	    rix,
+	    !! (rs->rs_flags & HAL_RX_2040),
+	    (rix & 0x80) ?
+	      (! (rs->rs_flags & HAL_RX_GI)) : rt->info[rix].shortPreamble);
 	/* NB: << 9 is to cvt to TU and /2 */
 	nextslot = (rstamp - txtime) + (sc->sc_tdmabintval << 9);
 
Index: if_athvar.h
===================================================================
--- if_athvar.h	(revision 250783)
+++ if_athvar.h	(working copy)
@@ -627,7 +627,8 @@
 	 */
 	u_int32_t		sc_use_ent  : 1,
 				sc_rx_stbc  : 1,
-				sc_tx_stbc  : 1;
+				sc_tx_stbc  : 1,
+				sc_hasenforcetxop : 1; /* support enforce TxOP */
 
 
 	int			sc_cabq_enable;	/* Enable cabq transmission */
@@ -1256,6 +1257,14 @@
 	ath_hal_setcapability(_ah, HAL_CAP_INTMIT, \
 	HAL_CAP_INTMIT_ENABLE, _v, NULL)
 
+#define	ath_hal_hasenforcetxop(_ah) \
+	(ath_hal_getcapability(_ah, HAL_CAP_ENFORCE_TXOP, 0, NULL) == HAL_OK)
+#define	ath_hal_getenforcetxop(_ah) \
+	(ath_hal_getcapability(_ah, HAL_CAP_ENFORCE_TXOP, 1, NULL) == HAL_OK)
+#define	ath_hal_setenforcetxop(_ah, _v) \
+	ath_hal_setcapability(_ah, HAL_CAP_ENFORCE_TXOP, 1, _v, NULL)
+
+
 /* EDMA definitions */
 #define	ath_hal_hasedma(_ah) \
 	(ath_hal_getcapability(_ah, HAL_CAP_ENHANCED_DMA_SUPPORT,	\
Index: if_ath.c
===================================================================
--- if_ath.c	(revision 250795)
+++ if_ath.c	(working copy)
@@ -670,6 +670,7 @@
 	sc->sc_hastsfadd = ath_hal_hastsfadjust(ah);
 	sc->sc_rxslink = ath_hal_self_linked_final_rxdesc(ah);
 	sc->sc_rxtsf32 = ath_hal_has_long_rxdesc_tsf(ah);
+	sc->sc_hasenforcetxop = ath_hal_hasenforcetxop(ah);
 	if (ath_hal_hasfastframes(ah))
 		ic->ic_caps |= IEEE80211_C_FF;
 	wmodes = ath_hal_getwirelessmodes(ah);
@@ -1586,6 +1587,15 @@
 	/* Let spectral at in case spectral is enabled */
 	ath_spectral_enable(sc, ic->ic_curchan);
 
+	/*
+	 * If we're doing TDMA, enforce the TXOP limitation for chips that
+	 * support it.
+	 */
+	if (sc->sc_hasenforcetxop && sc->sc_tdma)
+		ath_hal_setenforcetxop(sc->sc_ah, 1);
+	else
+		ath_hal_setenforcetxop(sc->sc_ah, 0);
+
 	/* Restore the LED configuration */
 	ath_led_config(sc);
 	ath_hal_setledstate(ah, HAL_LED_INIT);
@@ -2034,6 +2044,15 @@
 	ath_spectral_enable(sc, ic->ic_curchan);
 
 	/*
+	 * If we're doing TDMA, enforce the TXOP limitation for chips that
+	 * support it.
+	 */
+	if (sc->sc_hasenforcetxop && sc->sc_tdma)
+		ath_hal_setenforcetxop(sc->sc_ah, 1);
+	else
+		ath_hal_setenforcetxop(sc->sc_ah, 0);
+
+	/*
 	 * Likewise this is set during reset so update
 	 * state cached in the driver.
 	 */
@@ -2348,6 +2367,15 @@
 	/* Let spectral at in case spectral is enabled */
 	ath_spectral_enable(sc, ic->ic_curchan);
 
+	/*
+	 * If we're doing TDMA, enforce the TXOP limitation for chips that
+	 * support it.
+	 */
+	if (sc->sc_hasenforcetxop && sc->sc_tdma)
+		ath_hal_setenforcetxop(sc->sc_ah, 1);
+	else
+		ath_hal_setenforcetxop(sc->sc_ah, 0);
+
 	if (ath_startrecv(sc) != 0)	/* restart recv */
 		if_printf(ifp, "%s: unable to start recv logic\n", __func__);
 	/*
@@ -4869,6 +4897,15 @@
 		ath_spectral_enable(sc, chan);
 
 		/*
+		 * If we're doing TDMA, enforce the TXOP limitation for chips
+		 * that support it.
+		 */
+		if (sc->sc_hasenforcetxop && sc->sc_tdma)
+			ath_hal_setenforcetxop(sc->sc_ah, 1);
+		else
+			ath_hal_setenforcetxop(sc->sc_ah, 0);
+
+		/*
 		 * Re-enable rx framework.
 		 */
 		if (ath_startrecv(sc) != 0) {