Index: head/sys/dev/bwn/if_bwn.c
===================================================================
--- head/sys/dev/bwn/if_bwn.c
+++ head/sys/dev/bwn/if_bwn.c
@@ -2721,8 +2721,7 @@
 	BWN_LOCK(sc);
 	if (sc->sc_flags & BWN_FLAG_RUNNING) {
 		mac = (struct bwn_mac *)sc->sc_curmac;
-		bwn_set_slot_time(mac,
-		    (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20);
+		bwn_set_slot_time(mac, IEEE80211_GET_SLOTTIME(ic));
 	}
 	BWN_UNLOCK(sc);
 }
Index: head/sys/dev/otus/if_otus.c
===================================================================
--- head/sys/dev/otus/if_otus.c
+++ head/sys/dev/otus/if_otus.c
@@ -2423,7 +2423,7 @@
 
 	OTUS_LOCK_ASSERT(sc);
 
-	slottime = (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	slottime = IEEE80211_GET_SLOTTIME(ic);
 	otus_write(sc, AR_MAC_REG_SLOT_TIME, slottime << 10);
 	(void)otus_write_barrier(sc);
 }
Index: head/sys/dev/ral/rt2560.c
===================================================================
--- head/sys/dev/ral/rt2560.c
+++ head/sys/dev/ral/rt2560.c
@@ -2254,7 +2254,7 @@
 	uint32_t tmp;
 
 #ifndef FORCE_SLOTTIME
-	slottime = (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	slottime = IEEE80211_GET_SLOTTIME(ic);
 #else
 	/*
 	 * Setting slot time according to "short slot time" capability
@@ -2272,13 +2272,13 @@
 	 * (-1Mb~-2Mb lower) and the _whole_ BSS would stop using short
 	 * slot time.
 	 */
-	slottime = 20;
+	slottime = IEEE80211_DUR_SLOT;
 #endif
 
 	/* update the MAC slot boundaries */
 	tx_sifs = RAL_SIFS - RT2560_TXRX_TURNAROUND;
 	tx_pifs = tx_sifs + slottime;
-	tx_difs = tx_sifs + 2 * slottime;
+	tx_difs = IEEE80211_DUR_DIFS(tx_sifs, slottime);
 	eifs = (ic->ic_curmode == IEEE80211_MODE_11B) ? 364 : 60;
 
 	tmp = RAL_READ(sc, RT2560_CSR11);
Index: head/sys/dev/ral/rt2661.c
===================================================================
--- head/sys/dev/ral/rt2661.c
+++ head/sys/dev/ral/rt2661.c
@@ -2090,7 +2090,7 @@
 	uint8_t slottime;
 	uint32_t tmp;
 
-	slottime = (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	slottime = IEEE80211_GET_SLOTTIME(ic);
 
 	tmp = RAL_READ(sc, RT2661_MAC_CSR9);
 	tmp = (tmp & ~0xff) | slottime;
Index: head/sys/dev/ral/rt2860.c
===================================================================
--- head/sys/dev/ral/rt2860.c
+++ head/sys/dev/ral/rt2860.c
@@ -3048,7 +3048,7 @@
 
 	tmp = RAL_READ(sc, RT2860_BKOFF_SLOT_CFG);
 	tmp &= ~0xff;
-	tmp |= (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	tmp |= IEEE80211_GET_SLOTTIME(ic);
 	RAL_WRITE(sc, RT2860_BKOFF_SLOT_CFG, tmp);
 }
 
Index: head/sys/dev/usb/wlan/if_rum.c
===================================================================
--- head/sys/dev/usb/wlan/if_rum.c
+++ head/sys/dev/usb/wlan/if_rum.c
@@ -2068,7 +2068,7 @@
 	struct ieee80211com *ic = &sc->sc_ic;
 	uint8_t slottime;
 
-	slottime = (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	slottime = IEEE80211_GET_SLOTTIME(ic);
 
 	rum_modbits(sc, RT2573_MAC_CSR9, slottime, 0xff);
 
Index: head/sys/dev/usb/wlan/if_run.c
===================================================================
--- head/sys/dev/usb/wlan/if_run.c
+++ head/sys/dev/usb/wlan/if_run.c
@@ -5186,7 +5186,7 @@
 
 	run_read(sc, RT2860_BKOFF_SLOT_CFG, &tmp);
 	tmp &= ~0xff;
-	tmp |= (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	tmp |= IEEE80211_GET_SLOTTIME(ic);
 	run_write(sc, RT2860_BKOFF_SLOT_CFG, tmp);
 }
 
Index: head/sys/dev/usb/wlan/if_ural.c
===================================================================
--- head/sys/dev/usb/wlan/if_ural.c
+++ head/sys/dev/usb/wlan/if_ural.c
@@ -1771,7 +1771,7 @@
 	struct ieee80211com *ic = &sc->sc_ic;
 	uint16_t slottime, sifs, eifs;
 
-	slottime = (ic->ic_flags & IEEE80211_F_SHSLOT) ? 9 : 20;
+	slottime = IEEE80211_GET_SLOTTIME(ic);
 
 	/*
 	 * These settings may sound a bit inconsistent but this is what the
Index: head/sys/dev/usb/wlan/if_urtw.c
===================================================================
--- head/sys/dev/usb/wlan/if_urtw.c
+++ head/sys/dev/usb/wlan/if_urtw.c
@@ -4291,18 +4291,18 @@
 	if (sc->sc_flags & URTW_RTL8187B) {
 		urtw_write8_m(sc, URTW_SIFS, 0x22);
 		if (IEEE80211_IS_CHAN_ANYG(ic->ic_curchan))
-			urtw_write8_m(sc, URTW_SLOT, 0x9);
+			urtw_write8_m(sc, URTW_SLOT, IEEE80211_DUR_SHSLOT);
 		else
-			urtw_write8_m(sc, URTW_SLOT, 0x14);
+			urtw_write8_m(sc, URTW_SLOT, IEEE80211_DUR_SLOT);
 		urtw_write8_m(sc, URTW_8187B_EIFS, 0x5b);
 		urtw_write8_m(sc, URTW_CARRIER_SCOUNT, 0x5b);
 	} else {
 		urtw_write8_m(sc, URTW_SIFS, 0x22);
 		if (sc->sc_state == IEEE80211_S_ASSOC &&
 		    ic->ic_flags & IEEE80211_F_SHSLOT)
-			urtw_write8_m(sc, URTW_SLOT, 0x9);
+			urtw_write8_m(sc, URTW_SLOT, IEEE80211_DUR_SHSLOT);
 		else
-			urtw_write8_m(sc, URTW_SLOT, 0x14);
+			urtw_write8_m(sc, URTW_SLOT, IEEE80211_DUR_SLOT);
 		if (IEEE80211_IS_CHAN_ANYG(ic->ic_curchan)) {
 			urtw_write8_m(sc, URTW_DIFS, 0x14);
 			urtw_write8_m(sc, URTW_EIFS, 0x5b - 0x14);
Index: head/sys/dev/usb/wlan/if_urtwn.c
===================================================================
--- head/sys/dev/usb/wlan/if_urtwn.c
+++ head/sys/dev/usb/wlan/if_urtwn.c
@@ -3621,8 +3621,7 @@
 	int ac;
 
 	acm = 0;
-	slottime = (ic->ic_flags & IEEE80211_F_SHSLOT) ?
-	    IEEE80211_DUR_SHSLOT : IEEE80211_DUR_SLOT;
+	slottime = IEEE80211_GET_SLOTTIME(ic);
 
 	URTWN_LOCK(sc);
 	for (ac = WME_AC_BE; ac < WME_NUM_AC; ac++) {
Index: head/sys/net80211/ieee80211_phy.h
===================================================================
--- head/sys/net80211/ieee80211_phy.h
+++ head/sys/net80211/ieee80211_phy.h
@@ -53,6 +53,10 @@
 #define IEEE80211_DUR_SHSLOT	9	/* ERP short slottime */
 #define IEEE80211_DUR_OFDM_SLOT	9	/* OFDM slottime */
 
+#define IEEE80211_GET_SLOTTIME(ic) \
+	((ic->ic_flags & IEEE80211_F_SHSLOT) ? \
+	    IEEE80211_DUR_SHSLOT : IEEE80211_DUR_SLOT)
+
 /*
  * DIFS (microseconds).
  */