static __le16 ieee80211_duration(struct ieee80211_tx_data *tx,
				 struct sk_buff *skb, int group_addr,
				 int next_frag_len)
{
	int rate, mrate, erp, dur, i, shift = 0;
	struct ieee80211_rate *txrate;
	struct ieee80211_local *local = tx->local;
	struct ieee80211_supported_band *sband;
	struct ieee80211_hdr *hdr;
	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
	struct ieee80211_chanctx_conf *chanctx_conf;
	u32 rate_flags = 0;

	rcu_read_lock();
	chanctx_conf = rcu_dereference(tx->sdata->vif.chanctx_conf);
	if (chanctx_conf) {
		shift = ieee80211_chandef_get_shift(&chanctx_conf->def);
		rate_flags = ieee80211_chandef_rate_flags(&chanctx_conf->def);
	}
	rcu_read_unlock();

	/* assume HW handles this */
	if (tx->rate.flags & IEEE80211_TX_RC_MCS)
		return 0;

	/* uh huh? */
	if (WARN_ON_ONCE(tx->rate.idx < 0))
		return 0;

	sband = local->hw.wiphy->bands[info->band];
	txrate = &sband->bitrates[tx->rate.idx];

	erp = txrate->flags & IEEE80211_RATE_ERP_G;

	/*
	 * data and mgmt (except PS Poll):
	 * - during CFP: 32768
	 * - during contention period:
	 *   if addr1 is group address: 0
	 *   if more fragments = 0 and addr1 is individual address: time to
	 *      transmit one ACK plus SIFS
	 *   if more fragments = 1 and addr1 is individual address: time to
	 *      transmit next fragment plus 2 x ACK plus 3 x SIFS
	 *
	 * IEEE 802.11, 9.6:
	 * - control response frame (CTS or ACK) shall be transmitted using the
	 *   same rate as the immediately previous frame in the frame exchange
	 *   sequence, if this rate belongs to the PHY mandatory rates, or else
	 *   at the highest possible rate belonging to the PHY rates in the
	 *   BSSBasicRateSet
	 */
	hdr = (struct ieee80211_hdr *)skb->data;
	if (ieee80211_is_ctl(hdr->frame_control)) {
		/* TODO: These control frames are not currently sent by
		 * mac80211, but should they be implemented, this function
		 * needs to be updated to support duration field calculation.
		 *
		 * RTS: time needed to transmit pending data/mgmt frame plus
		 *    one CTS frame plus one ACK frame plus 3 x SIFS
		 * CTS: duration of immediately previous RTS minus time
		 *    required to transmit CTS and its SIFS
		 * ACK: 0 if immediately previous directed data/mgmt had
		 *    more=0, with more=1 duration in ACK frame is duration
		 *    from previous frame minus time needed to transmit ACK
		 *    and its SIFS
		 * PS Poll: BIT(15) | BIT(14) | aid
		 */
		return 0;
	}

	/* data/mgmt */
	if (0 /* FIX: data/mgmt during CFP */)
		return cpu_to_le16(32768);

	if (group_addr) /* Group address as the destination - no ACK */
		return 0;

	/* Individual destination address:
	 * IEEE 802.11, Ch. 9.6 (after IEEE 802.11g changes)
	 * CTS and ACK frames shall be transmitted using the highest rate in
	 * basic rate set that is less than or equal to the rate of the
	 * immediately previous frame and that is using the same modulation
	 * (CCK or OFDM). If no basic rate set matches with these requirements,
	 * the highest mandatory rate of the PHY that is less than or equal to
	 * the rate of the previous frame is used.
	 * Mandatory rates for IEEE 802.11g PHY: 1, 2, 5.5, 11, 6, 12, 24 Mbps
	 */
	rate = -1;
	/* use lowest available if everything fails */
	mrate = sband->bitrates[0].bitrate;
	for (i = 0; i < sband->n_bitrates; i++) {
		struct ieee80211_rate *r = &sband->bitrates[i];

		if (r->bitrate > txrate->bitrate)
			break;

		if ((rate_flags & r->flags) != rate_flags)
			continue;

		if (tx->sdata->vif.bss_conf.basic_rates & BIT(i))
			rate = DIV_ROUND_UP(r->bitrate, 1 << shift);

		switch (sband->band) {
		case IEEE80211_BAND_2GHZ: {
			u32 flag;
			if (tx->sdata->flags & IEEE80211_SDATA_OPERATING_GMODE)
				flag = IEEE80211_RATE_MANDATORY_G;
			else
				flag = IEEE80211_RATE_MANDATORY_B;
			if (r->flags & flag)
				mrate = r->bitrate;
			break;
		}
		case IEEE80211_BAND_5GHZ:
			if (r->flags & IEEE80211_RATE_MANDATORY_A)
				mrate = r->bitrate;
			break;
		case IEEE80211_BAND_60GHZ:
			/* TODO, for now fall through */
		case IEEE80211_NUM_BANDS:
			WARN_ON(1);
			break;
		}
	}
	if (rate == -1) {
		/* No matching basic rate found; use highest suitable mandatory
		 * PHY rate */
		rate = DIV_ROUND_UP(mrate, 1 << shift);
	}

	/* Don't calculate ACKs for QoS Frames with NoAck Policy set */
	if (ieee80211_is_data_qos(hdr->frame_control) &&
	    *(ieee80211_get_qos_ctl(hdr)) & IEEE80211_QOS_CTL_ACK_POLICY_NOACK)
		dur = 0;
	else
		/* Time needed to transmit ACK
		 * (10 bytes + 4-byte FCS = 112 bits) plus SIFS; rounded up
		 * to closest integer */
		dur = ieee80211_frame_duration(sband->band, 10, rate, erp,
				tx->sdata->vif.bss_conf.use_short_preamble,
				shift);

	if (next_frag_len) {
		/* Frame is fragmented: duration increases with time needed to
		 * transmit next fragment plus ACK and 2 x SIFS. */
		dur *= 2; /* ACK + SIFS */
		/* next fragment */
		dur += ieee80211_frame_duration(sband->band, next_frag_len,
				txrate->bitrate, erp,
				tx->sdata->vif.bss_conf.use_short_preamble,
				shift);
	}

	return cpu_to_le16(dur);
}