void ieee80211_check_fast_xmit(struct sta_info *sta)
{
	struct ieee80211_fast_tx build = {}, *fast_tx = NULL, *old;
	struct ieee80211_local *local = sta->local;
	struct ieee80211_sub_if_data *sdata = sta->sdata;
	struct ieee80211_hdr *hdr = (void *)build.hdr;
	struct ieee80211_chanctx_conf *chanctx_conf;
	__le16 fc;

	if (!ieee80211_hw_check(&local->hw, SUPPORT_FAST_XMIT))
		return;

	/* Locking here protects both the pointer itself, and against concurrent
	 * invocations winning data access races to, e.g., the key pointer that
	 * is used.
	 * Without it, the invocation of this function right after the key
	 * pointer changes wouldn't be sufficient, as another CPU could access
	 * the pointer, then stall, and then do the cache update after the CPU
	 * that invalidated the key.
	 * With the locking, such scenarios cannot happen as the check for the
	 * key and the fast-tx assignment are done atomically, so the CPU that
	 * modifies the key will either wait or other one will see the key
	 * cleared/changed already.
	 */
	spin_lock_bh(&sta->lock);
	if (ieee80211_hw_check(&local->hw, SUPPORTS_PS) &&
	    !ieee80211_hw_check(&local->hw, SUPPORTS_DYNAMIC_PS) &&
	    sdata->vif.type == NL80211_IFTYPE_STATION)
		goto out;

	if (!test_sta_flag(sta, WLAN_STA_AUTHORIZED))
		goto out;

	if (test_sta_flag(sta, WLAN_STA_PS_STA) ||
	    test_sta_flag(sta, WLAN_STA_PS_DRIVER) ||
	    test_sta_flag(sta, WLAN_STA_PS_DELIVER) ||
	    test_sta_flag(sta, WLAN_STA_CLEAR_PS_FILT))
		goto out;

	if (sdata->noack_map)
		goto out;

	/* fast-xmit doesn't handle fragmentation at all */
	if (local->hw.wiphy->frag_threshold != (u32)-1 &&
	    !ieee80211_hw_check(&local->hw, SUPPORTS_TX_FRAG))
		goto out;

	rcu_read_lock();
	chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
	if (!chanctx_conf) {
		rcu_read_unlock();
		goto out;
	}
	build.band = chanctx_conf->def.chan->band;
	rcu_read_unlock();

	fc = cpu_to_le16(IEEE80211_FTYPE_DATA | IEEE80211_STYPE_DATA);

	switch (sdata->vif.type) {
	case NL80211_IFTYPE_ADHOC:
		/* DA SA BSSID */
		build.da_offs = offsetof(struct ieee80211_hdr, addr1);
		build.sa_offs = offsetof(struct ieee80211_hdr, addr2);
		memcpy(hdr->addr3, sdata->u.ibss.bssid, ETH_ALEN);
		build.hdr_len = 24;
		break;
	case NL80211_IFTYPE_STATION:
		if (test_sta_flag(sta, WLAN_STA_TDLS_PEER)) {
			/* DA SA BSSID */
			build.da_offs = offsetof(struct ieee80211_hdr, addr1);
			build.sa_offs = offsetof(struct ieee80211_hdr, addr2);
			memcpy(hdr->addr3, sdata->u.mgd.bssid, ETH_ALEN);
			build.hdr_len = 24;
			break;
		}

		if (sdata->u.mgd.use_4addr) {
			/* non-regular ethertype cannot use the fastpath */
			fc |= cpu_to_le16(IEEE80211_FCTL_FROMDS |
					  IEEE80211_FCTL_TODS);
			/* RA TA DA SA */
			memcpy(hdr->addr1, sdata->u.mgd.bssid, ETH_ALEN);
			memcpy(hdr->addr2, sdata->vif.addr, ETH_ALEN);
			build.da_offs = offsetof(struct ieee80211_hdr, addr3);
			build.sa_offs = offsetof(struct ieee80211_hdr, addr4);
			build.hdr_len = 30;
			break;
		}
		fc |= cpu_to_le16(IEEE80211_FCTL_TODS);
		/* BSSID SA DA */
		memcpy(hdr->addr1, sdata->u.mgd.bssid, ETH_ALEN);
		build.da_offs = offsetof(struct ieee80211_hdr, addr3);
		build.sa_offs = offsetof(struct ieee80211_hdr, addr2);
		build.hdr_len = 24;
		break;
	case NL80211_IFTYPE_AP_VLAN:
		if (sdata->wdev.use_4addr) {
			fc |= cpu_to_le16(IEEE80211_FCTL_FROMDS |
					  IEEE80211_FCTL_TODS);
			/* RA TA DA SA */
			memcpy(hdr->addr1, sta->sta.addr, ETH_ALEN);
			memcpy(hdr->addr2, sdata->vif.addr, ETH_ALEN);
			build.da_offs = offsetof(struct ieee80211_hdr, addr3);
			build.sa_offs = offsetof(struct ieee80211_hdr, addr4);
			build.hdr_len = 30;
			break;
		}
		fallthrough;
	case NL80211_IFTYPE_AP:
		fc |= cpu_to_le16(IEEE80211_FCTL_FROMDS);
		/* DA BSSID SA */
		build.da_offs = offsetof(struct ieee80211_hdr, addr1);
		memcpy(hdr->addr2, sdata->vif.addr, ETH_ALEN);
		build.sa_offs = offsetof(struct ieee80211_hdr, addr3);
		build.hdr_len = 24;
		break;
	default:
		/* not handled on fast-xmit */
		goto out;
	}

	if (sta->sta.wme) {
		build.hdr_len += 2;
		fc |= cpu_to_le16(IEEE80211_STYPE_QOS_DATA);
	}

	/* We store the key here so there's no point in using rcu_dereference()
	 * but that's fine because the code that changes the pointers will call
	 * this function after doing so. For a single CPU that would be enough,
	 * for multiple see the comment above.
	 */
	build.key = rcu_access_pointer(sta->ptk[sta->ptk_idx]);
	if (!build.key)
		build.key = rcu_access_pointer(sdata->default_unicast_key);
	if (build.key) {
		bool gen_iv, iv_spc, mmic;

		gen_iv = build.key->conf.flags & IEEE80211_KEY_FLAG_GENERATE_IV;
		iv_spc = build.key->conf.flags & IEEE80211_KEY_FLAG_PUT_IV_SPACE;
		mmic = build.key->conf.flags &
			(IEEE80211_KEY_FLAG_GENERATE_MMIC |
			 IEEE80211_KEY_FLAG_PUT_MIC_SPACE);

		/* don't handle software crypto */
		if (!(build.key->flags & KEY_FLAG_UPLOADED_TO_HARDWARE))
			goto out;

		/* Key is being removed */
		if (build.key->flags & KEY_FLAG_TAINTED)
			goto out;

		switch (build.key->conf.cipher) {
		case WLAN_CIPHER_SUITE_CCMP:
		case WLAN_CIPHER_SUITE_CCMP_256:
			if (gen_iv)
				build.pn_offs = build.hdr_len;
			if (gen_iv || iv_spc)
				build.hdr_len += IEEE80211_CCMP_HDR_LEN;
			break;
		case WLAN_CIPHER_SUITE_GCMP:
		case WLAN_CIPHER_SUITE_GCMP_256:
			if (gen_iv)
				build.pn_offs = build.hdr_len;
			if (gen_iv || iv_spc)
				build.hdr_len += IEEE80211_GCMP_HDR_LEN;
			break;
		case WLAN_CIPHER_SUITE_TKIP:
			/* cannot handle MMIC or IV generation in xmit-fast */
			if (mmic || gen_iv)
				goto out;
			if (iv_spc)
				build.hdr_len += IEEE80211_TKIP_IV_LEN;
			break;
		case WLAN_CIPHER_SUITE_WEP40:
		case WLAN_CIPHER_SUITE_WEP104:
			/* cannot handle IV generation in fast-xmit */
			if (gen_iv)
				goto out;
			if (iv_spc)
				build.hdr_len += IEEE80211_WEP_IV_LEN;
			break;
		case WLAN_CIPHER_SUITE_AES_CMAC:
		case WLAN_CIPHER_SUITE_BIP_CMAC_256:
		case WLAN_CIPHER_SUITE_BIP_GMAC_128:
		case WLAN_CIPHER_SUITE_BIP_GMAC_256:
			WARN(1,
			     "management cipher suite 0x%x enabled for data\n",
			     build.key->conf.cipher);
			goto out;
		default:
			/* we don't know how to generate IVs for this at all */
			if (WARN_ON(gen_iv))
				goto out;
			/* pure hardware keys are OK, of course */
			if (!(build.key->flags & KEY_FLAG_CIPHER_SCHEME))
				break;
			/* cipher scheme might require space allocation */
			if (iv_spc &&
			    build.key->conf.iv_len > IEEE80211_FAST_XMIT_MAX_IV)
				goto out;
			if (iv_spc)
				build.hdr_len += build.key->conf.iv_len;
		}

		fc |= cpu_to_le16(IEEE80211_FCTL_PROTECTED);
	}

	hdr->frame_control = fc;

	memcpy(build.hdr + build.hdr_len,
	       rfc1042_header,  sizeof(rfc1042_header));
	build.hdr_len += sizeof(rfc1042_header);

	fast_tx = kmemdup(&build, sizeof(build), GFP_ATOMIC);
	/* if the kmemdup fails, continue w/o fast_tx */
	if (!fast_tx)
		goto out;

 out:
	/* we might have raced against another call to this function */
	old = rcu_dereference_protected(sta->fast_tx,
					lockdep_is_held(&sta->lock));
	rcu_assign_pointer(sta->fast_tx, fast_tx);
	if (old)
		kfree_rcu(old, rcu_head);
	spin_unlock_bh(&sta->lock);
}