mac80211: allow drivers to support S/G

If drivers want to support S/G (really just gather DMA on TX) then
we can now easily support this on the fast-xmit path since it just
needs to write to the ethernet header (and already has a check for
that being possible.)

However, disallow this on the regular TX path (which has to handle
fragmentation, software crypto, etc.) by calling skb_linearize().

Also allow the related HIGHDMA since that's not interesting to the
code in mac80211 at all anyway.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -1942,8 +1942,8 @@ enum ieee80211_hw_flags {
  *	Use the %IEEE80211_RADIOTAP_VHT_KNOWN_* values.
  *
  * @netdev_features: netdev features to be set in each netdev created
- *	from this HW. Note only HW checksum features are currently
- *	compatible with mac80211. Other feature bits will be rejected.
+ *	from this HW. Note that not all features are usable with mac80211,
+ *	other features will be rejected during HW registration.
  *
  * @uapsd_queues: This bitmap is included in (re)association frame to indicate
  *	for each access category if it is uAPSD trigger-enabled and delivery-

net/mac80211/main.c

@@ -840,7 +840,7 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 
 	/* Only HW csum features are currently compatible with mac80211 */
 	feature_whitelist = NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM |
-			    NETIF_F_HW_CSUM;
+			    NETIF_F_HW_CSUM | NETIF_F_SG | NETIF_F_HIGHDMA;
 	if (WARN_ON(hw->netdev_features & ~feature_whitelist))
 		return -EINVAL;
 

net/mac80211/tx.c

@@ -2864,6 +2864,12 @@ void __ieee80211_subif_start_xmit(struct sk_buff *skb,
 			goto out;
 	}
 
+	/* we cannot process non-linear frames on this path */
+	if (skb_linearize(skb)) {
+		kfree_skb(skb);
+		goto out;
+	}
+
 	/* the frame could be fragmented, software-encrypted, and other things
 	 * so we cannot really handle checksum offload with it - fix it up in
 	 * software before we handle anything else.