mac80211: support reporting A-MSDU subframes individually

Some devices may not be able to report A-MSDUs in
single buffers. Drivers for such devices were
forced to re-assemble A-MSDUs which would then
be eventually disassembled by mac80211. This could
lead to CPU cache thrashing and poor performance.

Since A-MSDU has a single sequence number all
subframes share it. This was in conflict with
retransmission/duplication recovery
(IEEE802.11-2012: 9.3.2.10).

Patch introduces a new flag that is meant to be
set for all individually reported A-MSDU subframes
except the last one. This ensures the
last_seq_ctrl is updated after the last subframe
is processed. If an A-MSDU is actually a duplicate
transmission all reported subframes will be
properly discarded.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
[johannes: add braces that were missing even before]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -829,6 +829,15 @@ ieee80211_tx_info_clear_status(struct ieee80211_tx_info *info)
  * @RX_FLAG_STBC_MASK: STBC 2 bit bitmask. 1 - Nss=1, 2 - Nss=2, 3 - Nss=3
  * @RX_FLAG_10MHZ: 10 MHz (half channel) was used
  * @RX_FLAG_5MHZ: 5 MHz (quarter channel) was used
+ * @RX_FLAG_AMSDU_MORE: Some drivers may prefer to report separate A-MSDU
+ *	subframes instead of a one huge frame for performance reasons.
+ *	All, but the last MSDU from an A-MSDU should have this flag set. E.g.
+ *	if an A-MSDU has 3 frames, the first 2 must have the flag set, while
+ *	the 3rd (last) one must not have this flag set. The flag is used to
+ *	deal with retransmission/duplication recovery properly since A-MSDU
+ *	subframes share the same sequence number. Reported subframes can be
+ *	either regular MSDU or singly A-MSDUs. Subframes must not be
+ *	interleaved with other frames.
  */
 enum mac80211_rx_flags {
 	RX_FLAG_MMIC_ERROR		= BIT(0),
@@ -859,6 +868,7 @@ enum mac80211_rx_flags {
 	RX_FLAG_STBC_MASK		= BIT(26) | BIT(27),
 	RX_FLAG_10MHZ			= BIT(28),
 	RX_FLAG_5MHZ			= BIT(29),
+	RX_FLAG_AMSDU_MORE		= BIT(30),
 };
 
 #define RX_FLAG_STBC_SHIFT		26

net/mac80211/rx.c

@@ -995,8 +995,9 @@ ieee80211_rx_h_check(struct ieee80211_rx_data *rx)
 				rx->sta->num_duplicates++;
 			}
 			return RX_DROP_UNUSABLE;
-		} else
+		} else if (!(status->flag & RX_FLAG_AMSDU_MORE)) {
 			rx->sta->last_seq_ctrl[rx->seqno_idx] = hdr->seq_ctrl;
+		}
 	}
 
 	if (unlikely(rx->skb->len < 16)) {