mac80211: do not compute offset from ssn in Rx AMPDU reordering buffer

Currently, frames that go into the reordering buffer are stored at
index ieee80211_sn_sub(sn, tid_rx->ssn) % tid_rx->buf_size.

The offset calculation to the starting sequence number (SSN) is
useless and just adds overhead so simply use sn % tid_rx->buf_size.

This means the reordering buffer will start to be filled somewhere
in the middle (at SSN % buf_size) and continue to get used from
there, but there's no reason to start from the beginning.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -729,9 +729,7 @@ static void ieee80211_release_reorder_frames(struct ieee80211_sub_if_data *sdata
 	lockdep_assert_held(&tid_agg_rx->reorder_lock);
 
 	while (ieee80211_sn_less(tid_agg_rx->head_seq_num, head_seq_num)) {
-		index = ieee80211_sn_sub(tid_agg_rx->head_seq_num,
-					 tid_agg_rx->ssn) %
-							tid_agg_rx->buf_size;
+		index = tid_agg_rx->head_seq_num % tid_agg_rx->buf_size;
 		ieee80211_release_reorder_frame(sdata, tid_agg_rx, index,
 						frames);
 	}
@@ -757,8 +755,7 @@ static void ieee80211_sta_reorder_release(struct ieee80211_sub_if_data *sdata,
 	lockdep_assert_held(&tid_agg_rx->reorder_lock);
 
 	/* release the buffer until next missing frame */
-	index = ieee80211_sn_sub(tid_agg_rx->head_seq_num,
-				 tid_agg_rx->ssn) % tid_agg_rx->buf_size;
+	index = tid_agg_rx->head_seq_num % tid_agg_rx->buf_size;
 	if (!tid_agg_rx->reorder_buf[index] &&
 	    tid_agg_rx->stored_mpdu_num) {
 		/*
@@ -793,15 +790,11 @@ static void ieee80211_sta_reorder_release(struct ieee80211_sub_if_data *sdata,
 	} else while (tid_agg_rx->reorder_buf[index]) {
 		ieee80211_release_reorder_frame(sdata, tid_agg_rx, index,
 						frames);
-		index =	ieee80211_sn_sub(tid_agg_rx->head_seq_num,
-					 tid_agg_rx->ssn) %
-							tid_agg_rx->buf_size;
+		index =	tid_agg_rx->head_seq_num % tid_agg_rx->buf_size;
 	}
 
 	if (tid_agg_rx->stored_mpdu_num) {
-		j = index = ieee80211_sn_sub(tid_agg_rx->head_seq_num,
-					     tid_agg_rx->ssn) %
-							tid_agg_rx->buf_size;
+		j = index = tid_agg_rx->head_seq_num % tid_agg_rx->buf_size;
 
 		for (; j != (index - 1) % tid_agg_rx->buf_size;
 		     j = (j + 1) % tid_agg_rx->buf_size) {
@@ -861,8 +854,7 @@ static bool ieee80211_sta_manage_reorder_buf(struct ieee80211_sub_if_data *sdata
 
 	/* Now the new frame is always in the range of the reordering buffer */
 
-	index = ieee80211_sn_sub(mpdu_seq_num,
-				 tid_agg_rx->ssn) % tid_agg_rx->buf_size;
+	index = mpdu_seq_num % tid_agg_rx->buf_size;
 
 	/* check if we already stored this frame */
 	if (tid_agg_rx->reorder_buf[index]) {