mac80211: add RX_FLAG_MACTIME_PLCP_START

The timestamp given by iwlwifi is at the beginning of the
frame over the air, at (or during) the SYNC field. Allow
such timestamps to be given to mac80211, at least (for now)
for frames with non-HT/VHT preambles.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 7d0479e..fb90d9c 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -4,7 +4,7 @@
  * Copyright 2006-2007	Jiri Benc <jbenc@suse.cz>
  * Copyright 2007	Johannes Berg <johannes@sipsolutions.net>
  * Copyright 2013-2014  Intel Mobile Communications GmbH
- * Copyright (C) 2015	Intel Deutschland GmbH
+ * Copyright (C) 2015-2016	Intel Deutschland GmbH
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
@@ -2671,6 +2671,18 @@
 		sband = local->hw.wiphy->bands[status->band];
 		bitrate = sband->bitrates[status->rate_idx].bitrate;
 		ri.legacy = DIV_ROUND_UP(bitrate, (1 << shift));
+
+		if (status->flag & RX_FLAG_MACTIME_PLCP_START) {
+			/* TODO: handle HT/VHT preambles */
+			if (status->band == IEEE80211_BAND_5GHZ) {
+				ts += 20 << shift;
+				mpdu_offset += 2;
+			} else if (status->flag & RX_FLAG_SHORTPRE) {
+				ts += 96;
+			} else {
+				ts += 192;
+			}
+		}
 	}
 
 	rate = cfg80211_calculate_bitrate(&ri);