ath5k: add notes about rx timestamp

add comments about the fact that we don't know when exactly the atheros
hardware takes the RX timestamp.

drivers/net/wireless/ath5k/base.c:      Changes-licensed-under: 3-Clause-BSD
Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath5k/base.c

@@ -1844,6 +1844,17 @@ ath5k_tasklet_rx(unsigned long data)
 		 * 15bit only. that means TSF extension has to be done within
 		 * 32768usec (about 32ms). it might be necessary to move this to
 		 * the interrupt handler, like it is done in madwifi.
+		 *
+		 * Unfortunately we don't know when the hardware takes the rx
+		 * timestamp (beginning of phy frame, data frame, end of rx?).
+		 * The only thing we know is that it is hardware specific...
+		 * On AR5213 it seems the rx timestamp is at the end of the
+		 * frame, but i'm not sure.
+		 *
+		 * NOTE: mac80211 defines mactime at the beginning of the first
+		 * data symbol. Since we don't have any time references it's
+		 * impossible to comply to that. This affects IBSS merge only
+		 * right now, so it's not too bad...
 		 */
 		rxs.mactime = ath5k_extend_tsf(sc->ah, rs.rs_tstamp);
 		rxs.flag |= RX_FLAG_TSFT;