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>

1 files changed, 11 insertions, 0 deletions

diff --git a/drivers/net/wireless/ath5k/base.c b/drivers/net/wireless/ath5k/base.c
index 73e7e613a3c9..5eacc7cdd9e3 100644
--- a/drivers/net/wireless/ath5k/base.c
+++ b/drivers/net/wireless/ath5k/base.c
@@ -1844,6 +1844,17 @@ accept:
                  * 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;