mac80211: fix comment regarding aggregation buf_size

The description for buf_size was misleading and
just said you couldn't TX larger aggregates, but
of course you can't TX aggregates in a way that
would exceed the window either, which is possible
even if the aggregates are shorter than that.

Expand the description, thanks to Emmanuel for
explaining this to me.

Cc: Emmanuel Grumbach <egrumbach@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index cefe1b3..965f1b1 100644 (file)
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -1753,8 +1753,19 @@ enum ieee80211_ampdu_mlme_action {
  *     that TX/RX_STOP can pass NULL for this parameter.
  *     The @buf_size parameter is only valid when the action is set to
  *     %IEEE80211_AMPDU_TX_OPERATIONAL and indicates the peer's reorder
- *     buffer size (number of subframes) for this session -- aggregates
- *     containing more subframes than this may not be transmitted to the peer.
+ *     buffer size (number of subframes) for this session -- the driver
+ *     may neither send aggregates containing more subframes than this
+ *     nor send aggregates in a way that lost frames would exceed the
+ *     buffer size. If just limiting the aggregate size, this would be
+ *     possible with a buf_size of 8:
+ *      - TX: 1.....7
+ *      - RX:  2....7 (lost frame #1)
+ *      - TX:        8..1...
+ *     which is invalid since #1 was now re-transmitted well past the
+ *     buffer size of 8. Correct ways to retransmit #1 would be:
+ *      - TX:       1 or 18 or 81
+ *     Even "189" would be wrong since 1 could be lost again.
+ *
  *     Returns a negative error code on failure.
  *     The callback can sleep.
  *