[RFC PATCH] mac80211: clip ADDBA instead of bailing out

[Alexandre Ferrieux <alexandre.ferrieux@gmail.com>]

When a Linux Wifi{4,5} device talks to a Wifi6 AP, if the AP proposes a Block
Acknowledgement aggregation size (ADDBA) exceeding its expectations, the code in
mac80211 just bails out, rejecting the aggregation. This yields a big
performance penalty on the ack path, which is observable in comparison with
other OSes (Windows and MacOS) which "play smarter" and accept the proposal with
a "clipped" size.

A typical scenario would be:

  AP -> Device : ADDBA_request(size=256)

Current Linux reaction:

  Device -> AP : ADDBA_reply(failure)

Other OSes reaction:

  Device -> AP : ADDBA_reply(size=64)

Note that the IEEE802.11 standard allows for both reactions, but it sounds
really suboptimal to be bailing out instead of clipping. The patch below does
the latter.

Signed-off-by: Alexandre Ferrieux <alexandre.ferrieux@gmail.com>
---

diff --git a/net/mac80211/agg-rx.c b/net/mac80211/agg-rx.c
index f3fbe5a4395e..264dad847842 100644
--- a/net/mac80211/agg-rx.c
+++ b/net/mac80211/agg-rx.c
@@ -317,18 +317,20 @@ void __ieee80211_start_rx_ba_session(struct sta_info *sta,
                max_buf_size = IEEE80211_MAX_AMPDU_BUF_HT;

        /* sanity check for incoming parameters:
-        * check if configuration can support the BA policy
-        * and if buffer size does not exceeds max value */
+        * check if configuration can support the BA policy */
        /* XXX: check own ht delayed BA capability?? */
        if (((ba_policy != 1) &&
-            (!(sta->sta.deflink.ht_cap.cap & IEEE80211_HT_CAP_DELAY_BA))) ||
-           (buf_size > max_buf_size)) {
-               status = WLAN_STATUS_INVALID_QOS_PARAM;
+            (!(sta->sta.deflink.ht_cap.cap & IEEE80211_HT_CAP_DELAY_BA)))) {
+               status = WLAN_STATUS_INVALID_QOS_PARAM;
                ht_dbg_ratelimited(sta->sdata,
                                   "AddBA Req with bad params from %pM on tid
%u. policy %d, buffer size %d\n",
                                   sta->sta.addr, tid, ba_policy, buf_size);
                goto end;
        }
+       if (buf_size > max_buf_size) {
+         buf_size = max_buf_size ; // Clip instead of bailing out
+       }
+
        /* determine default buffer size */
        if (buf_size == 0)
                buf_size = max_buf_size;