ath9k: adjust ack timeout workaround patch based on upstream submission

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@19638 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/mac80211/patches/520-ath9k_ack_timeout_workaround.patch

@@ -1,13 +1,19 @@
 --- a/drivers/net/wireless/ath/ath9k/hw.c
 +++ b/drivers/net/wireless/ath/ath9k/hw.c
-@@ -1233,6 +1233,11 @@ void ath9k_hw_init_global_settings(struc
+@@ -1233,6 +1233,17 @@ void ath9k_hw_init_global_settings(struc
  	/* As defined by IEEE 802.11-2007 17.3.8.6 */
  	slottime = ah->slottime + 3 * ah->coverage_class;
  	acktimeout = slottime + sifstime;
 +
-+	/* Workaround for a hw issue */
++	/*
++	 * Workaround for early ACK timeouts, add an offset to match the
++	 * initval's 64us ack timeout value.
++	 * This was initially only meant to work around an issue with delayed
++	 * BA frames in some implementations, but it has been found to fix ACK
++	 * timeout issues in other cases as well.
++	 */
 +	if (conf->channel && conf->channel->band == IEEE80211_BAND_2GHZ)
-+		acktimeout = max(64, acktimeout);
++		acktimeout += 64 - sifstime - ah->slottime;
 +
  	ath9k_hw_setslottime(ah, slottime);
  	ath9k_hw_set_ack_timeout(ah, acktimeout);