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From: Felix Fietkau <nbd@nbd.name>
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X-Mailman-Approved-At: Mon, 28 Nov 2016 08:47:10 -0500
Subject: Re: [Make-wifi-fast] [PATCH v7] mac80211: Move reorder-sensitive TX
 handlers to after TXQ dequeue.
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Date: Mon, 05 Sep 2016 17:49:44 -0000
X-Original-Date: Mon, 5 Sep 2016 19:49:42 +0200
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On 2016-09-05 13:30, Toke Høiland-Jørgensen wrote:
> The TXQ intermediate queues can cause packet reordering when more than
> one flow is active to a single station. Since some of the wifi-specific
> packet handling (notably sequence number and encryption handling) is
> sensitive to re-ordering, things break if they are applied before the
> TXQ.
> 
> This splits up the TX handlers and fast_xmit logic into two parts: An
> early part and a late part. The former is applied before TXQ enqueue,
> and the latter after dequeue. The non-TXQ path just applies both parts
> at once.
> 
> Because fragments shouldn't be split up or reordered, the fragmentation
> handler is run after dequeue. Any fragments are then kept in the TXQ and
> on subsequent dequeues they take precedence over dequeueing from the FQ
> structure.
> 
> This approach avoids having to scatter special cases for when TXQ is
> enabled, at the cost of making the fast_xmit and TX handler code
> slightly more complex.
In my test, this one completely breaks ath9k with the txq patch.
One or two packets go through, then tx stalls completely.

- Felix