Subject: Re: [RFC v2] mac80211: add A-MSDU tx support
To: Emmanuel Grumbach <egrumbach@gmail.com>
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Cc: linux-wireless <linux-wireless@vger.kernel.org>,
	Johannes Berg <johannes@sipsolutions.net>
From: Felix Fietkau <nbd@openwrt.org>
Message-ID: <56B744EC.7090803@openwrt.org> (sfid-20160207_142209_087602_62F98B5F)
Date: Sun, 7 Feb 2016 14:21:48 +0100
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On 2016-02-07 12:56, Emmanuel Grumbach wrote:
>>> well.. Yes, you can't assume that you'll have one descriptor for one
>>> MSDU payload  (unless the driver doesn't advertise SG to the
>>> netstack).
>> Okay, please make a suggestion describing the exact kinds of limits you
>> would need for iwlwifi.
> 
> Are athX devices able to handle MPDUs with any number of frags? Say if
> you have 30 different physically contiguous fragments, the DMA would
> be able to load all these into one single packet and send it to the
> air?
I think athX devices have no limitations there. I'm not testing this
with atheros devices though - ath9k does not have mac80211
per-sta-per-tid queueing support yet. I'm working with MediaTek MT76x2
chipsets with my mt76 driver, which I will upstream soon.

> iwlwifi currently has the limitation of 20 Transmit Buffers (BTs)
> which I mentioned earlier. I guess it'd be nice if the driver would be
> able to advertise how many fragments it can handle. Then, you'd need
> to stop the A-MSDU building if you'd cross this boundary?
> 
> You can look at skb_shinfo(skb)->nr_frags to know how many frags you
> have for each skb. On top of that, you need 1 frag for each subframe
> (subframe header).
I implemented all of your suggestions in RFC v3, let me know if
anything's missing.

- Felix