Re: [RFC v4] mac80211: add A-MSDU tx support

[Felix Fietkau <nbd@openwrt.org>]

On 2016-02-08 10:54, Krishna Chaitanya wrote:
> On Mon, Feb 8, 2016 at 2:56 PM, Emmanuel Grumbach <egrumbach@gmail.com> wrote:
>> On Mon, Feb 8, 2016 at 10:38 AM, Felix Fietkau <nbd@openwrt.org> wrote:
>>> Requires software tx queueing support. frag_list support (for zero-copy)
>>> is optional.
>>>
>>> Signed-off-by: Felix Fietkau <nbd@openwrt.org>
>>> ---
>>
>>
>> Ok - looks fine, but... and here comes the hard stuff.
>> The frame size in the PLCP is limited in a way that you can't - from a
>> spec POV - enable A-MSDU for low rates. Of course, you don't want to
>> do that for low rates at all regardless of the spec.
>> Since you build the A-MSDU in the mac80211 Tx queue which is not aware
>> of the link quality, how do we prevent A-MSDU if the rate is low /
>> dropping.
>> I'd even argue that when the rates get lower, you'll  have more
>> packets piling up in the software queue and ... even more chance to
>> get A-MSDU in the exact case where you really want to avoid it?
> 
> Similar to triggering AMPDU setup, we should put this control
> in RC (minstrel) to start/stop AMSDU based on link quality/if the rates
> drop below a pre-defined MCS (or) only for best-throughput rates.
I think starting/stopping A-MSDU based on the rate is a bad idea.
Even with low rates, using A-MSDU can be a good thing (especially for
TCP ACKs), it just needs different size limits.

- Felix