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Subject: Re: [PATCH] mac80211: allow bigger A-MSDU sizes in VHT, even if HT is
 limited
To:     Johannes Berg <johannes@sipsolutions.net>,
        linux-wireless@vger.kernel.org
References: <20200916164611.8022-1-nbd@nbd.name>
 <e122ae9b058e6f8c7c82d6ebef90517a604a3c64.camel@sipsolutions.net>
From:   Felix Fietkau <nbd@nbd.name>
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Message-ID: <e91d3c8e-2681-9b1d-8129-04a59ce6c20d@nbd.name>
Date:   Wed, 16 Sep 2020 20:31:47 +0200
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On 2020-09-16 19:44, Johannes Berg wrote:
> On Wed, 2020-09-16 at 18:46 +0200, Felix Fietkau wrote:
>> Some APs (e.g. Asus RT-AC88U) have been observed to report an HT MSDU size
>> limit of 3839 and a VHT limit of 7991. These APs can handle bigger frames
>> than 3839 bytes just fine, so we should remove the VHT limit based on the
>> HT capabilities. This improves tx throughput.
> 
> I'm not really convinced that we can do this, since we don't track
> HT/VHT limits separately.
> 
> IOW, to be behaving according to the AP you mention, we'd have to know
> when building an A-MSDU whether it's going to use HT or VHT rates, but
> we don't track that at all ...
These comments don't make sense to me. A-MSDU limit is tracked per sta,
and on all the implementations I've looked at, HT rates are never used
to transmit to a peer that can do VHT. It would be unnecessary to do
that anyway, since VHT modulations are a superset of HT modulations with
just some header differences.

On another note, VHT already supports bigger A-MSDU limits than HT in
the code below that check.

- Felix