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Date:   Wed, 12 Jun 2019 16:21:29 +0200
From:   Stanislaw Gruszka <sgruszka@redhat.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Cc:     Lorenzo Bianconi <lorenzo@kernel.org>, nbd@nbd.name,
        kvalo@codeaurora.org, linux-wireless@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mt76: usb: do not always copy the first part of
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On Wed, Jun 12, 2019 at 02:59:05PM +0200, Stanislaw Gruszka wrote:
> > > If max RX AMSDU size is 3839B I do not see reason why we allocate
> > > MT_SG_MAX_SIZE=8 of MT_RX_BUF_SIZE=2k buffers for sg_en case.
> > > I thought the reason is that max AMSDU size is 16kB so it fit into
> > > 8 sg buffers of 2k.
> > > 
> > > In other words, for me, looks like either
> > > - we can not handle AMSDU for non sg case because we do not
> > > allocate big enough buffer
> > 
> > I think AMSDU is mandatory and we currently support it even for non-sg case
> > (since max rx AMSDU is 3839B)
> > 
> > > or
> > > - we can just use one PAGE_SIZE buffer for rx and remove sg
> > > buffers for rx completely 
> > 
> > using sg buffers we can support bigger rx AMSDU size in the future without using
> > huge buffers (e.g. we can try to use IEEE80211_MAX_MPDU_LEN_HT_7935 with
> > mt76x2u)
> 
> I think it would be simpler just to allocate 2 pages for 7935B .

And if we could determine that there is no true need to use sg for rx,
I think best approach here would be revert f8f527b16db5 in v5.2 to fix
regression and remove rx sg in -next. That would make code simpler,
allocate 4k instead 16k per packet, allow to use build_skb (4096 - 3839
give enough space for shared info) and not use usb hcd bounce buffer.

Stanislaw