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Date:   Wed, 12 Jun 2019 16:44:01 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stanislaw Gruszka <sgruszka@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
 received frames
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On Jun 12, Stanislaw Gruszka wrote:
> 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=3D8 of MT_RX_BUF_SIZE=3D2k 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.
> > > >=20
> > > > 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
> > >=20
> > > I think AMSDU is mandatory and we currently support it even for non-s=
g case
> > > (since max rx AMSDU is 3839B)
> > >=20
> > > > or
> > > > - we can just use one PAGE_SIZE buffer for rx and remove sg
> > > > buffers for rx completely=20
> > >=20
> > > using sg buffers we can support bigger rx AMSDU size in the future wi=
thout using
> > > huge buffers (e.g. we can try to use IEEE80211_MAX_MPDU_LEN_HT_7935 w=
ith
> > > mt76x2u)
> >=20
> > I think it would be simpler just to allocate 2 pages for 7935B .
>=20
> 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.

I do not think we should drop sg support since:
- it allow us to rx huge amsdu frames (e.g. IEEE80211_MAX_MPDU_LEN_VHT_1145=
4)
  using multiple one page buffer. I think there will be new usb devices whe=
re we can
  increase amsdu size (we can increase it even on mt76x2u usb 3.0 devices)
- without SG we can't use build_skb() without copying a given size of the p=
acket since the
  space needed for skb_shared_info is 320B on a x86_64 device
- the fix for f8f527b16db5 has been already tested

Regards,
Lorenzo

>=20
> Stanislaw

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