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Date:   Wed, 12 Jun 2019 14:28:48 +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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[...]
>=20
> My sense of humor declined quite drastically lastly ;-/
>=20
> > > > but we can be more cautious since probably copying
> > > > the first 128B will not make any difference
> > >=20
> > > Not sure if I understand what you mean.
> >=20
> > Please correct me if I am wrong but I think max amsdu rx size is 3839B =
for
> > mt76. For the sg_en case this frame will span over multiple sg buffers =
since
> > sg buffer size is 2048B (2 sg buffers). Moreover if we do not take into=
 account
> > skb_shared_info when configuring the sg buffer size we will need to alw=
ays copy
> > the first 128B of the first buffer since received len will be set to 20=
48 and
> > the following if condition will always fail:
> >=20
> > if (SKB_WITH_OVERHEAD(buf_size) >=3D MT_DMA_HDR_LEN + len) {
> > }
>=20
> Ok, that I understand.
>=20
> > > > > And skb_shered_info is needed only in first buffer IIUC.
> > > > >=20
> > > > > Also this patch seems to make first patch unnecessary except for
> > > > > non sg_en case (in which I think rx AMSDU is broken anyway),
> > > > > so I would prefer just to apply first patch.
> > > >=20
> > > > I do not think rx AMSDU is broken for non sg_en case since the max =
rx value
> > > > allowed should be 3839 IIRC and we alloc one page in this case
> > >=20
> > > If that's the case we should be fine, but then I do not understand
> > > why we allocate 8*2k buffers for sg_en case, isn't that AP can
> > > sent AMSDU frame 16k big?
> >=20
> > Sorry I did not get what you mean here, could you please explain?
>=20
> 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

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=20

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)

Regards,
Lorenzo

>=20
> Do I miss something ?
>=20
> Stanislaw

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