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Date:   Wed, 12 Jun 2019 12:49:22 +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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References: <cover.1559293385.git.lorenzo@kernel.org>
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> On Wed, Jun 12, 2019 at 11:53:03AM +0200, Lorenzo Bianconi wrote:
> > > On Fri, May 31, 2019 at 11:38:23AM +0200, Lorenzo Bianconi wrote:
> >=20
> > [...]
> >=20
> > > >  	}
> > > > =20
> > > >  	urb->num_sgs =3D max_t(int, i, urb->num_sgs);
> > > > -	urb->transfer_buffer_length =3D urb->num_sgs * q->buf_size,
> > > > +	urb->transfer_buffer_length =3D urb->num_sgs * data_size;
> > > >  	sg_init_marker(urb->sg, urb->num_sgs);
> > > > =20
> > > >  	return i ? : -ENOMEM;
> > > > @@ -611,8 +611,12 @@ static int mt76u_alloc_rx(struct mt76_dev *dev)
> > > >  	if (!q->entry)
> > > >  		return -ENOMEM;
> > > > =20
> > > > -	q->buf_size =3D dev->usb.sg_en ? MT_RX_BUF_SIZE : PAGE_SIZE;
> > > > +	if (dev->usb.sg_en)
> > > > +		q->buf_size =3D MT_BUF_WITH_OVERHEAD(MT_RX_BUF_SIZE);
> > >=20
> > > I strongly recommend to not doing this. While this should work
> > > in theory creating buffer with size of 2k + some bytes might
> > > trigger various bugs in dma mapping or other low level code.
> >=20
> > even in practice actually :)
>=20
> I wouldn't be sure about this. It's not common to have buffers of
> such size and crossing pages boundaries. It really can trigger
> nasty bugs on various IOMMU drivers.

I was just joking, I mean that it worked in the tests I carried out, but I
agree it can trigger some issues in buggy IOMMU drivers

>=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.

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 acc=
ount
skb_shared_info when configuring the sg buffer size we will need to always =
copy
the first 128B of the first buffer since received len will be set to 2048 a=
nd
the following if condition will always fail:

if (SKB_WITH_OVERHEAD(buf_size) >=3D MT_DMA_HDR_LEN + len) {
}

>=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 v=
alue
> > 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?

Sorry I did not get what you mean here, could you please explain?

Regards,
Lorenzo

>=20
> Stanislaw
>=20

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