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Date:   Wed, 12 Jun 2019 12:25:05 +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
 received frames
Message-ID: <20190612102502.GB4431@redhat.com>
References: <cover.1559293385.git.lorenzo@kernel.org>
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 <20190612091036.GB2965@redhat.com>
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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:
> 
> [...]
> 
> > >  	}
> > >  
> > >  	urb->num_sgs = max_t(int, i, urb->num_sgs);
> > > -	urb->transfer_buffer_length = urb->num_sgs * q->buf_size,
> > > +	urb->transfer_buffer_length = urb->num_sgs * data_size;
> > >  	sg_init_marker(urb->sg, urb->num_sgs);
> > >  
> > >  	return i ? : -ENOMEM;
> > > @@ -611,8 +611,12 @@ static int mt76u_alloc_rx(struct mt76_dev *dev)
> > >  	if (!q->entry)
> > >  		return -ENOMEM;
> > >  
> > > -	q->buf_size = dev->usb.sg_en ? MT_RX_BUF_SIZE : PAGE_SIZE;
> > > +	if (dev->usb.sg_en)
> > > +		q->buf_size = MT_BUF_WITH_OVERHEAD(MT_RX_BUF_SIZE);
> > 
> > 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.
> 
> even in practice actually :)

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.

> but we can be more cautious since probably copying
> the first 128B will not make any difference

Not sure if I understand what you mean.

> > And skb_shered_info is needed only in first buffer IIUC.
> > 
> > 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.
> 
> 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

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?

Stanislaw