Re: [PATCH v2 2/2] mt76: usb: do not always copy the first part of received frames

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Wed, Jun 12, 2019 at 12:49:22PM +0200, Lorenzo Bianconi wrote:
> > 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.
> 
> 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

My sense of humor declined quite drastically lastly ;-/

> > > 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.
> 
> 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 always copy
> the first 128B of the first buffer since received len will be set to 2048 and
> the following if condition will always fail:
> 
> if (SKB_WITH_OVERHEAD(buf_size) >= MT_DMA_HDR_LEN + len) {
> }

Ok, that I understand.

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

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
or
- we can just use one PAGE_SIZE buffer for rx and remove sg
buffers for rx completely 

Do I miss something ?

Stanislaw