Re: [PATCH v2 1/2] mt76: usb: fix rx A-MSDU support

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> Hi and sorry for late comment.
> 

Hi Stanislaw,

> On Fri, May 31, 2019 at 11:38:22AM +0200, Lorenzo Bianconi wrote:
> > Commit f8f527b16db5 ("mt76: usb: use EP max packet aligned buffer sizes
> > for rx") breaks A-MSDU support. When A-MSDU is enable the device can
> > receive frames up to q->buf_size but they will be discarded in
> > mt76u_process_rx_entry since there is no enough room for
> > skb_shared_info. Fix the issue reallocating the skb and copying in the
> > linear area the first 128B of the received frames and in the frag_list
> > the remaining part.
> > 
> > Fixes: f8f527b16db5 ("mt76: usb: use EP max packet aligned buffer sizes for rx")
> > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > ---
> >  drivers/net/wireless/mediatek/mt76/mt76.h |  1 +
> >  drivers/net/wireless/mediatek/mt76/usb.c  | 52 +++++++++++++++++++----
> >  2 files changed, 44 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/net/wireless/mediatek/mt76/mt76.h b/drivers/net/wireless/mediatek/mt76/mt76.h
> > index 97a1296562d0..74d4edf941d6 100644
> > --- a/drivers/net/wireless/mediatek/mt76/mt76.h
> > +++ b/drivers/net/wireless/mediatek/mt76/mt76.h
> > @@ -30,6 +30,7 @@
> >  #define MT_TX_RING_SIZE     256
> >  #define MT_MCU_RING_SIZE    32
> >  #define MT_RX_BUF_SIZE      2048
> > +#define MT_SKB_HEAD_LEN     128
> >  
> >  struct mt76_dev;
> >  struct mt76_wcid;
> > diff --git a/drivers/net/wireless/mediatek/mt76/usb.c b/drivers/net/wireless/mediatek/mt76/usb.c
> > index bbaa1365bbda..2bfc8214c0d8 100644
> > --- a/drivers/net/wireless/mediatek/mt76/usb.c
> > +++ b/drivers/net/wireless/mediatek/mt76/usb.c
> > @@ -429,6 +429,48 @@ static int mt76u_get_rx_entry_len(u8 *data, u32 data_len)
> >  	return dma_len;
> >  }
> >  
> > +static struct sk_buff *
> > +mt76u_build_rx_skb(u8 *data, int len, int buf_size,
> > +		   int *nsgs)
> > +{
> > +	int data_len = min(len, MT_SKB_HEAD_LEN);
> > +	struct sk_buff *skb;
> > +
> > +	if (SKB_WITH_OVERHEAD(buf_size) >= MT_DMA_HDR_LEN + len) {
> > +		/* fast path */
> > +		skb = build_skb(data, buf_size);
> > +		if (!skb)
> > +			return NULL;
> > +
> > +		skb_reserve(skb, MT_DMA_HDR_LEN);
> > +		__skb_put(skb, len);
> > +
> > +		return skb;
> > +	}
> > +
> > +	/* slow path, not enough space for data and
> > +	 * skb_shared_info
> > +	 */
> > +	skb = alloc_skb(data_len, GFP_ATOMIC);
> > +	if (!skb)
> > +		return NULL;
> > +
> > +	skb_put_data(skb, data + MT_DMA_HDR_LEN, data_len);
> mt7601u and iwlmvm just copy hdrlen + 8 and put the rest
> of the buffer in fragment, which supose to be more efficient,
> see comment in iwl_mvm_pass_packet_to_mac80211().

Right here we copy 128B instead of 32 but I think it is good to have L3 and L4
header in the linear area of the skb since otherwise the stack will need to
align them

>  
> > +	data += (data_len + MT_DMA_HDR_LEN);
> > +	len -= data_len;
> > +	if (len > 0) {
> > +		struct page *page = virt_to_head_page(data);
> > +		int offset = data - (u8 *)page_address(page);
> u8 cast not needed.
> 
> > +		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
> > +				page, offset, len, buf_size);
> > +	} else {
> > +		*nsgs = 0;
> This seems to be not necessary or a bug (use first buffer 2 times). 

maybe I have been a little bit too paranoid here :)

Regards,
Lorenzo

> 
> Stanislaw

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