Re: [PATCH v3 wireless-drivers 1/3] mt76: usb: fix rx A-MSDU support

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Thu, Jun 13, 2019 at 11:43:11PM +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  | 49 ++++++++++++++++++-----
>  2 files changed, 41 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/wireless/mediatek/mt76/mt76.h b/drivers/net/wireless/mediatek/mt76/mt76.h
> index 8ecbf81a906f..889b76deb703 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..12d60d31cb51 100644
> --- a/drivers/net/wireless/mediatek/mt76/usb.c
> +++ b/drivers/net/wireless/mediatek/mt76/usb.c
> @@ -429,6 +429,45 @@ 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)
> +{
> +	struct sk_buff *skb;
> +
> +	if (SKB_WITH_OVERHEAD(buf_size) < MT_DMA_HDR_LEN + len) {
> +		struct page *page;
> +		int offset;
> +
> +		/* slow path, not enough space for data and
> +		 * skb_shared_info
> +		 */
> +		skb = alloc_skb(MT_SKB_HEAD_LEN, GFP_ATOMIC);
> +		if (!skb)
> +			return NULL;
> +
> +		skb_put_data(skb, data + MT_DMA_HDR_LEN, MT_SKB_HEAD_LEN);

I looked how rx amsdu is processed in mac80211 and it is decomposed and
copied into newly allocated individual skb's, see ieee80211_amsdu_to_8023s()

So copy L3 & L4 headers doesn't do anything good here, actually seems to
be better to have them in frag as __ieee80211_amsdu_copy_frag() do some
magic to avid copy.

Stanislaw