Date:   Mon, 11 Feb 2019 10:47:10 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org
Subject: Re: [RFC] mt76x02u: correct pad for fragments
Message-ID: <20190211094708.GA3467@localhost.localdomain>
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> Modifying skb->len & data_len doesn't look correct. Issue is not critical
> we just pad 2 times, except first padding is not filled with zeros.
> 
> However I'm not sure if we should not add pad to all skb's in frags
> list.

Hi Stanislaw,

IIRC the first packet of a A-MSDU burst keeps track of the burst length 
so if we add the a pad to last one we need to update the skb->len/data_len
of the first one. No need to pad all skbs in the frag list, just last one

Regards,
Lorenzo

> Additionally remove unlikely(pad) condition, we always pad for at least
> four bytes what is needed by HW.
> 
> Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
> ---
>  .../wireless/mediatek/mt76/mt76x02_usb_core.c | 22 +++++++------------
>  1 file changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c b/drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c
> index dc2226c722dd..c7ca2d93720a 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt76x02_usb_core.c
> @@ -49,21 +49,15 @@ int mt76x02u_skb_dma_info(struct sk_buff *skb, int port, u32 flags)
>  	       FIELD_PREP(MT_TXD_INFO_DPORT, port) | flags;
>  	put_unaligned_le32(info, skb_push(skb, sizeof(info)));
>  
> -	pad = round_up(skb->len, 4) + 4 - skb->len;
> -	skb_walk_frags(skb, iter) {
> +	skb_walk_frags(skb, iter)
>  		last = iter;
> -		if (!iter->next) {
> -			skb->data_len += pad;
> -			skb->len += pad;
> -			break;
> -		}
> -	}
> -
> -	if (unlikely(pad)) {
> -		if (skb_pad(last, pad))
> -			return -ENOMEM;
> -		__skb_put(last, pad);
> -	}
> +
> +	/* Add zero pad of 4 - 7 bytes at the end of buffer */
> +	pad = round_up(skb->len, 4) + 4 - skb->len;
> +	if (skb_pad(last, pad))
> +		return -ENOMEM;
> +	__skb_put(last, pad);
> +
>  	return 0;
>  }
>  
> -- 
> 2.19.2
>