netdev - Re: [PATCH] net: usb: ax88179_178a: fix packet alignment padding

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Date:   Tue, 16 Jun 2020 17:08:23 +0800
From:   Jeremy Kerr <jk@...abs.org>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, allan@...x.com.tw, freddy@...x.com.tw,
        pfink@...ist-es.de, linux-usb@...r.kernel.org, louis@...x.com.tw
Subject: Re: [PATCH] net: usb: ax88179_178a: fix packet alignment padding

Hi David,

> > Those last two bytes - 96 1f - aren't part of the original packet.
> 
> Does this happen for non-tail packets in a multi-packet cluster?

I believe so, yes. I haven't been able to reliably reproduce the multi-
packet behaviour though, so input from ASIX would be good.

> 
> Because that code in this loop makes the same calculations:
> 
>                 ax_skb = skb_clone(skb, GFP_ATOMIC);
>                 if (ax_skb) {
>                         ax_skb->len = pkt_len;
>                         ax_skb->data = skb->data + 2;
>                         skb_set_tail_pointer(ax_skb, pkt_len);
>                         ax_skb->truesize = pkt_len + sizeof(struct sk_buff);
>                         ax88179_rx_checksum(ax_skb, pkt_hdr);
>                         usbnet_skb_return(dev, ax_skb);
> 
> So if your change is right, it should be applied to this code block
> as well.

Yep, my patch changes that block too (or did I miss something?)

> And do we know that it's two extra tail bytes always?  Or some kind
> of alignment padding the chip performs for every sub-packet?

I've assumed it's a constant two bytes of prefix padding, as that's all
I've seen. But it would be great to have more detail from ASIX if
possible.

Cheers,


Jeremy