netdev - Re: [RFC PATCH 3/7] net: add option to get information about timestamped packets

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Date:   Tue, 25 Apr 2017 13:23:41 -0400
From:   Willem de Bruijn <willemb@...gle.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Denny Page <dennypage@...com>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [RFC PATCH 3/7] net: add option to get information about
 timestamped packets

On Tue, Apr 25, 2017 at 9:56 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> On Mon, Apr 24, 2017 at 11:18:13AM -0400, Willem de Bruijn wrote:
>> On Mon, Apr 24, 2017 at 5:00 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
>> > Would "skb->data - skb->head -
>> > skb->mac_header + skb->len" always work as the L2 length for received
>> > packets at the time when the cmsg is prepared?
>>
>> (skb->data - skb->head) - skb->mac_header computes the length
>> of data before the mac, such as reserve?
>
> data - head includes the reserve, but mac_header does too, so I think
> it should be just the length of MAC header and everything up to the
> data.
>
>> Do you mean skb->data -
>> skb->mac_header (or - skb_mac_offset(skb))?
>
> That would give me a pointer? If I used skb_mac_offset(), the total
> length would be just skb->len - skb_mac_offset()?

It appears so. The only existing caller first checks
skb_mac_header_was_set(skb).