netdev - Re: [PATCH net-next v4 1/7] igb: clear out tstamp after sending the packet

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Date:   Thu, 20 Jun 2019 06:47:25 -0400
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Vedang Patel <vedang.patel@...el.com>, netdev@...r.kernel.org
Cc:     jeffrey.t.kirsher@...el.com, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, jiri@...nulli.us,
        intel-wired-lan@...ts.osuosl.org, vinicius.gomes@...el.com,
        l@...ileo.org, jakub.kicinski@...ronome.com, m-karicheri2@...com,
        sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH net-next v4 1/7] igb: clear out tstamp after sending the
 packet



On 6/19/19 10:40 AM, Vedang Patel wrote:
> skb->tstamp is being used at multiple places. On the transmit side, it
> is used to determine the launchtime of the packet. It is also used to
> determine the software timestamp after the packet has been transmitted.
> 
> So, clear out the tstamp value after it has been read so that we do not
> report false software timestamp on the receive side.
> 
> Signed-off-by: Vedang Patel <vedang.patel@...el.com>
> ---
>  drivers/net/ethernet/intel/igb/igb_main.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index fc925adbd9fa..f66dae72fe37 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -5688,6 +5688,7 @@ static void igb_tx_ctxtdesc(struct igb_ring *tx_ring,
>  	 */
>  	if (tx_ring->launchtime_enable) {
>  		ts = ns_to_timespec64(first->skb->tstamp);
> +		first->skb->tstamp = 0;

Please provide more explanations.

Why only this driver would need this ?


>  		context_desc->seqnum_seed = cpu_to_le32(ts.tv_nsec / 32);
>  	} else {
>  		context_desc->seqnum_seed = 0;
>