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

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Message-ID: <799b2d75-a60d-4d93-b8d7-c29442b73dce@gmail.com>
Date:   Thu, 20 Jun 2019 17:56:28 -0400
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Patel, Vedang" <vedang.patel@...el.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>,
        "l@...ileo.org" <l@...ileo.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Murali Karicheri <m-karicheri2@...com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: Re: [PATCH net-next v4 1/7] igb: clear out tstamp after sending the
 packet



On 6/20/19 1:32 PM, Patel, Vedang wrote:
> 

> Ahh.. that’s clearly a false statement. Skb->tstamp is cleared so
> that it is not interpreted as a software timestamp when trying to
> send the Hardware TX timestamp to the userspace. I will rephrase the
> commit message in the next version.
> 
> Some more details: The problem occurs when using the txtime-assist
> mode of taprio with packets which also request the hardware transmit
> timestamp (e.g. PTP packets). Whenever txtime-assist mode is set,
> taprio will assign a hardware transmit timestamp to all the packets
> (in skb->tstamp). PTP packets will also request the hardware transmit
> timestamp be sent to the userspace after packet is transmitted.
> 
> Whenever a new timestamp is detected by the driver (this work is done
> in igb_ptp_tx_work() which calls igb_ptp_tx_hwtstamps() in
> igb_ptp.c[1]), it will queue the timestamp in the ERR_QUEUE for the
> userspace to read. When the userspace is ready, it will issue a
> recvmsg() call to collect this timestamp. The problem is in this
> recvmsg() call. If the skb->tstamp is not cleared out, it will be
> interpreted as a software timestamp and the hardware tx timestamp
> will not be successfully sent to the userspace. Look at
> skb_is_swtx_tstamp() and the callee function __sock_recv_timestamp()
> in net/socket.c for more details.


That amount of details in the changelog would be really nice ;)