netdev - Re: [PATCH net-next v6 1/8] igb: clear out skb->tstamp after reading the txtime

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Date:   Wed, 26 Jun 2019 20:06:32 +0000
From:   "Patel, Vedang" <vedang.patel@...el.com>
To:     "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        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>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "Brown, Aaron F" <aaron.f.brown@...el.com>
Subject: Re: [PATCH net-next v6 1/8] igb: clear out skb->tstamp after
 reading the txtime

Hi Jeff,

> On Jun 26, 2019, at 12:44 PM, Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com> wrote:
> 
> On Tue, 2019-06-25 at 15:07 -0700, Vedang Patel wrote:
>> If a packet which is utilizing the launchtime feature (via SO_TXTIME
>> socket
>> option) also requests the hardware transmit timestamp, the hardware
>> timestamp is not delivered to the userspace. This is because the
>> value in
>> skb->tstamp is mistaken as the software timestamp.
>> 
>> Applications, like ptp4l, request a hardware timestamp by setting the
>> SOF_TIMESTAMPING_TX_HARDWARE socket option. 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.
>> 
>> Signed-off-by: Vedang Patel <vedang.patel@...el.com>
>> ---
>> drivers/net/ethernet/intel/igb/igb_main.c | 1 +
>> 1 file changed, 1 insertion(+)
> 
> Since this fix is really not needed for the rest of the patch series,
> if you have to do another version of the series, can you drop this
> patch from any future version?  I don't want to keep spinning my
> validation team on a updated version of this patch, that is not really
> being updated.
> 
> I plan to take this version of the patch and will hold onto it for my
> next 1GbE updates to Dave.

This patch is needed for ptp4l to function properly when txtime-assist mode is enabled. So, dropping this patch will break the series. When are you planning to submit the next set of updates to Dave? If a new version is needed, I can plan to send it out after you send your updates.

Thanks,
Vedang