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

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Date:   Tue, 25 Jun 2019 15:07:12 -0700
From:   Vedang Patel <vedang.patel@...el.com>
To:     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, eric.dumazet@...il.com,
        aaron.f.brown@...el.com, Vedang Patel <vedang.patel@...el.com>
Subject: [PATCH net-next v6 1/8] igb: clear out skb->tstamp after reading the txtime

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(+)

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;
 		context_desc->seqnum_seed = cpu_to_le32(ts.tv_nsec / 32);
 	} else {
 		context_desc->seqnum_seed = 0;
-- 
2.7.3