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Date:   Sun, 10 Jan 2021 19:35:26 +0900
From:   Vincent Mailhol <>
To:     Marc Kleine-Budde <>,
Cc:     Vincent Mailhol <>,
        Wolfgang Grandegger <>,
        "David S. Miller" <>,
        Jakub Kicinski <>, (open list:NETWORKING DRIVERS), (open list)
Subject: [PATCH 1/1] can: dev: add software tx timestamps

Call skb_tx_timestamp() within can_put_echo_skb() so that a software
tx timestamp gets attached on the skb.

There two main reasons to include this call in can_put_echo_skb():

  * It easily allow to enable the tx timestamp on all devices with
    just one small change.

  * According to Documentation/networking/timestamping.rst, the tx
    timestamps should be generated in the device driver as close as
    possible, but always prior to passing the packet to the network
    interface. During the call to can_put_echo_skb(), the skb gets
    cloned meaning that the driver should not dereference the skb
    variable anymore after can_put_echo_skb() returns. This makes
    can_put_echo_skb() the very last place we can use the skb without
    having to access the echo_skb[] array.


  * By default, skb_tx_timestamp() does nothing. It needs to be
    activated by passing the SOF_TIMESTAMPING_TX_SOFTWARE flag either
    through socket options or control messages.

  * The hardware rx timestamp of a local loopback message is the
    hardware tx timestamp. This means that there are no needs to
    implement SOF_TIMESTAMPING_TX_HARDWARE for CAN sockets.


Support for the error queue in CAN RAW sockets (which is needed for tx
timestamps) was introduced in:

Signed-off-by: Vincent Mailhol <>
 drivers/net/can/dev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
index 3486704c8a95..3904e0874543 100644
--- a/drivers/net/can/dev.c
+++ b/drivers/net/can/dev.c
@@ -484,6 +484,8 @@ int can_put_echo_skb(struct sk_buff *skb, struct net_device *dev,
 		/* save this skb for tx interrupt echo handling */
 		priv->echo_skb[idx] = skb;
+		skb_tx_timestamp(skb);
 	} else {
 		/* locking problem with netif_stop_queue() ?? */
 		netdev_err(dev, "%s: BUG! echo_skb %d is occupied!\n", __func__, idx);

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