netdev - [net-next 16/17] can: dev: can_put_echo_skb(): add software tx timestamps

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Date:   Wed, 13 Jan 2021 22:14:09 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, linux-can@...r.kernel.org,
        kernel@...gutronix.de,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Marc Kleine-Budde <mkl@...gutronix.de>
Subject: [net-next 16/17] can: dev: can_put_echo_skb(): add software tx timestamps

From: Vincent Mailhol <mailhol.vincent@...adoo.fr>

Call skb_tx_timestamp() within can_put_echo_skb() so that a software tx
timestamp gets attached to 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.

Remark: 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.

References:

 * Support for the error queue in CAN RAW sockets (which is needed for
   tx timestamps) was introduced in:
   https://git.kernel.org//torvalds/c/eb88531bdbfaafb827192d1fc6c5a3fcc4fadd96

  * Put the call to skb_tx_timestamp() just before adding it to the
    array:
    https://lore.kernel.org/r/043c3ea1-6bdd-59c0-0269-27b2b5b36cec@victronenergy.com

  * About Tx hardware timestamps
    https://lore.kernel.org/r/20210111171152.GB11715@hoboy.vegasvil.org

Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
Link: https://lore.kernel.org/r/20210112095437.6488-2-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@...gutronix.de>
---
 drivers/net/can/dev/skb.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/can/dev/skb.c b/drivers/net/can/dev/skb.c
index 53683d4312f1..6a64fe410987 100644
--- a/drivers/net/can/dev/skb.c
+++ b/drivers/net/can/dev/skb.c
@@ -65,6 +65,8 @@ int can_put_echo_skb(struct sk_buff *skb, struct net_device *dev,
 		/* save frame_len to reuse it when transmission is completed */
 		can_skb_prv(skb)->frame_len = frame_len;
 
+		skb_tx_timestamp(skb);
+
 		/* save this skb for tx interrupt echo handling */
 		priv->echo_skb[idx] = skb;
 	} else {
-- 
2.29.2