netdev - [PATCH v4 1/1] can: dev: add software tx timestamps

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Message-Id: <20210112095437.6488-2-mailhol.vincent@wanadoo.fr>
Date:   Tue, 12 Jan 2021 18:54:37 +0900
From:   Vincent Mailhol <mailhol.vincent@...adoo.fr>
To:     Marc Kleine-Budde <mkl@...gutronix.de>, linux-can@...r.kernel.org,
        Jeroen Hofstee <jhofstee@...tronenergy.com>,
        Richard Cochran <richardcochran@...il.com>
Cc:     Wolfgang Grandegger <wg@...ndegger.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "open list : NETWORKING DRIVERS" <netdev@...r.kernel.org>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>
Subject: [PATCH v4 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.

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/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eb88531bdbfaafb827192d1fc6c5a3fcc4fadd96

  * Put the call to skb_tx_timestamp() just before adding it to the
    array: https://lkml.org/lkml/2021/1/10/54

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

Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
 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 {
-- 
2.26.2