Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Vincent Mailhol <mailhol.vincent@wanadoo.fr>
To:     Marc Kleine-Budde <mkl@pengutronix.de>, linux-can@vger.kernel.org
Cc:     Vincent Mailhol <mailhol.vincent@wanadoo.fr>,
        Wolfgang Grandegger <wg@grandegger.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        netdev@vger.kernel.org (open list:NETWORKING DRIVERS),
        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH 1/1] can: dev: add software tx timestamps
Date:   Sun, 10 Jan 2021 19:35:26 +0900
Message-Id: <20210110103526.61047-2-mailhol.vincent@wanadoo.fr>
In-Reply-To: <20210110103526.61047-1-mailhol.vincent@wanadoo.fr>
References: <20210110103526.61047-1-mailhol.vincent@wanadoo.fr>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

Remarks:

  * 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.

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

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
 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);
-- 
2.26.2