[PATCH v3 1/1] can: dev: add software tx timestamps

[Vincent Mailhol]

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

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

Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
 drivers/net/can/dev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
index 3486704c8a95..850759c7677f 100644
--- a/drivers/net/can/dev.c
+++ b/drivers/net/can/dev.c
@@ -481,6 +481,7 @@ int can_put_echo_skb(struct sk_buff *skb, struct net_device *dev,
 		skb->pkt_type = PACKET_BROADCAST;
 		skb->ip_summed = CHECKSUM_UNNECESSARY;
 		skb->dev = dev;
+		skb_tx_timestamp(skb);
 
 		/* save this skb for tx interrupt echo handling */
 		priv->echo_skb[idx] = skb;
-- 
2.26.2