[PATCH v2 0/1] Add software TX timestamps to the CAN devices

With the ongoing work to add BQL to Socket CAN, I figured out that it
would be nice to have an easy way to mesure the latency.

And one easy way to do so it to check the round trip time of the
packet by doing the difference between the software rx timestamp and
the software tx timestamp.

rx timestamps are already available. This patch gives the missing
piece: add a tx software timestamp feature to the CAN devices.

Of course, the tx software timestamp might also be used for other
purposes such as performance measurements of the different queuing
disciplines (e.g. by checking the difference between the kernel tx
software timestamp and the userland tx software timestamp).

v2 reflects the comments that Jeroen made in
https://lkml.org/lkml/2021/1/10/54

Vincent Mailhol (1):
  can: dev: add software tx timestamps

 drivers/net/can/dev.c | 2 ++
 1 file changed, 2 insertions(+)

-- 
2.26.2

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 <redacted>
---
 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