[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).

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

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

Hello Jeroen,

On Sun. 10 Jan 2021 at 20:29, Jeroen Hofstee <jhofstee@victronenergy.com> wrote:
>
> Hello Vincent,
>
> On 1/10/21 11:35 AM, Vincent Mailhol wrote:
> > Call skb_tx_timestamp() within can_put_echo_skb() so that a software
> > tx timestamp gets attached on the skb.
> >
> [..]
> >
> > 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);
>
> Personally, I would put the skb_tx_timestamp, before adding it to the array:
>
>          /* make settings for echo to reduce code in irq context */
>          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;

I agree that it is better like that from an aesthetic point of
view. The reason to put it at the very end was to really to
blindly follow the doc and do the timestamp as late as possible.

>
> I don't think it actually matters though.

Indeed, but will still follow your suggestion though. Putting it
before would just delay the timestamp by a few assembly
instructions: it is negligible enough.


Yours sincerely,
Vincent Mailhol