On 18.06.2021 20:23:39, Oliver Hartkopp wrote:
> > Even if the Linux Kernel doesn't drop any messages, not all CAN
> > controllers support that feature. On the Linux side we try our best, but
> > some USB attached devices don't report a TX complete event back, so the
> > driver triggers the CAN echo skb after the USB transfer has been
> > completed.
> > 
> > We don't have a feature flag to query if the Linux driver support proper
> > CAN echo on TX complete notification.
> 
> We have. It is set in struct netdevice.flags and called IFF_ECHO.
> 
> https://elixir.bootlin.com/linux/v5.12.11/source/net/can/af_can.c#L257

The flag tells the rest of the stack, that the driver takes care of
generating the CAN echo packages.

Several USB based driver set the IFF_ECHO flag, but the USB device don't
signal the TX-complete to the host. These drivers generate the CAN echo
frame after the successful USB TX transmission. This is better than
letting the networking stack generate the CAN echo frame, but it's not
100% perfect.

> E.g. the slcan driver does not have this bit set.

Marc

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