Re: Write canfd_frame to can interface

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 29.07.2021 12:03:56, thomas@the-wagner.de wrote:
> I have been working on getting my device compatible with both, CAN and
> CAN FD.
> 
> For receiving this is working straight forward. My physical interface
> is CAN FD capable and no matter whether I set it up as
>   ip link set can0 type can bitrate 500000 fd off
> or
>   ip link set can0 type can bitrate 500000 fd dbitrate 2000000 off
> in code I can always just use the canfd_frame struct and set the
> CAN_RAW_FD_FRAMES option. Doing this I can receive CAN and CAN FD
> frames in both modes without having to fall back to the can_frame
> struct (as explained in the docs).
> 
> For sending I expected a similar behavior. I set the CAN_RAW_FD_FRAMES
> option and always sent using the canfd_frame struct. Sadly, this fails
> while writing on the interface when it is not in FD-mode with an
> Invalid Argument error. To make this work without falling back to the
> can_frame struct I just do
>   write(sock, &canfdf, sizeof(struct can_frame));
> where canfdf is a canfd_frame. Not setting CAN_RAW_FD_FRAMES when the
> interface is in CAN mode but sending using the full canfd_frame won't
> work too.
> 
> Is this expected behavior?

Yes.

> Shouldn't the error only be returned if the
> canfd_frame I pass has more than 8 bytes when the interface is not in
> FD-mode?

A CAN-2.0 frame with 8 bytes is something different than a CAN-FD frame
with 8 bytes. The kernel uses the length of the frame to decide if it is
a CAN-2.0 or CAN-FD frame. If your CAN controller has switched CAN-FD
off, it cannot send CAN-FD frames, thus you get an error.

Does that make sense?

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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