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From: Marc Kleine-Budde <>
To: Thomas Wagner <>
Subject: Re: Write canfd_frame to can interface
Date: Thu, 29 Jul 2021 14:14:17 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <00a601d78471$9e6f3ca0$db4db5e0$>

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On 29.07.2021 14:02:34, Thomas Wagner wrote:
> Hello Marc,
> On 2021-07-29 12:55, Marc Kleine-Budde wrote:
> > On 29.07.2021 12:03:56, wrote:
> >> 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?
> Sure!
> I see how a CAN-2.0 frame with 8 bytes differs from a CAN-FD frame with
> 8-bytes, but when I receive into a canfd_frame I can't differentiate like that
> anymore. In userspace an 8B CAN-2.0 frame and an 8B CAN-FD frame look just
> the same, no matter the interface running with FD on or off.
> ... which is wrong as I just noticed. Paying attention to the actual bytes read
> by the socket I can see the 16 vs. 72B that make up a can_frame vs. a
> canfd_frame respectively. Even when always writing into a canfd_frame.

With the C language the type information of your read buffer doesn't
leave the scope of your function (write() uses a void * for the buffer).
The length information is checked in the kernel, but only if it's large enough:

> The same differentiation I must make when sending...

ACK - If you an idea how to improve the documentation, let me know!


Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   |  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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      reply	other threads:[~2021-07-29 12:14 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 10:03 Write canfd_frame to can interface thomas
2021-07-29 10:55 ` Marc Kleine-Budde
2021-07-29 12:02   ` Thomas Wagner
2021-07-29 12:14     ` Marc Kleine-Budde [this message]

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