From: Marc Kleine-Budde <mkl@pengutronix.de>
To: Thomas Wagner <thomas@the-wagner.de>
Cc: linux-can@vger.kernel.org
Subject: Re: Write canfd_frame to can interface
Date: Thu, 29 Jul 2021 14:14:17 +0200 [thread overview]
Message-ID: <20210729121417.kysljj4636hmhem2@pengutronix.de> (raw)
In-Reply-To: <00a601d78471$9e6f3ca0$db4db5e0$@the-wagner.de>
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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, thomas@the-wagner.de 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:
https://elixir.bootlin.com/linux/v5.13/source/net/can/raw.c#L850
> The same differentiation I must make when sending...
ACK - If you an idea how to improve the documentation, let me know!
regards,
Marc
--
Pengutronix e.K. | Marc Kleine-Budde |
Embedded Linux | https://www.pengutronix.de |
Vertretung West/Dortmund | Phone: +49-231-2826-924 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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prev parent 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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