linux-kernel - Re: [RFC][PATCH 0/2] LIN support for Linux

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Message-ID: <ed685ce0-16e2-3f7c-173a-ac14f32d9ca6@hartkopp.net>
Date:   Wed, 30 Nov 2022 22:02:23 +0100
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Christoph Fritz <christoph.fritz@...dev.de>
Cc:     Pavel Pisa <pisa@....felk.cvut.cz>,
        Richard Weinberger <richard@....at>,
        Andreas Lauser <andreas.lauser@...tion.io>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/2] LIN support for Linux

Hi Christoph, all,

On 28.11.22 11:16, Christoph Fritz wrote:

>> IIRC the implementation of the master/slave timings was the biggest
> 
> Currently sllin only supports master mode, I guess because of the tight
> timing constraints.

I think this has to be corrected.

In the master mode the SocketCAN Broadcast Manager (BCM) is configured 
to periodically send LIN headers
(according to LIN schedule table).

https://www.kernel.org/doc/html/latest/networking/can.html#broadcast-manager-protocol-sockets-sock-dgram

This is a very easy approach to precisely send the the LIN frames from 
kernel space and also atomically change the content of (all) the 
configured LIN frames while the schedule table is continuously processed.

Sending LIN frames directly from *userspace* (and handling timers there) 
was *never* intended for real LIN communication - although the examples 
(with cangen) in the document look like this.

Same applies to the slave mode:

If you check out 
https://raw.githubusercontent.com/wiki/lin-bus/linux-lin/sllin-doc.pdf 
on page 11 you are able to enable the slave mode with

	insmod ./sllin.ko master=0

The 'trick' about this mode is that the RTR-functionality of the BCM is 
able process the incoming CAN/LIN identifier and *instantly* send back 
some pre-defined data for that specific LIN-ID, so that the SLLIN driver 
sends/answers the 'data' section of the received LIN-ID within the 
required timing constrains for LIN slaves.

Not sure if the info about this concept got lost somehow, but the 
CAN_BCM is the key for handling the LIN protocol and offload the LIN 
scheduling (master/slave) to the kernel for the comparably dumb tty 
interfaces.

Best regards,
Oliver