netdev - Re: [RFC PATCH v3 8/9] can: slcan: add support to set bit time register (btr)

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Date:   Wed, 27 Jul 2022 19:21:01 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Dario Binacchi <dario.binacchi@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org, linux-can@...r.kernel.org,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        michael@...rulasolutions.com,
        Amarula patchwork <linux-amarula@...rulasolutions.com>,
        Jeroen Hofstee <jhofstee@...tronenergy.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        netdev@...r.kernel.org,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Max Staudt <max@...as.org>
Subject: Re: [RFC PATCH v3 8/9] can: slcan: add support to set bit time
 register (btr)

On 27.07.2022 17:55:10, Dario Binacchi wrote:
> Hello Marc,
> 
> On Wed, Jul 27, 2022 at 1:31 PM Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> >
> > On 26.07.2022 23:02:16, Dario Binacchi wrote:
> > > It allows to set the bit time register with tunable values.
> > > The setting can only be changed if the interface is down:
> > >
> > > ip link set dev can0 down
> > > ethtool --set-tunable can0 can-btr 0x31c
> > > ip link set dev can0 up
> >
> > As far as I understand, setting the btr is an alternative way to set the
> > bitrate, right?
> 
> I thought of a non-standard bitrate or, in addition to the bitrate, the
> possibility of enabling some specific CAN controller options. Maybe Oliver
> could help us come up with the right answer.
> 
> This is the the slcan source code:
> https://github.com/linux-can/can-utils/blob/cad1cecf1ca19277b5f5db39f8ef6f8ae426191d/slcand.c#L331
> btr case cames after speed but they don't seem to be considered alternative.
> 
> > I don't like the idea of poking arbitrary values into a
> > hardware from user space.
> 
> However this is already possible through the slcand and slcan_attach
> applications.
> Furthermore, the driver implements the LAWICEL ASCII protocol for CAN
> frame transport over serial lines,
> and this is one of the supported commands.
> 
> >
> > Do you have a use case for this?
> 
> I use the applications slcand and slcan_attach as a reference, I try to make the
> driver independent from them for what concerns the CAN setup. And the bit time
> register setting is the last dependency.

Ok - We avoided writing bit timing registers from user space into the
hardware for all existing drivers. If there isn't a specific use case,
let's skip this patch. If someone comes up with a use case we can think
of a proper solution.

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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