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

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Message-ID: <CABGWkvo0B8XM+5qLhz3zY2DzyUrEQtQyJnd91VweUWDUcjyr5A@mail.gmail.com>
Date:   Fri, 29 Jul 2022 08:52:07 +0200
From:   Dario Binacchi <dario.binacchi@...rulasolutions.com>
To:     Max Staudt <max@...as.org>, Marc Kleine-Budde <mkl@...gutronix.de>
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
Subject: Re: [RFC PATCH v3 8/9] can: slcan: add support to set bit time
 register (btr)

Hello Marc and Max,

On Thu, Jul 28, 2022 at 12:57 PM Max Staudt <max@...as.org> wrote:
>
> On Thu, 28 Jul 2022 12:50:49 +0200
> Marc Kleine-Budde <mkl@...gutronix.de> wrote:
>
> > On 28.07.2022 12:23:04, Dario Binacchi wrote:
> > > > > Does it make sense to use the device tree
> > > >
> > > > The driver doesn't support DT and DT only works for static serial
> > > > interfaces.
> >
> > Have you seen my remarks about Device Tree?
>
> Dario, there seems to be a misunderstanding about the Device Tree.
>
> It is used *only* for hardware that is permanently attached, present at
> boot, and forever after. Not for dyamically added stuff, and definitely
> not for ldiscs that have to be attached manually by the user.
>
>
> The only exception to this is if you have an embedded device with an
> slcan adapter permanently attached to one of its UARTs. Then you can
> use the serdev ldisc adapter to attach the ldisc automatically at boot.

It is evident that I am lacking some skills (I will try to fix it :)).
I think it is
equally clear that it is not worth going down this path.

>
> If you are actively developing for such a use case, please let us know,
> so we know what you're after and can help you better :)
>

I don't have a use case, other than to try, if possible, to make the driver
autonomous from slcand / slcan_attach for the CAN bus setup.

Returning to Marc's previous analysis:
"... Some USB CAN drivers query the bit timing const from the USB device."

Can we think of taking the gs_usb driver as inspiration for getting/setting the
bit timings?

https://elixir.bootlin.com/linux/latest/source/drivers/net/can/usb/gs_usb.c#L951
https://elixir.bootlin.com/linux/latest/source/drivers/net/can/usb/gs_usb.c#L510

and, as done with patches:

can: slcan: extend the protocol with CAN state info
can: slcan: extend the protocol with error info

further extend the protocol to get/set the bit timing from / to the adapter ?
In the case of non-standard bit rates, the driver would try, depending on the
firmware of the adapter, to calculate and set the bit timings autonomously.

Thanks and regards,
Dario

>
> Max



-- 

Dario Binacchi

Embedded Linux Developer

dario.binacchi@...rulasolutions.com

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