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

From: Dario Binacchi <dario.binacchi@amarulasolutions.com>
To: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: linux-kernel@vger.kernel.org, linux-can@vger.kernel.org,
	Oliver Hartkopp <socketcan@hartkopp.net>,
	michael@amarulasolutions.com,
	Amarula patchwork <linux-amarula@amarulasolutions.com>,
	Jeroen Hofstee <jhofstee@victronenergy.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Wolfgang Grandegger <wg@grandegger.com>,
	netdev@vger.kernel.org,
	Vincent Mailhol <mailhol.vincent@wanadoo.fr>,
	Max Staudt <max@enpas.org>
Subject: Re: [RFC PATCH v3 8/9] can: slcan: add support to set bit time register (btr)
Date: Wed, 27 Jul 2022 17:55:10 +0200	[thread overview]
Message-ID: <CABGWkvrmbQcCHdZ_ANb+_196d9HsAxAHc4QS94R19v5STHcbiA@mail.gmail.com> (raw)
In-Reply-To: <20220727113054.ffcckzlcipcxer2c@pengutronix.de>

Hello Marc,

On Wed, Jul 27, 2022 at 1:31 PM Marc Kleine-Budde <mkl@pengutronix.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.

Thanks and regards,
Dario

>
> Marc
>
> --
> Pengutronix e.K.                 | Marc Kleine-Budde           |
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-- 

Dario Binacchi

Embedded Linux Developer

dario.binacchi@amarulasolutions.com

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