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

From: Marc Kleine-Budde <mkl@pengutronix.de>
To: Dario Binacchi <dario.binacchi@amarulasolutions.com>
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 19:21:01 +0200	[thread overview]
Message-ID: <20220727172101.iw3yiynni6feft4v@pengutronix.de> (raw)
In-Reply-To: <CABGWkvrmbQcCHdZ_ANb+_196d9HsAxAHc4QS94R19v5STHcbiA@mail.gmail.com>

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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@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.

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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