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

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Date:   Thu, 28 Jul 2022 11:02:28 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Dario Binacchi <dario.binacchi@...rulasolutions.com>
Cc:     Max Staudt <max@...as.org>, 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)

On 28.07.2022 09:36:21, Dario Binacchi wrote:
> > Most of the other CAN drivers write the BTR values into the register of
> > the hardware. How are these BTR values transported into the driver?
> >
> > There are 2 ways:
> >
> > 1) - user space configures a bitrate
> >    - the kernel calculates with the "struct can_bittiming_const" [1] given
> >      by driver and the CAN clock rate the low level timing parameters.
> >
> >      [1] https://elixir.bootlin.com/linux/v5.18/source/include/uapi/linux/can/netlink.h#L47
> >
> > 2) - user space configures low level bit timing parameter
> >      (Sample point in one-tenth of a percent, Time quanta (TQ) in
> >       nanoseconds, Propagation segment in TQs, Phase buffer segment 1 in
> >       TQs, Phase buffer segment 2 in TQs, Synchronisation jump width in
> >       TQs)
> >     - the kernel calculates the Bit-rate prescaler from the given TQ and
> >       CAN clock rate
> >
> > Both ways result in a fully calculated "struct can_bittiming" [2]. The
> > driver translates this into the hardware specific BTR values and writes
> > the into the registers.
> >
> > If you know the CAN clock and the bit timing const parameters of the
> > slcan's BTR register you can make use of the automatic BTR calculation,
> > too. Maybe the framework needs some tweaking if the driver supports both
> > fixed CAN bit rate _and_ "struct can_bittiming_const".
> 
> Does it make sense to use the device tree

The driver doesn't support DT and DT only works for static serial
interfaces.

> to provide the driver with those
> parameters required for the automatic calculation of the BTR (clock rate,
> struct can_bittiming_const, ...) that depend on the connected
> controller?

The device tree usually says it's a CAN controller compatible to X and
the following clock(s) are connected. The driver for CAN controller X
knows the bit timing const. Some USB CAN drivers query the bit timing
const from the USB device.

> In this way the solution should be generic and therefore scalable. I
> think we should also add some properties to map the calculated BTR
> value on the physical register of the controller.

The driver knows how to map the "struct can_bittiming" to the BTR
register values of the hardware.

What does the serial protocol say to the BTR values? Are these standard
SJA1000 layout with 8 MHz CAN clock or are those adapter specific?

> Or, use the device tree to extend the bittates supported by the controller
> to the fixed ones (struct can_priv::bitrate_const)?

The serial protocol defines fixed bit rates, no need to describe them in
the DT:

|           0            10 Kbit/s
|           1            20 Kbit/s
|           2            50 Kbit/s
|           3           100 Kbit/s
|           4           125 Kbit/s
|           5           250 Kbit/s
|           6           500 Kbit/s
|           7           800 Kbit/s
|           8          1000 Kbit/s

Are there more bit rates?

regards,
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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