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

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 27.07.2022 19:28:39, Max Staudt wrote:
> On Wed, 27 Jul 2022 13:30:54 +0200
> Marc Kleine-Budde <mkl@pengutronix.de> wrote:
> 
> > As far as I understand, setting the btr is an alternative way to set the
> > bitrate, right? I don't like the idea of poking arbitrary values into a
> > hardware from user space.
> 
> I agree with Marc here.
> 
> This is a modification across the whole stack, specific to a single
> device, when there are ways around.
> 
> If I understand correctly, the CAN232 "S" command sets one of the fixed
> bitrates, whereas "s" sets the two BTR registers. Now the question is,
> what do BTR0/BTR1 mean, and what are they? If they are merely a divider
> in a CAN controller's master clock, like in ELM327, then you could
> 
>   a) Calculate the BTR values from the bitrate userspace requests, or

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".

[2] https://elixir.bootlin.com/linux/v5.18/source/include/uapi/linux/can/netlink.h#L31

>   b) pre-calculate a huge table of possible bitrates and present them
>      all to userspace. Sounds horrible, but that's what I did in can327,
>      haha. Maybe I should have reigned them in a little, to the most
>      useful values.

If your adapter only supports fixed values, then that's the only way to
go.

>   c) just limit the bitrates to whatever seems most useful (like the
>      "S" command's table), and let users complain if they really need
>      something else. In the meantime, they are still free to slcand or
>      minicom to their heart's content before attaching slcan, thanks to
>      your backwards compatibility efforts.

In the early stages of the non-mainline CAN framework we had tables for
BTR values for some fixed bit rates, but that turned out to be not
scaleable.

> In short, to me, this isn't a deal breaker for your patch series.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
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