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

[Marc Kleine-Budde]

On 27.07.2022 22:12:13, Oliver Hartkopp wrote:
> 
> 
> On 27.07.22 20:24, Marc Kleine-Budde wrote:
> > On 27.07.2022 19:28:39, Max Staudt wrote:
> > > On Wed, 27 Jul 2022 13:30:54 +0200
> > > Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> 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.
> 
> The Microchip CAN BUS Analyzer Tool is another example for fixed bitrates:
> 
> https://elixir.bootlin.com/linux/v5.18.14/source/drivers/net/can/usb/mcba_usb.c#L156
> 
> So this might be the way to go here too ...

The slcan driver already supports fixed bitrates. This discussion is
about setting the BTR registers in one way or another.

Marc

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