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:

quoted

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

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