Messages in this thread | | | Date | Wed, 27 Jul 2022 22:12:13 +0200 | Subject | Re: [RFC PATCH v3 8/9] can: slcan: add support to set bit time register (btr) | From | Oliver Hartkopp <> |
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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@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.
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 ...
Best regards, Oliver
> >> In short, to me, this isn't a deal breaker for your patch series. > > Marc >
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