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Date: Thu, 28 Jul 2022 12:23:04 +0200
From: Dario Binacchi <dario.binacchi@...rulasolutions.com>
To: Marc Kleine-Budde <mkl@...gutronix.de>
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 Thu, Jul 28, 2022 at 11:02 AM Marc Kleine-Budde <mkl@...gutronix.de> wrote:
>
> 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?
I think they are adapter specific.
This is what the can232_ver3_Manual.pdf reports:
sxxyy[CR] Setup with BTR0/BTR1 CAN bit-rates where xx and yy is a hex
value. This command is only active if the CAN
channel is closed.
xx BTR0 value in hex
yy BTR1 value in hex
Example: s031C[CR]
Setup CAN with BTR0=0x03 & BTR1=0x1C
which equals to 125Kbit.
But I think the example is misleading because IMHO it depends on the
adapter's controller (0x31C -> 125Kbit).
>
> > 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?
No, the manual can232_ver3_Manual.pdf does not contain any others.
What about defining a device tree node for the slcan (foo adapter):
slcan {
compatible = "can,slcan";
/* bit rate btr0btr1 */
additional-bitrates = < 33333 0x0123
80000 0x4567
83333 0x89ab
150000 0xcd10
175000 0x2345
200000 0x6789>
};
So that the can_priv::bitrate_cons array (dynamically created) will
contain the bitrates
10000,
20000,
50000,
100000,
125000,
250000,
500000,
800000,
1000000 /* end of standards bitrates, use S command */
33333, /* use s command, btr 0x0123 */
80000, /* use s command, btr 0x4567 */
83333, /* use s command, btr 0x89ab */
150000, /* use s command, btr 0xcd10 */
175000, /* use s command, btr 0x2345 */
200000 /* use s command, btr 0x6789 */
};
So if a standard bitrate is requested, the S command is used,
otherwise the s command with the associated btr.
Thanks and regards,
Dario
>
> 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 |
--
Dario Binacchi
Embedded Linux Developer
dario.binacchi@...rulasolutions.com
__________________________________
Amarula Solutions SRL
Via Le Canevare 30, 31100 Treviso, Veneto, IT
T. +39 042 243 5310
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www.amarulasolutions.com
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