Re: [PATCH v2 1/5] can: add new CAN FD bittiming parameters: Transmitter Delay Compensation (TDC)

[Vincent MAILHOL <mailhol.vincent@wanadoo.fr>]

On Wed. 16 Jun 2021 at 18:54, Marc Kleine-Budde <mkl@pengutronix.de> wrote:
> On 24.02.2021 09:20:04, Vincent Mailhol wrote:
> > At high bit rates, the propagation delay from the TX pin to the RX pin
> > of the transceiver causes measurement errors: the sample point on the
> > RX pin might occur on the previous bit.
> >
> > This issue is addressed in ISO 11898-1 section 11.3.3 "Transmitter
> > delay compensation" (TDC).
> >
> > This patch adds two new structures: can_tdc and can_tdc_const in order
> > to implement this TDC.
> >
> > The structures are then added to can_priv.
> >
> > A controller supports TDC if an only if can_priv::tdc_const is not
> > NULL.
> >
> > TDC is active if and only if:
> >   - fd flag is on
> >   - can_priv::tdc.tdco is not zero.
> > It is the driver responsibility to check those two conditions are met.
> >
> > No new controller modes are introduced (i.e. no CAN_CTRL_MODE_TDC) in
> > order not to be redundant with above logic.
> >
> > The names of the parameters are chosen to match existing CAN
> > controllers specification. References:
> >   - Bosch C_CAN FD8:
> > https://www.bosch-semiconductors.com/media/ip_modules/pdf_2/c_can_fd8/users_manual_c_can_fd8_r210_1.pdf
> >   - Microchip CAN FD Controller Module:
> > http://ww1.microchip.com/downloads/en/DeviceDoc/MCP251XXFD-CAN-FD-Controller-Module-Family-Reference-Manual-20005678B.pdf
> >   - SAM E701/S70/V70/V71 Family:
> > https://www.mouser.com/datasheet/2/268/60001527A-1284321.pdf
> >
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> > ---
> >  include/linux/can/bittiming.h | 65 +++++++++++++++++++++++++++++++++++
> >  include/linux/can/dev.h       |  3 ++
> >  2 files changed, 68 insertions(+)
> >
> > diff --git a/include/linux/can/bittiming.h b/include/linux/can/bittiming.h
> > index 707575c668f4..91bf9f8926a7 100644
> > --- a/include/linux/can/bittiming.h
> > +++ b/include/linux/can/bittiming.h
> > @@ -1,5 +1,6 @@
> >  /* SPDX-License-Identifier: GPL-2.0-only */
> >  /* Copyright (c) 2020 Pengutronix, Marc Kleine-Budde <kernel@pengutronix.de>
> > + * Copyright (c) 2021 Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> >   */
> >
> >  #ifndef _CAN_BITTIMING_H
> > @@ -10,6 +11,70 @@
> >
> >  #define CAN_SYNC_SEG 1
> >
> > +/*
> > + * struct can_tdc - CAN FD Transmission Delay Compensation parameters
> > + *
> > + * At high bit rates, the propagation delay from the TX pin to the RX
> > + * pin of the transceiver causes measurement errors: the sample point
> > + * on the RX pin might occur on the previous bit.
> > + *
> > + * To solve this issue, ISO 11898-1 introduces in section 11.3.3
> > + * "Transmitter delay compensation" a SSP (Secondary Sample Point)
> > + * equal to the distance, in time quanta, from the start of the bit
> > + * time on the TX pin to the actual measurement on the RX pin.
> > + *
> > + * This structure contains the parameters to calculate that SSP.
> > + *
> > + * @tdcv: Transmitter Delay Compensation Value. Distance, in time
> > + *   quanta, from when the bit is sent on the TX pin to when it is
> > + *   received on the RX pin of the transmitter. Possible options:
> > + *
> > + *     O: automatic mode. The controller dynamically measure @tdcv
>           ^
>
> I think this is supposed to be a 0, not a O?
> I'll send a patch

Absolutely. I am not sure how I did such a mistake. I was
probably not fully awake as there are two other grammar issues
there. I sent another patch to fix those.

Yours sincerely,
Vincent