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

From: Marc Kleine-Budde <mkl@pengutronix.de>
To: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: linux-can <linux-can@vger.kernel.org>,
	Oliver Hartkopp <socketcan@hartkopp.net>,
	Wolfgang Grandegger <wg@grandegger.com>
Subject: Re: [PATCH v2 1/5] can: add new CAN FD bittiming parameters: Transmitter Delay Compensation (TDC)
Date: Wed, 16 Jun 2021 11:54:10 +0200	[thread overview]
Message-ID: <20210616095410.lsvqnismp4uydrmh@pengutronix.de> (raw)
In-Reply-To: <20210224002008.4158-2-mailhol.vincent@wanadoo.fr>

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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

> + *	  for each transmited CAN FD frame.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
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