Re: [PATCH v5 4/4] iplink_can: add new CAN FD bittiming parameters: Transmitter Delay Compensation (TDC)

Marc Kleine-Budde
2021-08-16T13:51:26Z

[-- Attachment #1: Type: text/plain, Size: 4053 bytes --]

On 14.08.2021 19:17:28, 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 brings command line support to nine TDC parameters which
> were recently added to the kernel's CAN netlink interface in order to
> implement TDC:
> - IFLA_CAN_TDC_TDCV_MIN: Transmitter Delay Compensation Value
> minimum value
> - IFLA_CAN_TDC_TDCV_MAX: Transmitter Delay Compensation Value
> maximum value
> - IFLA_CAN_TDC_TDCO_MIN: Transmitter Delay Compensation Offset
> minimum value
> - IFLA_CAN_TDC_TDCO_MAX: Transmitter Delay Compensation Offset
> maximum value
> - IFLA_CAN_TDC_TDCF_MIN: Transmitter Delay Compensation Filter
> window minimum value
> - IFLA_CAN_TDC_TDCF_MAX: Transmitter Delay Compensation Filter
> window maximum value
> - IFLA_CAN_TDC_TDCV: Transmitter Delay Compensation Value
> - IFLA_CAN_TDC_TDCO: Transmitter Delay Compensation Offset
> - IFLA_CAN_TDC_TDCF: Transmitter Delay Compensation Filter window
>
> All those new parameters are nested together into the attribute
> IFLA_CAN_TDC.
>
> A tdc-mode parameter allow to specify how to operate. Valid options
> are:
>
> * auto: the transmitter automatically measures TDCV. As such, TDCV
> values can not be manually provided. In this mode, the user must
> specify TDCO and may also specify TDCF if supported.
>
> * manual: Use the TDCV value provided by the user are used. In this
> mode, the user must specify both TDCV and TDCO and may also
> specify TDCF if supported.
>
> * off: TDC is explicitly disabled.
>
> * tdc-mode parameter omitted (default mode): the kernel decides
> whether TDC should be enabled or not and if so, it calculates the
> TDC values. TDC parameters are an expert option and the average
> user is not expected to provide those, thus the presence of this
> "default mode".
>
> TDCV is always reported in manual mode. In auto mode, TDCV is reported
> only if the value is available. Especially, the TDCV might not be
> available if the controller has no feature to report it or if the
> value in not yet available (i.e. no data sent yet and measurement did
> not occur).
>
> TDCF is reported only if tdcf_max is not zero (i.e. if supported by the controller).
>
> For reference, here are a few samples of how the output looks like:
>
> $ ip link set can0 type can bitrate 1000000 dbitrate 8000000 fd on tdco 7 tdcf 8 tdc-mode auto
>
> $ ip --details link show can0
> 1: can0: <NOARP,ECHO> mtu 72 qdisc noop state DOWN mode DEFAULT group default qlen 10
> link/can promiscuity 0 minmtu 0 maxmtu 0
> can <FD,TDC_AUTO> state STOPPED (berr-counter tx 0 rx 0) restart-ms 0
^^^^^^^^
This is just the supported mode(s), right?

> bitrate 1000000 sample-point 0.750
> tq 12 prop-seg 29 phase-seg1 30 phase-seg2 20 sjw 1 brp 1
> ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp_inc 1
> dbitrate 8000000 dsample-point 0.700
> dtq 12 dprop-seg 3 dphase-seg1 3 dphase-seg2 3 dsjw 1 dbrp 1
> tdco 7 tdcf 8
> ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 dbrp_inc 1
> tdco 0..127 tdcf 0..127
> clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535

Is there a way to figure out, which tdc mode is currently active?

AFAICS just implicitly:
- tdco + tdcv -> manual
- tdco -> automatic
- neither -> off

correct?

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 |

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]