This RFC is a follow-up on the discussion started in: https://lore.kernel.org/linux-can/20210112130538.14912-1-mailhol.vincent@xxxxxxxxxx/T/#t * Scope of this RFC * I want to discuss 1/ which TDC values we should provide to the user and 2/ how to calculate those. In this RFC, I will not directly discuss how to actually modify the netlink ABI. * The code * /** * 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 and needs to be * corrected. * * 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 to the 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. * * 0: Automatic mode (default). Use the value dynamically * measured by the controller. * * Other values: manual mode. Use the fixed provided value. * * N.B. when using the automatic mode, the dynamically measured * value might not be visible to the kernel. * * @tdco: Transmitter Delay Compensation Offset. Offset value, in time * quanta, defining the delay between the start of the bit * reception on the CANRX pin of the transceiver and the SSP such * as SSP = @tdcv + @tdco. * * @tdcf: Transmitter Delay Compensation Filter window. Defines the * minimum value for the SSP position, in time quanta. If SSP is * less than @tdcf, then no compensation delay occurs and the * normal sampling point is used instead. The feature is enabled * if and only if @tdcf is configured to a value greater than @tdco. */ struct can_tdc { u16 tdc; u16 tdco; u16 tdcf; }; /** * struct can_tdc_const - CAN hardware-dependent bit-timing constant * for TDC * * @tdcv_max: Transmitter Delay Compensation Value maximum value, * should be set to zero if the controller does not support * manual mode for tdcv. * @tdco_max: Transmitter Delay Compensation Offset maximum value. * Should be set to zero if the controller does not support TDC. * @tdcf_max: Transmitter Delay Compensation Filter window maximum * value. Should be set to zero if the controller does not * support this feature. */ struct can_tdc_const { u16 tdcv_max; u16 tdco_max; u16 tdcf_max; }; /* Do the bittiming calculation of the tdc parameters */ static void can_set_tdc(const struct can_bittiming *dbt, struct can_tdc *tdc, const struct can_tdc_const *tdc_const) { /* As specified in ISO 11898-1 section 11.3.3 "Transmitter * delay compensation" (TDC) is only applicable if data BRP is * one or two. */ if ((dbt->brp == 1) || (dbt->brp == 2)) { memset(tdc, 0, sizeof(*tdc)); return; } tdc->tdcv = 0; /* Convert the sample point from tenth of a percent to time quanta */ tdc->tdco = min(can_bit_time(dbt) * dbt->sample_point / 100, tdc_const->tdco_max); tdc->tdcf = 0; } * Explanation on the code * The only way to calculate tdcv is through measurement which is done by the controller. This parameter is only here to receive a fixed value that would be given by the user through the netlink interface. The calculation of tdco is of my conception. I could not find any reference formula. My logic is to just reuse the normal sample point so that at the end ssp = tdcv + sp. Another method might set ssp to the middle of the bit: tdc->tdco = can_bit_time(dbt) / 2 My current proposal is not to use tdcf by default (leave it to zero) if doing bittiming calculation. Meaning that this value will only be used if configured by the user through the netlink interface. The reason for this choice are: * the lack of test environment: on my hardware (the ETAS ES582.1), everything works up fine to 8 Mbps just by using the tdco. * I could not find a good reference to calculate that value. Any ideas of how to calculate tdcf is welcome! * References * This is a collection of specifications and references which I used while writing this RFC. I believe that all TDC use cases are covered in this RFC. - 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 - ISO 11898-1 Thank you for your comments, Yours sincerely, Vincent
