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SubjectRe: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates
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On 08/18/2017 02:39 PM, Franklin S Cooper Jr wrote:
> During test transmitting using CAN-FD at high bitrates (4 Mbps) only
> resulted in errors. Scoping the signals I noticed that only a single bit
> was being transmitted and with a bit more investigation realized the actual
> MCAN IP would go back to initialization mode automatically.
>
> It appears this issue is due to the MCAN needing to use the Transmitter
> Delay Compensation Mode as defined in the MCAN User's Guide. When this
> mode is used the User's Guide indicates that the Transmitter Delay
> Compensation Offset register should be set. The document mentions that this
> register should be set to (1/dbitrate)/2*(Func Clk Freq).
>
> Additional CAN-CIA's "Bit Time Requirements for CAN FD" document indicates
> that this TDC mode is only needed for data bit rates above 2.5 Mbps.
> Therefore, only enable this mode and only set TDCO when the data bit rate
> is above 2.5 Mbps.
>
> Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
> ---
> I'm pretty surprised that this hasn't been implemented already since
> the primary purpose of CAN-FD is to go beyond 1 Mbps and the MCAN IP
> supports up to 10 Mbps.
>
> So it will be nice to get comments from users of this driver to understand
> if they have been able to use CAN-FD beyond 2.5 Mbps without this patch.
> If they haven't what did they do to get around it if they needed higher
> speeds.
>
> Meanwhile I plan on testing this using a more "realistic" CAN bus to insure
> everything still works at 5 Mbps which is the max speed of my CAN
> transceiver.

ping. Anyone has any thoughts on this?
>
> drivers/net/can/m_can/m_can.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
> index f4947a7..720e073 100644
> --- a/drivers/net/can/m_can/m_can.c
> +++ b/drivers/net/can/m_can/m_can.c
> @@ -126,6 +126,12 @@ enum m_can_mram_cfg {
> #define DBTP_DSJW_SHIFT 0
> #define DBTP_DSJW_MASK (0xf << DBTP_DSJW_SHIFT)
>
> +/* Transmitter Delay Compensation Register (TDCR) */
> +#define TDCR_TDCO_SHIFT 8
> +#define TDCR_TDCO_MASK (0x7F << TDCR_TDCO_SHIFT)
> +#define TDCR_TDCF_SHIFT 0
> +#define TDCR_TDCF_MASK (0x7F << TDCR_TDCO_SHIFT)
> +
> /* Test Register (TEST) */
> #define TEST_LBCK BIT(4)
>
> @@ -977,6 +983,8 @@ static int m_can_set_bittiming(struct net_device *dev)
> const struct can_bittiming *dbt = &priv->can.data_bittiming;
> u16 brp, sjw, tseg1, tseg2;
> u32 reg_btp;
> + u32 enable_tdc = 0;
> + u32 tdco;
>
> brp = bt->brp - 1;
> sjw = bt->sjw - 1;
> @@ -991,9 +999,23 @@ static int m_can_set_bittiming(struct net_device *dev)
> sjw = dbt->sjw - 1;
> tseg1 = dbt->prop_seg + dbt->phase_seg1 - 1;
> tseg2 = dbt->phase_seg2 - 1;
> +
> + /* TDC is only needed for bitrates beyond 2.5 MBit/s
> + * Specified in the "Bit Time Requirements for CAN FD" document
> + */
> + if (dbt->bitrate > 2500000) {
> + enable_tdc = DBTP_TDC;
> + /* Equation based on Bosch's M_CAN User Manual's
> + * Transmitter Delay Compensation Section
> + */
> + tdco = priv->can.clock.freq / (dbt->bitrate * 2);
> + m_can_write(priv, M_CAN_TDCR, tdco << TDCR_TDCO_SHIFT);
> + }
> +
> reg_btp = (brp << DBTP_DBRP_SHIFT) | (sjw << DBTP_DSJW_SHIFT) |
> (tseg1 << DBTP_DTSEG1_SHIFT) |
> - (tseg2 << DBTP_DTSEG2_SHIFT);
> + (tseg2 << DBTP_DTSEG2_SHIFT) | enable_tdc;
> +
> m_can_write(priv, M_CAN_DBTP, reg_btp);
> }
>
>

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