netdev - Re: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates

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Date:   Wed, 13 Sep 2017 16:58:12 -0500
From:   Franklin S Cooper Jr <fcooper@...com>
To:     <wg@...ndegger.com>, <mkl@...gutronix.de>, <mario.huettel@....net>,
        <socketcan@...tkopp.net>, <quentin.schulz@...e-electrons.com>,
        <edumazet@...gle.com>, <linux-can@...r.kernel.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Nori, Sekhar" <nsekhar@...com>
Subject: Re: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates



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@...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);
>  	}
>  
>