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SubjectRe: [PATCH v6 4/6] can: m_can: Support higher speed CAN-FD bitrates
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Hi,

On Tuesday 02 January 2018 07:05 PM, Marc Kleine-Budde wrote:
> On 12/22/2017 02:31 PM, Faiz Abbas wrote:
>> From: Franklin S Cooper Jr <fcooper@ti.com>
>>
>> During test transmitting using CAN-FD at high bitrates (> 2 Mbps)
>> would fail. 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 with the correct value for the transmitter delay
>> compensation offset (tdco). What impacts the tdco value isn't 100% clear
>> but to calculate it you use an equation defined in the MCAN User's Guide.
>>
>> The user guide mentions that this register needs to be set based on clock
>> values, secondary sample point and the data bitrate. One of the key
>> variables that can't automatically be determined is the secondary
>> sample point (ssp). This ssp is similar to the sp but is specific to this
>> transmitter delay compensation mode. The guidelines for configuring
>> ssp is rather vague but via some CAN test it appears for DRA76x that putting
>> the value same as data sampling point works.
>>
>> The CAN-CIA's "Bit Time Requirements for CAN FD" paper presented at
>> the International CAN Conference 2013 indicates that this TDC mode is
>> only needed for data bit rates above 2.5 Mbps. Therefore, only enable
>> this mode when the data bit rate is above 2.5 Mbps.
>>
>> Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
>> Signed-off-by: Sekhar Nori <nsekhar@ti.com>
>> Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
>> ---
>> drivers/net/can/m_can/m_can.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 40 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
>> index 53e764f..371ffc1 100644
>> --- a/drivers/net/can/m_can/m_can.c
>> +++ b/drivers/net/can/m_can/m_can.c
>> @@ -127,6 +127,12 @@ 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_TDCF_SHIFT)
>> +
>> /* Test Register (TEST) */
>> #define TEST_LBCK BIT(4)
>>
>> @@ -991,7 +997,8 @@ static int m_can_set_bittiming(struct net_device *dev)
>> const struct can_bittiming *bt = &priv->can.bittiming;
>> const struct can_bittiming *dbt = &priv->can.data_bittiming;
>> u16 brp, sjw, tseg1, tseg2;
>> - u32 reg_btp;
>> + u32 reg_btp, tdco, ssp;
>
> Please move the tdco and the ssp into "if (dbt->bitrate > 2500000)" scope.

Ok.

>
> Initialize "reg_btp = 0", see below.
>
>> + bool enable_tdc = false;
>
> Please remove, see below.
>
>>
>> brp = bt->brp - 1;
>> sjw = bt->sjw - 1;
>> @@ -1006,9 +1013,41 @@ 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.
>> + * This is mentioned in the "Bit Time Requirements for CAN FD"
>> + * paper presented at the International CAN Conference 2013
>> + */
>> + if (dbt->bitrate > 2500000) {
>> + /* Use the same value of secondary sampling point
>> + * as the data sampling point
>> + */
>> + ssp = dbt->sample_point;
>> +
>> + /* Equation based on Bosch's M_CAN User Manual's
>> + * Transmitter Delay Compensation Section
>> + */
>> + tdco = (priv->can.clock.freq / 1000) *
>> + ssp / dbt->bitrate;
>> +
>> + /* Max valid TDCO value is 127 */
>> + if (tdco > 127) {
>> + netdev_warn(dev, "TDCO value of %u is beyond maximum limit. Disabling Transmitter Delay Compensation mode\n",
>
> "maximum limit"? Either "maximum" or "limit" should be enough. If the
> value is above 127, does it make sense to disable the tdco completely?

I guess we can put the closest possible value (127) so that the ssp
calculated by the IP is as close to the chosen one as possible.

>
>> + tdco);
>> + } else {
>> + enable_tdc = true;
>
> Why not set "reg_btp |= DBTP_TDC;" here directly?
>
>> + 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);
>
> Adjust this to "reg_btp |=".
>
>> +
>> + if (enable_tdc)
>> + reg_btp |= DBTP_TDC;
>
> Please remove.

Sure, will remove.

Thanks,
Faiz


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