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Date:   Thu, 26 May 2022 15:51:33 +0000
From:   Srinivas Neeli <sneeli@...inx.com>
To:     Vincent Mailhol <vincent.mailhol@...il.com>
CC:     "wg@...ndegger.com" <wg@...ndegger.com>,
        "mkl@...gutronix.de" <mkl@...gutronix.de>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        Appana Durga Kedareswara Rao <appanad@...inx.com>,
        Srinivas Goud <sgoud@...inx.com>,
        Michal Simek <michals@...inx.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        git <git@...inx.com>
Subject: RE: [PATCH] can: xilinx_can: Add Transmitter delay compensation (TDC)
 feature support

Hi Vincent,

> -----Original Message-----
> From: Vincent Mailhol <vincent.mailhol@...il.com>
> Sent: Friday, May 13, 2022 6:54 AM
> To: Srinivas Neeli <sneeli@...inx.com>
> Cc: wg@...ndegger.com; mkl@...gutronix.de; davem@...emloft.net;
> edumazet@...gle.com; Appana Durga Kedareswara Rao
> <appanad@...inx.com>; Srinivas Goud <sgoud@...inx.com>; Michal Simek
> <michals@...inx.com>; kuba@...nel.org; pabeni@...hat.com; linux-
> can@...r.kernel.org; netdev@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; git
> <git@...inx.com>
> Subject: Re: [PATCH] can: xilinx_can: Add Transmitter delay compensation
> (TDC) feature support
> 
> On Fri. 13 May 2022 at 07:30, Srinivas Neeli <srinivas.neeli@...inx.com>
> wrote:
> > Added Transmitter delay compensation (TDC) feature support.
> > In the case of higher measured loop delay with higher baud rates,
> > observed bit stuff errors.
> > By enabling the TDC feature in a controller, will compensate for the
> > measure loop delay in the receive path.
> > TDC feature requires BRP values can be 1 or 2.
> > The current CAN framework does not limit the brp so while using TDC,
> > have to restrict BRP values.
> > Ex:
> > ip link set can0 type can tq 12 prop-seg 39 phase-seg1 20 phase-seg2
> > 20 sjw 20 dtq 12 dprop-seg 5 dphase-seg1 6 dphase-seg2 4 dsjw 4 fd on
> > loopback on tdco 12 tdc-mode auto
> 
> Did you experience some cases in which you had BRP > 2 and saw
> transmission errors due to the absence of delay compensation? Could you
> show the calculated values?
> Usually, you start to observe but stuff error at high bitrates (e.g.
> ~5MBPS), and for such bitrates, time quanta has to be small which then
> results in a small BRP.

yes, we observed errors with higher baud rates(4 and 5 MBPS).
Observation:
BRPA 1Mbps Sampling 75%
BRPD 5MBPS Sampling 75%
On NXP PHY observed a delay of 160 ns. so observing the failure.
After enabling the TDC feature to work fine.

> 
> > Signed-off-by: Srinivas Neeli <srinivas.neeli@...inx.com>
> > ---
> >  drivers/net/can/xilinx_can.c | 30 +++++++++++++++++++++++++-----
> >  1 file changed, 25 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/can/xilinx_can.c
> > b/drivers/net/can/xilinx_can.c index e2b15d29d15e..7af518fbed02 100644
> > --- a/drivers/net/can/xilinx_can.c
> > +++ b/drivers/net/can/xilinx_can.c
> > @@ -1,7 +1,7 @@
> >  // SPDX-License-Identifier: GPL-2.0-or-later
> >  /* Xilinx CAN device driver
> >   *
> > - * Copyright (C) 2012 - 2014 Xilinx, Inc.
> > + * Copyright (C) 2012 - 2022 Xilinx, Inc.
> >   * Copyright (C) 2009 PetaLogix. All rights reserved.
> >   * Copyright (C) 2017 - 2018 Sandvik Mining and Construction Oy
> >   *
> > @@ -133,6 +133,8 @@ enum xcan_reg {
> >  #define XCAN_DLCR_BRS_MASK             0x04000000 /* BRS Mask in DLC */
> >
> >  /* CAN register bit shift - XCAN_<REG>_<BIT>_SHIFT */
> > +#define XCAN_BRPR_TDCO_SHIFT_CANFD     8  /* Transmitter Delay
> Compensation Offset */
> 
> Having CANFD in the name is redundant (TDC implies CANFD).
> #define XCAN_BRPR_TDCO_SHIFT 8
update in V2.

> 
> > +#define XCAN_BRPR_TDCE_SHIFT_CANFD     16 /* Transmitter Delay
> Compensation (TDC) Enable */
> 
> Why not:
> #define XCAN_BRPR_TDC_ENABLE BIT(16)
update in V2.

> 
> >  #define XCAN_BTR_SJW_SHIFT             7  /* Synchronous jump width */
> >  #define XCAN_BTR_TS2_SHIFT             4  /* Time segment 2 */
> >  #define XCAN_BTR_SJW_SHIFT_CANFD       16 /* Synchronous jump width
> */
> > @@ -259,7 +261,7 @@ static const struct can_bittiming_const
> xcan_bittiming_const_canfd2 = {
> >         .tseg2_min = 1,
> >         .tseg2_max = 128,
> >         .sjw_max = 128,
> > -       .brp_min = 2,
> > +       .brp_min = 1,
> 
> Was there any reason to have brp_min = 2 in the first place?
> I think this change  should be a different patch. If the brp_min = 2 is just a
> typo, you might also want to backport it to stable branches.

On early silicon engineering samples we observed bit shrinking issue when we use brp =1 , hence we updated brp_min =2.
As in production silicon this issue is fixed we are planning to revert the patch.

> 
> >         .brp_max = 256,
> >         .brp_inc = 1,
> >  };
> > @@ -272,11 +274,21 @@ static struct can_bittiming_const
> xcan_data_bittiming_const_canfd2 = {
> >         .tseg2_min = 1,
> >         .tseg2_max = 16,
> >         .sjw_max = 16,
> > -       .brp_min = 2,
> > +       .brp_min = 1,
> >         .brp_max = 256,
> >         .brp_inc = 1,
> >  };
> >
> > +/* Transmission Delay Compensation constants for CANFD2.0 and Versal
> > +*/ static const struct can_tdc_const xcan_tdc_const = {
> > +       .tdcv_min = 0,
> > +       .tdcv_max = 0, /* Manual mode not supported. */
> 
> Right, had a look at the datasheet and xilinx indeed does not support setting
> TDCV.
> However, xilinx still has a TDCV register to report the measured transmission
> delay.
> 
> Socket CAN's TDC framework provides can_priv::do_get_auto_tdcv() to
> report the measured value through the netlink interface:
> https://elixir.bootlin.com/linux/v5.17/source/include/linux/can/dev.h#L87
> 
> Can you implement this call back function?
Will implement in V2.

> 
> > +       .tdco_min = 0,
> > +       .tdco_max = 64,
> > +       .tdcf_min = 0, /* Filter window not supported */
> > +       .tdcf_max = 0,
> > +};
> > +
> >  /**
> >   * xcan_write_reg_le - Write a value to the device register little endian
> >   * @priv:      Driver private data structure
> > @@ -425,6 +437,11 @@ static int xcan_set_bittiming(struct net_device
> *ndev)
> >             priv->devtype.cantype == XAXI_CANFD_2_0) {
> >                 /* Setting Baud Rate prescalar value in F_BRPR Register */
> >                 btr0 = dbt->brp - 1;
> > +               if (can_tdc_is_enabled(&priv->can)) {
> > +                       btr0 = btr0 |
> > +                       (priv->can.tdc.tdco) << XCAN_BRPR_TDCO_SHIFT_CANFD |
> > +                       1 << XCAN_BRPR_TDCE_SHIFT_CANFD;
> 
> I don't think the parenthesis around (priv->can.tdc.tdco) are needed.
Yes, will update.
> 
>                        btr0 = btr0 |
>                        priv->can.tdc.tdco << XCAN_BRPR_TDCO_SHIFT |
>                       XCAN_BRPR_TDC_ENABLE
> 
> (c.f. above for macro names)
> 
> > +               }
> >
> >                 /* Setting Time Segment 1 in BTR Register */
> >                 btr1 = dbt->prop_seg + dbt->phase_seg1 - 1; @@
> > -1747,13 +1764,16 @@ static int xcan_probe(struct platform_device *pdev)
> >                 priv->can.data_bittiming_const =
> >                         &xcan_data_bittiming_const_canfd;
> >
> > -       if (devtype->cantype == XAXI_CANFD_2_0)
> > +       if (devtype->cantype == XAXI_CANFD_2_0) {
> >                 priv->can.data_bittiming_const =
> >                         &xcan_data_bittiming_const_canfd2;
> > +               priv->can.tdc_const = &xcan_tdc_const;
> > +       }
> >
> >         if (devtype->cantype == XAXI_CANFD ||
> >             devtype->cantype == XAXI_CANFD_2_0)
> > -               priv->can.ctrlmode_supported |= CAN_CTRLMODE_FD;
> > +               priv->can.ctrlmode_supported |= CAN_CTRLMODE_FD |
> > +                                               CAN_CTRLMODE_TDC_AUTO;
> >
> >         priv->reg_base = addr;
> >         priv->tx_max = tx_max;
> > --
> > 2.25.1
> >

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