Re: [PATCH v2 2/2] can: netlink: add interface for CAN-FD Transmitter Delay Compensation (TDC)

[Vincent MAILHOL <mailhol.vincent@wanadoo.fr>]

On Wed. 16 Jun 2021 at 18:46, Marc Kleine-Budde <mkl@pengutronix.de> wrote:
> On 04.06.2021 00:15:50, Vincent Mailhol wrote:
> [...]
>
> > +static size_t can_tdc_get_size(const struct net_device *dev)
> > +{
> > +     struct can_priv *priv = netdev_priv(dev);
> > +     size_t size;
> > +
> > +     if (!priv->tdc_const)
> > +             return 0;
> > +
> > +     size = nla_total_size(0);                       /* nest IFLA_CAN_TDC */
> > +     size += nla_total_size(sizeof(u32));            /* IFLA_CAN_TDCV_MAX */
> > +     size += nla_total_size(sizeof(u32));            /* IFLA_CAN_TDCO_MAX */
> > +     size += nla_total_size(sizeof(u32));            /* IFLA_CAN_TDCF_MAX */
> > +
> > +     if (priv->tdc.tdco) {
>
> Naively I'd say, iff the device has tdc_const give the user space the
> tdc parameters, regardless if some value is 0 or not.
>
> What do you think?

I thought about that.
The first important remark is that if tdc.tdco is zero, then TDC
is off (c.f. documentation of struct can_tdc::tdco).

Let me illustrate my vision through examples.


** Case 1: link is not configured at all. **
Here, only the constant values are displayed.

# ip --details link show can0
1:  can0: <NOARP,ECHO> mtu 16 qdisc noop state DOWN mode DEFAULT group
default qlen 10
    link/can  promiscuity 0 minmtu 0 maxmtu 0
    can state STOPPED (berr-counter tx 0 rx 0) restart-ms 0
      ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp_inc 1
      ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 brp_inc 1
      tdcv_max 0 tdco_max 127 tdcf_max 127
      clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536
gso_max_segs 65535


** Case 2: only the nominal bitrate is configured. **
The data bittiming variables (including TDC) are not shown.

# ip --details link show can0
1:  can0: <NOARP,ECHO> mtu 16 qdisc noop state DOWN mode DEFAULT group
default qlen 10
    link/can  promiscuity 0 minmtu 0 maxmtu 0
    can state STOPPED (berr-counter tx 0 rx 0) restart-ms 0
      bitrate 500000 sample-point 0.875
      tq 12 prop-seg 69 phase-seg1 70phase-seg2 20  sjw 1
      ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp_inc 1
      ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 brp_inc 1
      tdcv_max 0 tdco_max 127 tdcf_max 127
      clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536
gso_max_segs 65535


** Case 3: both nominal and data bitrates are configured (but not TDC). **
Only the TDC variables are not shown.

# ip --details link show can0
1:  can0: <NOARP,ECHO> mtu 72 qdisc noop state DOWN mode DEFAULT group
default qlen 10
    link/can  promiscuity 0 minmtu 0 maxmtu 0
    can <FD> state STOPPED (berr-counter tx 0 rx 0) restart-ms 0
      bitrate 500000 sample-point 0.875
      tq 12 prop-seg 69 phase-seg1 70phase-seg2 20  sjw 1
      ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp_inc 1
      dbitrate 2000000 dsample-point 0.750
      dtq 12 dprop-seg 14 dphase-seg1 15 dphase-seg2 10 dsjw 1
      ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 brp_inc 1
      tdcv_max 0 tdco_max 127 tdcf_max 127
      clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536
gso_max_segs 65535


** Case 4: nominal and data bitrates and TDC are configured. **
Everything is shown.

# ip --details link show can0
1:  can0: <NOARP,ECHO> mtu 72 qdisc noop state DOWN mode DEFAULT group
default qlen 10
    link/can  promiscuity 0 minmtu 0 maxmtu 0
    can <FD> state STOPPED (berr-counter tx 0 rx 0) restart-ms 0
      bitrate 1000000 sample-point 0.750
      tq 12 prop-seg 29 phase-seg1 30phase-seg2 20  sjw 1
      ES582.1/ES584.1: tseg1 2..256 tseg2 2..128 sjw 1..128 brp 1..512 brp_inc 1
      dbitrate 5000000 dsample-point 0.750
      dtq 12 dprop-seg 5 dphase-seg1 6 dphase-seg2 4 dsjw 1
      tdcv 0 tdco 12 tdcf 0
      ES582.1/ES584.1: dtseg1 2..32 dtseg2 1..16 dsjw 1..8 dbrp 1..32 brp_inc 1
      tdcv_max 0 tdco_max 127 tdcf_max 127
      clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536
gso_max_segs 65535


I think that we can agree on Cases 1, 2 (it would not make sense
to display TDC without the data bittiming variables) and 4.

The edge case is Case 3. It depends if we consider the TDC as a
separate set or not. It is not silly to display the TDC whenever
fd is on. I prefer to keep it the way I did it. But I would not
object to changing this if you insist. That would mean:
-     if (priv->tdc.tdco) {
+     if (priv->data_bittiming.bitrate) {


Finally, I have one side comment. It seems to me that you did not
understand that the intent of
|     if (priv->tdc.tdco)
was to actually check whether TDC was on or off. In other words, my
code was unclear.

I am now thinking to introduce an helper macro:
static bool can_tdc_is_enabled(const struct can_priv *priv)
|{
|    return !!priv->tdc.tdco;
|}

The code would look more clear like that.
-     if (priv->tdc.tdco) {
+     if (can_tdc_is_enabled(priv) {


Yours sincerely,
Vincent