netdev - Re: [PATCH v5 2/7] can: bittiming: allow TDC{V,O} to be zero and add can_tdc_const::tdc{v,o,f}

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Date:   Mon, 16 Aug 2021 14:25:19 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     linux-can <linux-can@...r.kernel.org>,
        Stefan Mätje <Stefan.Maetje@....eu>,
        netdev <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] can: bittiming: allow TDC{V,O} to be zero and add
 can_tdc_const::tdc{v,o,f}_min

On 16.08.2021 19:24:43, Vincent MAILHOL wrote:
> On Mon. 16 Aug 2021 at 17:42, Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> > On 15.08.2021 12:32:43, Vincent Mailhol wrote:
> > > ISO 11898-1 specifies in section 11.3.3 "Transmitter delay
> > > compensation" that "the configuration range for [the] SSP position
> > > shall be at least 0 to 63 minimum time quanta."
> > >
> > > Because SSP = TDCV + TDCO, it means that we should allow both TDCV and
> > > TDCO to hold zero value in order to honor SSP's minimum possible
> > > value.
> > >
> > > However, current implementation assigned special meaning to TDCV and
> > > TDCO's zero values:
> > >   * TDCV = 0 -> TDCV is automatically measured by the transceiver.
> > >   * TDCO = 0 -> TDC is off.
> > >
> > > In order to allow for those values to really be zero and to maintain
> > > current features, we introduce two new flags:
> > >   * CAN_CTRLMODE_TDC_AUTO indicates that the controller support
> > >     automatic measurement of TDCV.
> > >   * CAN_CTRLMODE_TDC_MANUAL indicates that the controller support
> > >     manual configuration of TDCV. N.B.: current implementation failed
> > >     to provide an option for the driver to indicate that only manual
> > >     mode was supported.
> > >
> > > TDC is disabled if both CAN_CTRLMODE_TDC_AUTO and
> > > CAN_CTRLMODE_TDC_MANUAL flags are off, c.f. the helper function
> > > can_tdc_is_enabled() which is also introduced in this patch.
> >
> > Nitpick: We can only say that TDC is disabled, if the driver supports
> > the TDC interface at all, which is the case if tdc_const is set.
> 
> I would argue that saying that a device does not support TDC is
> equivalent to saying that TDC is always disabled for that device.
> Especially, the function can_tdc_is_enabled() can be used even if
> the device does not support TDC (even if there is no benefit
> doing so).
> 
> Do you still want me to rephrase this part?
> 
> > > Also, this patch adds three fields: tdcv_min, tdco_min and tdcf_min to
> > > struct can_tdc_const. While we are not convinced that those three
> > > fields could be anything else than zero, we can imagine that some
> > > controllers might specify a lower bound on these. Thus, those minimums
> > > are really added "just in case".
> >
> > I'm not sure, if we talked about the mcp251xfd's tcdo, valid values are
> > -64...63.
> 
> Yes! Stefan shed some light on this. The mcp251xfd uses a tdco
> value which is relative to the sample point.

I don't read the documentation this way....

> | SSP = TDCV + absolute TDCO
> |     = TDCV + SP + relative TDCO
> 
> Consequently:
> | relative TDCO = absolute TDCO - SP

In the mcp15xxfd family manual
(http://ww1.microchip.com/downloads/en/DeviceDoc/MCP251XXFD-CAN-FD-Controller-Module-Family-Reference-Manual-20005678B.pdf)
in the 2mbit/s data bit rate example in table 3-5 (page 21) it says:

| DTSEG1  15 DTQ
| DTSEG2   4 DTQ
| TDCO    15 DTQ

The mcp251xfd driver uses 15, the framework calculates 16 (== Sync Seg+
tseg1, which is correct), and relative tdco would be 0:

| mcp251xfd_set_bittiming: tdco=15, priv->tdc.tdc=16, relative_tdco=0

Here the output with the patched ip tool:

| 4: mcp251xfd0: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state UP mode DEFAULT group default qlen 10
|     link/can  promiscuity 0 minmtu 0 maxmtu 0 
|     can <FD,TDC_AUTO> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 100 
|           bitrate 500000 sample-point 0.875
|           tq 25 prop-seg 34 phase-seg1 35 phase-seg2 10 sjw 1 brp 1
|           mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_inc 1
|           dbitrate 2000000 dsample-point 0.750
|           dtq 25 dprop-seg 7 dphase-seg1 7 dphase-seg2 5 dsjw 1 dbrp 1
|           tdco 15
|           mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbrp_inc 1
|           tdco 0..127
|           clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 parentbus spi parentdev spi0.0


> Which is also why TDCO can be negative.
> 
> I added an helper function can_tdc_get_relative_tdco() in the
> fourth path of this series:
> https://lore.kernel.org/linux-can/20210814091750.73931-5-mailhol.vincent@wanadoo.fr/T/#u
> 
> Devices which use the absolute TDCO can directly use
> can_priv->tdc.tdco. Devices which use the relative TDCO such as
> the mcp251xfd should use this helper function instead.

Don't think so....

> However, you will still need to convert the TDCO valid range from
> relative values to absolute ones. In your case 0..127.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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