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

[Stefan Mätje]

Hi Vincent,

I would like to add some comments below:

Am Montag, den 16.08.2021, 14:25 +0200 schrieb Marc Kleine-Budde:
> On 16.08.2021 19:24:43, Vincent MAILHOL wrote:
> > On Mon. 16 Aug 2021 at 17:42, Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> 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....

@Vincent: I have to agree with Marc here. Perhaps my email
https://lore.kernel.org/linux-can/094d8a2eab2177e5a5143f96cf745b26897e1793.camel@xxxxxx/
was also misleading. I also referred there to a MicroChip Excel sheet
(https://ww1.microchip.com/downloads/en/DeviceDoc/MCP2517FD%20Bit%20Time%20Calculations%20-%20UG.xlsx)
that describes the calculation of the bit timing and the TDCO. The values calculated 
there correspond to the SPO from the above email. Microchip calculates the TDCO as
TDCO = (DPRSEG + DPH1SEG) * DBRP.
Thus, as already discussed, negative values are not purposeful.

Sorry, that that email was misleading. So far I've seen now only the ESDACC 
controller has a "relative" TDCO register value where a negative value may
be sensible.

> > > 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@xxxxxxxxxx/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....

@Vincent: Perhaps you should not implement this helper function as it is only needed
for the ESDACC so far.

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

Stefan