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Date:   Mon, 16 Aug 2021 15:43:42 +0200
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Cc:     linux-can <linux-can@vger.kernel.org>,
        Stefan =?utf-8?B?TcOkdGpl?= <Stefan.Maetje@esd.eu>,
        netdev <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.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
Message-ID: <20210816134342.w3bc5zjczwowcjr4@pengutronix.de>
References: <20210815033248.98111-1-mailhol.vincent@wanadoo.fr>
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 <CAMZ6RqK5t62UppiMe9k5jG8EYvnSbFW3doydhCvp72W_X2rXAw@mail.gmail.com>
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On 16.08.2021 14:33:09, Marc Kleine-Budde wrote:
> On 16.08.2021 14:25:19, Marc Kleine-Budde wrote:
> > > > I'm not sure, if we talked about the mcp251xfd's tcdo, valid values=
 are
> > > > -64...63.
> > >=20
> > > Yes! Stefan shed some light on this. The mcp251xfd uses a tdco
> > > value which is relative to the sample point.
> >=20
> > I don't read the documentation this way....
> >=20
> > > | SSP =3D TDCV + absolute TDCO
> > > |     =3D TDCV + SP + relative TDCO
> > >=20
> > > Consequently:
> > > | relative TDCO =3D absolute TDCO - SP
> >=20
> > In the mcp15xxfd family manual
> > (http://ww1.microchip.com/downloads/en/DeviceDoc/MCP251XXFD-CAN-FD-Cont=
roller-Module-Family-Reference-Manual-20005678B.pdf)
> > in the 2mbit/s data bit rate example in table 3-5 (page 21) it says:
> >=20
> > | DTSEG1  15 DTQ
> > | DTSEG2   4 DTQ
> > | TDCO    15 DTQ
> >=20
> > The mcp251xfd driver uses 15, the framework calculates 16 (=3D=3D Sync =
Seg+
> > tseg1, which is correct), and relative tdco would be 0:
> >=20
> > | mcp251xfd_set_bittiming: tdco=3D15, priv->tdc.tdc=3D16, relative_tdco=
=3D0
> >=20
> > Here the output with the patched ip tool:
>=20
> Sorry, the previous output was not using the sample points of the
> example in the data sheet, this is the fixed output:
>=20
> | 6: mcp251xfd0: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state U=
P mode DEFAULT group default qlen 10
> |     link/can  promiscuity 0 minmtu 0 maxmtu 0=20
> |     can <FD,TDC_AUTO> state ERROR-ACTIVE (berr-counter tx 0 rx 0) resta=
rt-ms 100=20
> |           bitrate 500000 sample-point 0.800
> |           tq 25 prop-seg 31 phase-seg1 32 phase-seg2 16 sjw 1 brp 1
> |           mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 br=
p_inc 1
> |           dbitrate 2000000 dsample-point 0.800
> |           dtq 25 dprop-seg 7 dphase-seg1 8 dphase-seg2 4 dsjw 1 dbrp 1
> |           tdco 16
> |           mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 d=
brp_inc 1
> |           tdco 0..127
> |           clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536=
 gso_max_segs 65535 parentbus spi parentdev spi0.0=20

The following sequence doesn't clear "tdcv" properly:

| ip link set dev mcp251xfd0 down; \
| ip link set mcp251xfd0 txqueuelen 10 up type can \
|     sample-point 0.8 bitrate 500000  \
|     dsample-point 0.8 dbitrate 2000000 fd on \
|     tdc-mode manual tdco 11 tdcv 22
|=20
| ip link set dev mcp251xfd0 down; \
| ip link set mcp251xfd0 txqueuelen 10 up type can \
|     sample-point 0.8 bitrate 500000  \
|     dsample-point 0.8 dbitrate 2000000 fd on

| Aug 16 15:10:43 rpi4b8 kernel: mcp251xfd spi0.0 mcp251xfd0: mcp251xfd_set=
_bittiming: tdco=3D11 tdcv=3D22 mode=3Dmanual
| Aug 16 15:10:43 rpi4b8 kernel: IPv6: ADDRCONF(NETDEV_CHANGE): mcp251xfd0:=
 link becomes ready
| Aug 16 15:10:59 rpi4b8 kernel: mcp251xfd spi0.0 mcp251xfd0: mcp251xfd_set=
_bittiming: tdco=3D16 tdcv=3D22 mode=3Dautomatic
| Aug 16 15:10:59 rpi4b8 kernel: IPv6: ADDRCONF(NETDEV_CHANGE): mcp251xfd0:=
 link becomes ready

neither does "tdc-mode off":

| Aug 16 15:12:18 rpi4b8 kernel: mcp251xfd spi0.0 mcp251xfd0: mcp251xfd_set=
_bittiming: tdco=3D11 tdcv=3D22 mode=3Doff
| Aug 16 15:12:18 rpi4b8 kernel: IPv6: ADDRCONF(NETDEV_CHANGE): mcp251xfd0:=
 link becomes ready

---

We have 4 operations:
- tdc-mode off                  switch off tdc altogether
- tdc-mode manual tdco X tdcv Y configure X and Y for tdco and tdcv
- tdc-mode auto tdco X          configure X tdco and
                                controller measures tdcv automatically
- /* nothing */                 configure default value for tdco
                                controller measures tdcv automatically

The /* nothing */ operation is what the old "ip" tool does, so we're
backwards compatible here (using the old "ip" tool on an updated
kernel/driver).

At first glance it's a bit non-intuitive that you have to specify
nothing for the "full" automatic mode.

Oh, I just noticed:

| ip link set dev mcp251xfd0 down; \
| ip link set mcp251xfd0 txqueuelen 10 up type can \
|     sample-point 0.8 bitrate 500000  \
|     dsample-point 0.8 dbitrate 2000000 fd on \
|     tdc-mode manual tdco 11 tdcv 22

followed by:

| ip link set dev mcp251xfd0 down; \
| ip link set mcp251xfd0 txqueuelen 10 up type can

We stay in manual mode:

| Aug 16 15:27:47 rpi4b8 kernel: mcp251xfd spi0.0 mcp251xfd0: mcp251xfd_set=
_bittiming: tdco=3D11 tdcv=3D22 mode=3Dmanual

| 8: 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=20
|     can <FD,TDC_AUTO,TDC_MANUAL> state ERROR-ACTIVE (berr-counter tx 0 rx=
 0) restart-ms 100=20
|           bitrate 500000 sample-point 0.800
|           tq 25 prop-seg 31 phase-seg1 32 phase-seg2 16 sjw 1 brp 1
|           mcp251xfd: tseg1 2..256 tseg2 1..128 sjw 1..128 brp 1..256 brp_=
inc 1
|           dbitrate 2000000 dsample-point 0.800
|           dtq 25 dprop-seg 7 dphase-seg1 8 dphase-seg2 4 dsjw 1 dbrp 1
|           tdcv 22 tdco 11
|           mcp251xfd: dtseg1 1..32 dtseg2 1..16 dsjw 1..16 dbrp 1..256 dbr=
p_inc 1
|           tdcv 0..63 tdco 0..63
|           clock 40000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 g=
so_max_segs 65535 parentbus spi parentdev spi0.0=20

I have to give "fd on" + the bit timing parameters to go to the full
automatic mode again:

| Aug 16 15:32:46 rpi4b8 kernel: mcp251xfd spi0.0 mcp251xfd0: mcp251xfd_set=
_bittiming: tdco=3D16 tdcv=3D22 mode=3Dautomatic


Does it make sense to let "mode auto" without a tdco value switch the
controller into full automatic mode and /* nothing */ not tough the tdc
config at all? If the full automatic mode is power on default mode, then
new kernel/drivers are compatible with old the old ip tool.

regards,
Marc

--=20
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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