Re: [PATCH 1/2] can: netlink: clear data_bittiming if fd is turned off

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

On Fri. 18 juin 2021 at 18:10, Marc Kleine-Budde <mkl@pengutronix.de> wrote:
>
> On 18.06.2021 17:19:03, Vincent Mailhol wrote:
> > When the FD is turned off through the netlink interface, the value
>
> values
>
> > still remain in data_bittiming and are displayed despite of the
> > feature being disabled.
> >
> > Example:
> >
> > $ ip link set can0 type can bitrate 500000 dbitrate 2000000 fd on
> > $ ip --details link show can0
> > 1:  can0: <NOARP,ECHO> mtu 72 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 10
> >     link/can  promiscuity 0 minmtu 0 maxmtu 0
> >     can <FD> state STOPPED restart-ms 0
> >         bitrate 500000 sample-point 0.875
> >         tq 12 prop-seg 69 phase-seg1 70 phase-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 dbrp-inc 1
> >         clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
> >
> > $ ip link set can0 type can bitrate 500000 fd off
> > $ ip --details link show can0
> > 1:  can0: <NOARP,ECHO> mtu 16 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 10
> >     link/can  promiscuity 0 minmtu 0 maxmtu 0
> >     can state STOPPED restart-ms 0
> >         bitrate 500000 sample-point 0.875
> >         tq 12 prop-seg 69 phase-seg1 70 phase-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 dbrp-inc 1
> >         clock 80000000 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
> >
> > Remark: once FD is turned off, it is not possible to just turn fd back
> > on and reuse the previously input data bittiming values:
>
> > $ ip link set can0 type can bitrate 500000 fd on
> > RTNETLINK answers: Operation not supported
> >
> > This means that the user will need to overwrite data bittiming with
>                                         ^^^^^^^^^
> set
>
> At least with your change it's more a "set again" than to overwrite.
>
> > fresh values in order to turn fd on again.
> >
> > Because old data bittiming values can not be reused, this patch just
> > clears priv->data_bittiming whenever FD is turned off. This way, the
> > data bittiming variables are not displayed anymore.
> >
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> > ---
> > Hi Marc,
> >
> > I suggest to rebase this patch before the netlink TDC series.
>
> makes sense - If you're OK with the changes, I'll add them while
> applying.

I am OK with the changes.

> Your patch makes the interface consistent, another option would be to
> allow FD mode if the data bit timing values have been set before.
> Opinions?

One part of me says it would make sense but let's also try to be
realistic. The only reason I spotted this issue is because I was
actively trying to find defects while testing my TDC
implementation. Under normal usage, I never had have such needs.

How many people will need to set fd on, then off, then on again?
Too few I think. Let's force those few people to always provide
the data bittiming values. The overhead which would be needed to
implement this in drivers/net/can/dev/netlink.c is just not worth it.

> Marc
>
> --
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