Re: [PATCH v2 05/10] can: netlink: add CAN_CTRLMODE_XL_TMS flag

[Vincent Mailhol <mailhol@kernel.org>]

Hello Oliver,

On 06/11/2025 at 09:42, Oliver Hartkopp wrote:
> Hello Vincent,
> 
> On 21.10.25 17:47, Vincent Mailhol wrote:
>> The Transceiver Mode Switching (TMS) indicates whether the CAN XL
>> controller shall use the PWM or NRZ encoding during the data phase.
>>
>> The term "transceiver mode switching" is used in both ISO 11898-1 and
>> CiA 612-2 (although only the latter one uses the abbreviation TMS). We
>> adopt the same naming convention here for consistency.
>>
>> Add the CAN_CTRLMODE_XL_TMS flag to the list of the CAN control modes.
>>
>> In the netlink interface, each boolean option is in reality a tristate
>> in disguise: on, off or omitted. For the moment, TMS is implemented as
>> below:
>>
>>    - CAN_CTRLMODE_XL_TMS is set to false: TMS is disabled.
>>    - CAN_CTRLMODE_XL_TMS is set to true: TMS is enabled.
>>    - CAN_CTRLMODE_XL_TMS is omitted: return -EOPNOTSUPP.
> 
> I would propose to follow the usual pattern:
> 
> - TMS off (default)
> - TMS on (when selected on the command line)

OK. "TMS omitted" will be interpreted as "TMS off" in v2.

>> For most of the other control modes, omitting a flag default to the
>> option turned off. It could also be possible to provide a default
>> behaviour if the TMS flag is omitted (i.e. either default to TMS off
>> or on). However, it is not clear for the moment which default
>> behaviour is preferable. If the usage shows a clear trend (for example
>> if the vast majority of the users want TMS on by default), it is still
>> possible to revisit that choice in the future. Whereas once a default
>> option is provided, we can not change it back without breaking the
>> interface.
>>
>> As a corollary, for the moment, the users will be forced to specify
>> the TMS in the ip tool when using CAN XL.
>>
>> Add can_validate_xl_flags() to check the coherency of the TMS flag.
>> That function will be reused in upcoming changes to validate the other
>> CAN XL flags.
>>
>> Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
>> ---
>> Question:
>>
>> Is it still possible to send Classical CAN frames when TMS is on? If
>> not, we need to also add this filter in can_dev_dropped_skb():
> 
> No.
> 
> I've now learned there are two "CANXL-only" modes:
> 
> 1. TMS on -> no CC/FD traffic
> 2. TMS off and ERR_SIG off -> no CC/FD traffic, because CC/FD require ERR_SIG on
> for a compliant transmission

I see. I was under the assumption that CC and FD could be used with error
signalling off in mixed mode. Thanks!

> And there is a "mixed-mode" with CC/FD/XL with TMS off ('ERR_SIG on' is default
> anyway).
> 
> This "mixed-mode" requires all bitrates for CC/FD/XL to be set and all these CAN
> protocols can be sent.

Why? Will your device reject the configuration if you omit the FD bitrate? I did
not see anything in this direction in the ISO standard.

Did you have any source for this? Maybe the CiA provided some clarification
which I am not aware of?


Yours sincerely,
Vincent Mailhol