Re: [RFC PATCH v5 2/2] can: reject CAN FD content when disabled on CAN XL interfaces

[Oliver Hartkopp <socketcan@hartkopp.net>]

On 10.09.25 18:19, Vincent Mailhol wrote:
> On 10/09/2025 at 17:48, Oliver Hartkopp wrote:
>> On 10.09.25 09:40, Vincent Mailhol wrote:
>>> On 10/09/2025 at 16:27, Oliver Hartkopp wrote:
>>
>>>>>       /* CAN XL is allowed on virtual interfaces if it fits the MTU */
>>>>>          if (!priv)
>>>>>              return dev->mtu == CANXL_MTU;
>>>>
>>>>           return can_is_canxl_dev_mtu(mtu);
>>>>
>>>> The MTU of CAN XL interfaces might vary.
>>>
>>> Maybe this is something that we discussed before, I do not remember, but how is
>>> it that the MTU can vary?
>>>
>>> MTU is the *Maximum* Transmission Unit. I understand that the size of a CAN XL
>>> frame is variable, but the MTU should be constant, right? Why can it vary?
>>
>> Depending on the realtime requirements the length of the CAN frames (and
>> therefore the time the bus is blocked) the MTU can be reduced. This is (like the
>> bitrate settings) a network architects decision which is enforced by setting the
>> MTU accordingly.
> 
> Is this an extension we offer in Socket CAN?

Yes.

> The standard says nothing about
> having the MTU configurable.
> 
> For CAN FD, we forcefully set the MTU in netlink.c
> 
> https://elixir.bootlin.com/linux/v6.16/source/drivers/net/can/dev/netlink.c#L228

Oh. I did not realize before that we can either modify the MTU with 
setting fd on/off and via setting the MTU in can_change_mtu()

https://elixir.bootlin.com/linux/v6.16/source/drivers/net/can/dev/dev.c#L313

?!?

The two APIs problem for changing the MTU?!?

I expected the default MTU for CAN FD capable interfaces to be CANFD_MTU 
which is obviously not the case.

> I will have to think of what are the implication for CAN XL.

I would define a default CANXL MTU (CANXL_MTU 2060) which might be 
changed with can_change_mtu().

And when ever we switch xl on this value is selected as device MTU.

Or the user can change the MTU as he needs it.
And when xl on is selected and the MTU is a can_is_canxl_dev_mtu() this 
value is used. When can_is_canxl_dev_mtu() is not true we take CANXL_MTU.

Something like this.

Best regards,
Oliver