Re: [PATCH v3 5/5] can: allow to change the device mtu for CAN FD capable devices

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 02/15/2014 06:29 PM, Oliver Hartkopp wrote:
>> On 02/15/2014 05:32 PM, Oliver Hartkopp wrote:
>>> The configuration for CAN FD depends on CAN_CTRLMODE_FD enabled in the driver
>>> specific ctrlmode_supported capabilities.
>>>
>>> The configuration can be done either with the 'fd { on | off }' option in the
>>> 'ip' tool from iproute2 or by setting the CAN netdevice MTU to CAN_MTU (16) or
>>> to CANFD_MTU (72).
>>
>> Please don't add two possibilities to activate canfd on a CAN network
>> card. There should only be one.
> 
> Yes it /should/ be like this.
> I thought about it some time but I didn't find a better solution.
> 
> The problem is that dev->mtu is the switch for the skb->len whether CAN frames
> or CAN FD frames are generated or received.
> 
> The MTU can be modified by 'ip' (independently from CAN specific settings).

It _can_ be modified (directly by the userspace) _if_ the change mtu
callback is implemented. Maybe we don't need the callback?

> OTOH the only way the CAN devices provide their capabilities is the
> ctrlmode_supported bitfield - and it does not make sense to provide
> CAN_CTRLMODE_FD in ctrlmode_supported and do not provide the configuration for it.
> 
> If you would skip the CAN_CTRLMODE_FD bit in ctrlmode_supported there's no way
> to identify a CAN FD capable device. You could only try to set the MTU to
> CANFD_MTU and see, if it fails. Not a good solution either.
> 
> 
>>> +EXPORT_SYMBOL_GPL(can_change_mtu);
>>
>> Who will be the user of this function? The individual network drivers?
>>
> 
> Yes. It manages the MTU and CAN_CTRLMODE_FD consistency and only allows the
> CANFD_MTU when the driver supports it.

And it has to be assigned by a FD capable driver to the net_device_ops,
right?

> Any other things to look at - or is the rest ok so far?

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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