Re: [RFC] can: Introducing CANFD for af_can & can-raw

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

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On 03/21/2012 01:51 PM, Kurt Van Dijck wrote:
>>>> What about
>>>> the RX path? We can just use the new 64 byte CAN frames and have to
>>>> tweak the CAN_RAW if the read from the userspace is too small.
>>> You may have noticed that I did not need to do anything
>>> in the receive path in order to receive longer frames.
>>>
>>> look at this block in raw_recvmsg:
>>>
>>> 	if (size < skb->len)
>>> 		msg->msg_flags |= MSG_TRUNC;
>>> 	else
>>> 		size = skb->len;
>>>
>>> Even today already, you can recv. with smaller or bigger buffers than
>>> the actual CAN frame.
>>
>> ...using an unmodified mainline kernel?
> 
> Must be. yes!
> I think that's good.

Indeed. I wasn't aware of this.

>>> I did try to let the kernel work with 64bytes, and still use
>>> 8byte frames in userspace. that all works, especially since noone
>>> seems to test for MSG_TRUNC.
>>
>> Have you checked _all_ CAN userspace applications? :P
>> We cannot break the ABI.
> I mentioned cansend & candump.
> I realize there's a lot of work ahead. I just started, and tried
> not to touch too much, and yet prove it can work.
> 
> I agree that we cannot break the current ABI.
> 
>>
>>> An improvement that just crosses my mind is something like:
>>>
>>> 	int real_len;
>>>
>>> 	real_len = offsetof(struct can_frame, data) +
>>> 		((struct can_frame)skb->data)->dlc;
>>> 	if (size < real_len)
>>> 		msg->msg_flags |= MSG_TRUNC;
>>> 	else
>>> 		size = real_len;
>>>
>>> Such would modify the ABI a bit more, but prevents copying unused bytes
>>> and avoids unnecessary MSG_TRUNC flags.
>>
>> I suggest to copy the full 8 bytes of data if dlc <= 8, for full ABI
>> compatibility.
> 
> Do we then agree that it is desired not to do this beyond the first 8 bytes?
> Moreover, we could depend this on the function parameter 'size'. If it
> is 16bytes, do as before, else do the new trick. Did I miss something then?

Yes, if 16 bytes is the length of the current struct can_frame.

Marc
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