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

[Alexander Stein <alexander.stein@systec-electronic.com>]

Am Mittwoch, 21. März 2012, 14:51:43 schrieb Marc Kleine-Budde:
> On 03/21/2012 02:29 PM, Alexander Stein wrote:
> >> Further, there are only certain dlc values allowed for CAN FD. We must
> >> decide if the enforce these values or simply do padding with "0"
> >> somewhere.> 
> > What I understand from the proposal is that DLCs >8 are optional. So you
> > might get hardware which still supports only 8 bytes. How can/should
> > this be handled?
> 
> I haven't read the pdf that Kurt linked in the original mail. Is the
> usage of dlc > 8 optional or the support of dlc > 8 in hardware?

Quoted from section 6:
> The CAN FD protocol allows frames with more than eight data bytes. It is
> not required that all CAN FD implementations support longer frames, CAN
> FD implementations may be limited to a subset of DATA FIELD length. A CAN FD
> implementation that supports only up to e.g. eight data bytes in a frame
> shall not treat longer received frames as an error, fault-free longer
> frames shall be acknowledged and shall take part in acceptance filtering.
> Received data bytes that exceed the CAN FD’s data handling capacity shall
> be discarded. A such limited CAN FD implementation that is requested to
> transmit a longer frame shall fill up the data bytes in the frame that
> exceed the data handling capacity with a constant byte pattern. This
> pattern shall be chosen so that it does not cause the insertion of STUFF
> BITS, e.g. 0xCC.

Sounds to me like hardware support of dlc > 8 is optional. Even in the fact 
that you want the controller to send more than e.g. 8 bytes the controller 
shall insert stuff bits (0xCC) for unsupported data bytes.

Alexander
-- 
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