Request for feedback for CAN message structure

[oss at c-mauderer.de]

Hello Pavel and Prashanth,

Am 20.06.22 um 22:57 schrieb Pavel Pisa:
> Hello Prashanth S and others,
> 
> excuse me for lower activity last weeks. We have exams period
> and I have lot of other duties and was even ill. I am at Embedded
> World in Nuremberg now, so may it be there is some chance to meet
> somebody other from RTEMS community.
> 
> As for the e-mail it is better to add one of my personal e-mails
> (pisa at cmp.felk.cvut.cz or ppisa at pikron.com)
> to notice me for something important, I do not check every
> message comming through my list subscription. I go through
> RTEMS, NuttX, CAN, etc. forums only when I have spare time.
> 
> On Monday 20 of June 2022 18:37:38 Prashanth S wrote:
>> This is a request for suggestions and feedback for the CAN message
>> structure.
>>
>> *CAN message structure that represents the can messages from application to
>> driver:*
>>
>> struct can_message {
>> uint32_t timestamp;
>> uint32_t id;                 // 32 bits to support extended id (29 bits)
>> uint16_t flags;            // RTR | BRS | EXT ...
>> uint8_t dlc;                 // (0 - 8) | 12 | 16 | 20 | 24 | 32 | 48 | 64
>> uint8_t res;                 // reserved for alignment.
>> uint8_t data[64];         // For supporting data transfer up to 64 bytes.
>> };
>>
>> This is the CAN messages structure created based on the suggestions in the
>> mail chain and looking through other CAN solutions (Nuttx, GRCAN, LinCAN).
> 
> Yes I like solution with explicit and aligned fields.
> I confirm that I think that length representing real data length is better
> and it has been chosen for CAN FD by SocketCAN.
> 
> I confirm that flags implemented with explicit sized field seem to be more
> safe to me. Bitfields are tricky due to endianing and have problem with
> initialization and copying when new field is added.
> 
> I am not sure if the field name "dlc" (data length code) is right
> when the field represents real length, may be "len" is a better
> match.
> 
> I think that fields order makes sense as well (alignment, purpose etc..).
> Only think to consider is possibility to swap
> 
>    uint32_t timestamp;
>    uint32_t id;
> 
> because id is the first and the most important and accessed field,
> so when the pointer to structure is give it can be small win
> on some targets that it can be accessed without offset addition.
> But that is probably neglectable and there can be arguments against.
> 
> Another think to consider is a size of "dlc" and or "len" code.
> There is ongoing standardization process for CAN XL and it
> would allow CAN frames up to 2kB and the length will be with
> byte granularity. But CAN XL adds datatype filed and much more
> options so it is probably out of the actual RTEMS scope
> and would require variable length read and write systemcalls
> because 2kB overhead for some 1 or 2 byte message is too high.
> 
> So even 8 bit length field is OK for now and may be res field
> can solve CAN XL compatibility one day.

Not sure where the field will be used but wouldn't it be better if 
length is 16 bit already if there is an upcoming standard that will 
support lengths that need it. Otherwise some drivers might use the "res" 
for something else or make some assumptions about the length of the 
field that make it harder to change it later.

Best regards

Christian

> 
> So generally I agree with the message structure, I declare
> minor weight vote to switch from "dlc" to "len".
> I think that switch to common message structure based API
> on RTEMS would be big win and that (at least for
> now) keeping character device like API with IOCTLs
> is simpler and matches better actual RTEMS CAN implementations.
> 
> Best wishes,
> 
> Pavel
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