Re: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates

[Marc Kleine-Budde]

On 10/19/2017 01:14 PM, Oliver Hartkopp wrote:
>>>>> Since we have a netlink socket interface to configure sample point, I
>>>>> wonder if that should be extended to configure SSP too (or at least the
>>>>> offset part of SSP)?
> 
> +1 too

The struct can_bittiming in defined in uapi, so we have to keep ABI
compatibility in mind.

>>>> Sekhar is right that ideally the user should be able to set the SSP at
>>>> runtime. However, my issue is that based on my experience CAN users
>>>> expect the driver to just work the majority of times. For unique use
>>>> cases where the driver calculated values don't work then the user should
>>>> be able to override it. This should only be done for a very small
>>>> percentage of CAN users. Unless you allow DT to provide a default SSP
>>>> many users of MCAN may find that the default SSP doesn't work and must
>>>> always use runtime overrides to get anything to work. I don't think that
>>>> is a good user experience which is why I don't like the idea.
>>>
>>> Fair enough. But not quite sure if CAN users expect CAN-FD to "just
>>> work" without doing any bittiming related setup.
>>
>>  From my point of view I'd rather buy a board from a HW vendor where
>> CAN-FD works, rather than a board where I have to tweak the bit-timing
>> for a simple CAN-FD test setup.
>>
>> As far as I see for the m_can driver it's a single tuple: "bitrate > 2.5
>> MBit/s -> 50%". Do we need an array of tuples in general?
>>
>> If good default values are transceiver and board specific, they can go
>> into the DT. We need a generic (this means driver agnostic) binding for
>> this. If this table needs to be tweaked for special purpose, then we can
>> add a netlink interface for this as well. >
>> Comments?
> 
> By now we calculate reasonable default values (e.g. for SP and SJW), you 
> can override by setting alternative values via netlink configuration.
> 
> I would tend to stay on this approach and not hide these things in DTs - 
> just because of someone wants to initialize his specific interface 'easier'.

If the values are not board specific, then it makes no sense to put them
into the DT.

> One tool, one place is IMHO still the best option.

Marc

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