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

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

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On 10/19/2017 01:09 PM, Sekhar Nori wrote:
> On Thursday 19 October 2017 02:43 PM, Marc Kleine-Budde wrote:
>> On 10/19/2017 07:07 AM, Sekhar Nori wrote:
>>>>>> Sounds reasonable. What's the status of this series?
>>>>>
>>>>> I have had some offline discussions with Franklin on this, and I am not
>>>>> fully convinced that DT is the way to go here (although I don't have the
>>>>> agreement with Franklin there).
>>>>
>>>> Probably the fundamental area where we disagree is what "default" SSP
>>>> value should be used. Based on a short (< 1 ft) point to point test
>>>> using a SSP of 50% worked fine. However, I'm not convinced that this
>>>> default value of 50% will work in a more "traditional" CAN bus at higher
>>>> speeds. Nor am I convinced that a SSP of 50% will work on every MCAN
>>>> board in even the simplest test cases.
>>>>
>>>> I believe that this default SSP should be a DT property that allows any
>>>> board to determine what default value works best in general.
>>>
>>> With that, I think, we are taking DT from describing board/hardware
>>> characteristics to providing default values that software should use.
>>
>> If the default value is board specific and cannot be calculated in
>> general or from other values present in the DT, then it's from my point
>> of view describing the hardware.
>>
>>> In any case, if Marc and/or Wolfgang are okay with it, binding
>>> documentation for such a property should be sent to DT maintainers for
>>> review.
>>>
>>>>>
>>>>> There are two components in configuring the secondary sample point. It
>>>>> is the transceiver loopback delay and an offset (example half of the bit
>>>>> time in data phase).
>>>>>
>>>>> While the transceiver loopback delay is pretty board dependent (and thus
>>>>> amenable to DT encoding), I am not quite sure the offset can be
>>>>> configured in DT because its not really board dependent.
>>>>>
>>>>> Unfortunately, offset calculation does not seem to be an exact science.
>>>>> There are recommendations ranging from using 50% of bit time to making
>>>>> it same as the sample point configured. This means users who need to
>>>>> change the SSP due to offset variations need to change  their DT even
>>>>> without anything changing on their board.
>>>>>
>>>>> 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)?
>>>>
>>>> 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?

Do we need more than one tuple here?

>> 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?
> 
> I dont know how a good default (other than 50% as the starting point)
> can be arrived at without doing any actual measurements on the actual
> network. Since we do know that the value has to be tweaked, agree that
> netlink interface has to be provided.
> 
> I wonder whether even if a DT binding for default is provided, everyone
> will end up setting it to 50% (since there is no way for them to predict
> any better). In effect, I am suggesting using a hardcoded value of 50%
> instead of introducing a binding without a clear need for it.

Ok, if the value is network and not board specific it doesn't belong
into the DT.

> Note that I am only talking about there "offset" part of SSP here. The
> transceiver loopback delay is calculated automatically by Bosch's MCAN
> IP. But if there are other IPs which don't do that, then yes, that
> should be a DT property IMO.

Ok, but let's wait for such an IP core to show up here :)

Marc

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