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

[Marc Kleine-Budde]

On 10/19/2017 10:04 AM, Ramesh Shanmugasundaram 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).
>>
>> 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)?
> 
> +1
> > I also wonder how a default TDCO setting in DT would help.

For a given board with a given transceiver and layout and proper values
in the supplied DT the higher bitrates would work out of the box.

> Is the driver going to hard code the Tq settings as well for the
> most commonly used bitrate?

What's Tq in this context. Time Quanta?

> A link up start-up script (using netlink) would help setting a
> default TDCO along with other params if this is the main concern.

There are various ways to setup these values. Plain old start-up scripts
are one of them, systemd-networkd or network manager are others.

Marc

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