netdev - Re: [PATCH net-next v2] candev: allow SJW user setting for bittiming calculation

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Message-ID: <4E7D8A4F.7010806@grandegger.com>
Date:	Sat, 24 Sep 2011 09:44:15 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Pavel Pisa <pisa@....felk.cvut.cz>
CC:	SocketCAN Core Mailing List <socketcan-core@...ts.berlios.de>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2] candev: allow SJW user setting for bittiming
 calculation

Hi Pavel,

On 09/23/2011 11:32 AM, Pavel Pisa wrote:
> Hello Oliver and Wolfgang,
...
> I hope that actual calculation works reasonably well.
> But if it should be enhanced, then it would worth
> to add additional parameter except crystal/controller
> clock source freqency to the concrete boards drivers.
> It should be measured round-trip delay of given/used
> transceiver/optocouplers combination. This would
> allow to have sampling point setup yet more independent
> on given HW and same value could be used for different
> bitrates. But there is still unknown parameter
> capacity/length of connected wires so there is still
> something left to user consideration.

The actual in-kernel bit-timing calculation is known to work well for
most, not all, CAN users, especially with "good" clock source
frequencies (a multiple of 8 MHz). That's fine and intended. The results
of that algorithm are not always satisfactory. Such cases should then be
handled in user-space, either manually or by an advanced algorithm. In
the BerliOS "can-utils" there we have already "can-calc-bit-timing". Any
enhancements and improvements would be nice to share, indeed. Do you
already have/use some code for the enhancement mentioned above?

Wolfgang.
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