From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oliver Hartkopp Subject: Re: [PATCH net-next v2] candev: allow SJW user setting for bittiming calculation Date: Thu, 22 Sep 2011 21:41:23 +0200 Message-ID: <4E7B8F63.6060108@hartkopp.net> References: <4E7B0DE6.9020807@hartkopp.net> <4E7B331F.3070801@grandegger.com> <4E7B61C8.8020506@hartkopp.net> <4E7B7A30.4030707@grandegger.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: SocketCAN Core Mailing List , Linux Netdev List To: Wolfgang Grandegger Return-path: In-Reply-To: <4E7B7A30.4030707-5Yr1BZd7O62+XT7JhA+gdA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: socketcan-core-bounces-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org Errors-To: socketcan-core-bounces-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org List-Id: netdev.vger.kernel.org Hi Wolfgang, i put Pavel and Andrey into CC, maybe they also have an opinion about this. I just talked to a specialist who told me, that a higher SJW is better - if possible. And if an expert told me so and i had a simple config interface to fullfill it, it would be great for me ;-) Here is a link to the patch and this discussion: http://patchwork.ozlabs.org/patch/115932/ Regards, Oliver On 09/22/11 20:10, Wolfgang Grandegger wrote: > On 09/22/2011 06:26 PM, Oliver Hartkopp wrote: >> On 09/22/11 15:07, Wolfgang Grandegger wrote: >> >>> Hi Oliver, >>> >>> On 09/22/2011 12:28 PM, Oliver Hartkopp wrote: >>>> This patch adds support for SJW user settings to not set the synchronization >>>> jump width (SJW) to 1 in any case when using the in-kernel bittiming >>>> calculation. >>>> >>>> The ip-tool from iproute2 already supports to pass the user defined SJW >>>> value. The given SJW value is sanitized with the controller specific sjw_max >>>> and the calculated tseg2 value. As the SJW can have values up to 4 providing >>>> this value will lead to the maximum possible SJW automatically. A higher SJW >>>> allows higher controller oscillator tolerances. >> >> (..) >> >> >>> As I already said, I expected "bt->sjw" to be always 0 when >>> can_calc_bittiming() is called. But the software somehow allows to >>> preset "bt->sjw", which is not intended as the help of the ip utility shows: >> >> >> What was the technical reason for this 'not intended' that time? > > Fiddling with SJW is something for *expert* only. You need to know > what's going on the bus (electrically). To be clear, I'm not such an expert. > >>> I actually hesitate to extend can_calc_bittiming(). I suggest using an >>> external tool to calculate proper bit-timing parameters and set them >>> with "ip link set DEVICE type can tq ...". >> >> >> Yes, i know the possibility to specify the bittiming via time-quanta (tq). > > This is what we called the "expert mode". > >> But i think the in-kernel bittiming calculation is pretty good and solves most >> usual cases. For me it's just an additional requirement to specify SJW as > > What's your reason to increase SJW? Likely because you get bus errors > with the standard settings. How should it be set? Likely you need to > fiddle with other parameters as well. For sure, no business for a normal > CAN user. > >> MIN(tseg2, max_sjw) to extend the clock source tolerance - which can be >> provided as an additional option to the bittiming calculation without pain >> (IMO). If i can 'tune' the sample point, why should i not have any influence >> to the synchronization jump width generation then? > > Bitrate and sample point is something obvious for normal users. > >> And btw. a help text can really be changed easily ;-) >> >> - [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] | >> + [ bitrate BITRATE [ sample-point SAMPLE-POINT] [sjw SJW] ] | > > Well, I have stronger arguments against it ;-). > > Any other opinions or suggestions? > > Wolfgang. > > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org > More majordomo info at http://vger.kernel.org/majordomo-info.html