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

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Date:   Thu, 19 Oct 2017 22:17:11 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Mario Hüttel <mario.huettel@....net>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Sekhar Nori <nsekhar@...com>,
        Franklin S Cooper Jr <fcooper@...com>,
        "Yang, Wenyou" <Wenyou.Yang@...rochip.com>, wg@...ndegger.com,
        quentin.schulz@...e-electrons.com, edumazet@...gle.com,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Wenyou Yang <wenyou.yang@...el.com>,
        Dong Aisheng <b29396@...escale.com>,
        "Quadros, Roger" <rogerq@...com>
Subject: Re: [RFC PATCH] can: m_can: Support higher speed CAN-FD bitrates

On 10/19/2017 09:54 PM, Mario Hüttel wrote:
> On 10/19/2017 08:35 PM, Oliver Hartkopp wrote:

>> We already have this 'dsample-point' implemented in the ip tool:
>>
>> $ ip link set vcan0 type can help
>> Usage: ip link set DEVICE type can
>>      [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
>>      [ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
>>         phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
>>
>>      [ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] |  <<-- here!
>>      [ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1
>>         dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ]
>>
>> But AFAIK m_can is not using that value in m_can_set_bittiming().
>>
> Actually I need some clarification. The sample point of the can core is
> between the two time segments.
> I always thought that the "sample point" options of the ip tool are used
> in the internal
> calculation of the two timing segments and is therefore no individual value.

You are right.

See picture at http://www.bittiming.can-wiki.info/

Usually you can give the bitrate and the sample point (which is at 75% 
aka 0.750 by default) and then the kernel-internal bitrate calculating 
algorithm calculates the tq prop-seg phase-seg1 phase-seg2 stuff.

Alternatively you can provide the tq prop-seg phase-seg1 phase-seg2 
stuff on your own which is set to the CAN controller registers then.

For that reason my remark "m_can is not using that value" was wrong as 
m_can just uses the tq prop-seg phase-seg1 phase-seg2 stuff - either 
from the bitrate calculation or provided by the user.

Thanks for the question ;-)

Best,
Oliver