netdev - Re: [PATCH v2 2/4] can: fixed-transceiver: Add documentation for CAN fixed transceiver bindings

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Date:   Tue, 1 Aug 2017 09:12:43 +0200
From:   Kurt Van Dijck <dev.kurt@...dijck-laurijssen.be>
To:     Oliver Hartkopp <socketcan@...tkopp.net>
Cc:     Franklin S Cooper Jr <fcooper@...com>,
        Andrew Lunn <andrew@...n.ch>, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, netdev@...r.kernel.org,
        linux-can@...r.kernel.org, wg@...ndegger.com, mkl@...gutronix.de,
        robh+dt@...nel.org, quentin.schulz@...e-electrons.com,
        sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v2 2/4] can: fixed-transceiver: Add documentation for CAN
 fixed transceiver bindings

> Hi Kurt,
> 
> On 07/28/2017 09:41 PM, Kurt Van Dijck wrote:
> 
> >>The transceiver is an analog device that needs to support faster
> >>switching frequency (FETs) including minimizing delay to support CAN-FD
> >>ie higher bitrate. From the transceiver perspective the bits for
> >>"arbitration" and "data" look exactly the same. Since it can't
> >>differentiate between the two (at the physical layer) then the actual
> >>limit isn't specific to which part/type of the CAN message is being
> >>sent. Rather its just a single overall max bitrate limit.
> >
> >I must disagree here.
> >The transceiver is an analog device that performs 2 functions:
> >propagate tx bits to CAN wire, and propagate CAN wire state
> >(dominant/recesive) to rx bits.
> >
> >I'll rephrase the above explanation to fit your argument:
> >During arbitration, both directions are required, and needs to propagate
> >within 1 bit time. The transceiver doesn't know, it just performs to
> >best effort.
> >During data, the round-trip timing requirement of layer2 is relaxed.
> >The transceiver still doesn't know, it still performs to best effort.
> >Due to the relaxed round-trip timing requirement, the same transceiver
> >can suddenly allow higher bitrates. The transceiver didn't change, the
> >requirement did change.
> >This is what I meant earlier with "layer2 has been adapted to circumvent
> >layer1 limitations"
> >

> I talked to our CAN transceiver & CAN physical layer specialist who was
> involved in the ISO activities too.
> 
> We just need ONE value: max-bitrate
> 
> The CAN transceiver is qualified to provide this bitrate. That's it.
> There's nothing special with the arbitration bitrate - we don't even need to
> outline any CAN FD capability.
> 
> The other things like 'loop delay compensation' are done in the CAN
> controller. The better the transceiver get's the bits 'in shape' the higher
> it can be qualified. But from the SoC/Controller/Linux view we only need the
> max-bitrate value to double check with the CAN controllers bitrate
> configuration request (which was Franklins intention).

I bet your physical layer specialist understands the details way better
than I do :-)

1 value it is then.

Kind regards,
Kurt