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

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Date:   Fri, 28 Jul 2017 20:33:28 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     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, dev.kurt@...dijck-laurijssen.be
Subject: Re: [PATCH v2 2/4] can: fixed-transceiver: Add documentation for CAN
 fixed transceiver bindings

Hi Kurt,

On 07/28/2017 03:02 PM, Kurt Van Dijck wrote:

>>> The word 'max-arbitration-bitrate' makes the difference very clear.
>>
>> I think you are mixing up ISO layer 1 and ISO layer 2.
> 
> In order to provide higher data throughput without putting extra limits
> on transceiver & wire, the requirement for the round-trip delay to be
> within 1 bittime has been eliminated, but only for the data phase when
> arbitration is over.
> So layer 2 (CAN FD) has been adapted to circumvent the layer 1
> (transceiver + wire) limitations.
> 
> In fact, the round-trip delay requirement never actually did matter for
> plain CAN during data bits either. CAN FD just makes use of that,
> but is therefore incompatible on the wire.
> 
> I forgot the precise wording, but this is the principle that Bosch
> explained on the CAN conference in Nurnberg several years ago, or at
> least this is how I remembered it :-)

I just checked an example for a CAN FD qualified transceiver

http://www.nxp.com/products/automotive-products/energy-power-management/can-transceivers/high-speed-can-transceiver-with-standby-mode:TJA1044

where it states:

The TJA1044T is specified for data rates up to 1 Mbit/s. Pending the 
release of ISO11898-2:2016 including CAN FD and SAE-J2284-4/5, 
additional timing parameters defining loop delay symmetry are specified 
for the TJA1044GT and TJA1044GTK. This implementation enables reliable 
communication in the CAN FD fast phase at data rates up to 5 Mbit/s.

and

TJA1044GT/TJA1044GTK

- Timing guaranteed for data rates up to 5 Mbit/s
- Improved TXD to RXD propagation delay of 210 ns

> I haven't followed the developments of transceivers, but with the above
> principle in mind, it's obvious that any transceiver allows higher
> bitrates during the data segment because the TX-to-RX line delay must
> not scale with the bitrate.
> In reality, maybe not all transceivers will mention this in their
> datasheet.
> 
> So whether you call it 'max-arbitration-bitrate' & 'max-data-bitrate'
> or 'max-bitrate' & 'max-data-bitrate' does not really matter (I prefer
> 1st) but you will one day need 2 bitrates.

The question to me is whether it is right option to specify two bitrates 
OR to specify one maximum bitrate and provide a property that a CAN FD 
capable propagation delay is available.

E.g.

	max-bitrate
	max-data-bitrate

or

	max-bitrate
	canfd-capable	// CAN FD capable propagation delay available


I assume the optimized propagation delay is 'always on' as the 
transceiver is not able to detect which kind of bits it is processing.
That's why I think providing two bitrates leads to a wrong view on the 
transceiver.

Regards,
Oliver