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

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Message-ID: <fe99189e-d077-ba65-4dfc-a4d8beee62b3@hartkopp.net>
Date:   Thu, 27 Jul 2017 20:47:37 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Franklin S Cooper Jr <fcooper@...com>, Andrew Lunn <andrew@...n.ch>
Cc:     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, dev.kurt@...dijck-laurijssen.be,
        sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v2 2/4] can: fixed-transceiver: Add documentation for CAN
 fixed transceiver bindings

On 07/26/2017 08:29 PM, Franklin S Cooper Jr wrote:
>

> I'm fine with switching to using bitrate instead of speed. Kurk was
> originally the one that suggested to use the term arbitration and data
> since thats how the spec refers to it. Which I do agree with. But your
> right that in the drivers (struct can_priv) we just use bittiming and
> data_bittiming (CAN-FD timings). I don't think adding "fd" into the
> property name makes sense unless we are calling it something like
> "max-canfd-bitrate" which I would agree is the easiest to understand.
>
> So what is the preference if we end up sticking with two properties?
> Option 1 or 2?
>
> 1)
> max-bitrate
> max-data-bitrate
>
> 2)
> max-bitrate
> max-canfd-bitrate
>
>

1

>> A CAN transceiver is limited in bandwidth. But you only have one RX and
>> one TX line between the CAN controller and the CAN transceiver. The
>> transceiver does not know about CAN FD - it has just a physical(!) layer
>> with a limited bandwidth. This is ONE limitation.
>>
>> So I tend to specify only ONE 'max-bitrate' property for the
>> fixed-transceiver binding.
>>
>> The fact whether the CAN controller is CAN FD capable or not is provided
>> by the netlink configuration interface for CAN controllers.
>
> Part of the reasoning to have two properties is to indicate that you
> don't support CAN FD while limiting the "arbitration" bit rate.

??

It's a physical layer device which only has a bandwidth limitation.
The transceiver does not know about CAN FD.

> With one
> property you can not determine this and end up having to make some
> assumptions that can quickly end up biting people.

Despite the fact that the transceiver does not know anything about ISO 
layer 2 (CAN/CAN FD) the properties should look like

	max-bitrate
	canfd-capable

then.

But when the tranceiver is 'canfd-capable' agnostic, why provide a 
property for it?

Maybe I'm wrong but I still can't follow your argumentation ideas.

Regards,
Oliver