netdev - Re: CAN-FD Transceiver Limitations

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Message-ID: <20170629154139.GC13221@lunn.ch>
Date:   Thu, 29 Jun 2017 17:41:39 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Franklin S Cooper Jr <fcooper@...com>
Cc:     linux-can@...r.kernel.org, netdev@...r.kernel.org,
        wg@...ndegger.com, mkl@...gutronix.de
Subject: Re: CAN-FD Transceiver Limitations

> Transceivers for CAN are not apart of any model. Traditional CAN didn't
> have a problem because all transceivers from my understanding supported
> the maximum speed of 1 Mbps defined by the spec. However, with the
> introduction of CAN Flexible Datarate mode it seems that for
> transceivers that supported CAN-FD the maximum supported speeds vary.

So transceivers are dumb devices, nothing to configure, so no need to
have a driver for them.

> Now that I think of it
> you also can't determine if the transceiver supports CAN-FD in the first
> place. IP that supports CAN-FD is backwards compatible with standard
> CAN. Therefore, its feasible that you may even use a transceiver that
> doesn't support CAN-FD. So I would think something like the below would
> be needed.
> 
> mcan@0 {
> 	...
> 	fixed-transceiver {
> 	      max-canfd-speed = <2000>
> 	};
> 	...
> };

Are there likely to be other transceiver properties? Adding a subnode
may not make sense if this is going to be the only property.

Also, 2KHz is not very fast :-)

Taking a quick look in Documentation/devicetree/bindings/net/can, it
seems a bit of a wild west. No standardization, no central binding
which CAN drivers are expected to support, etc. This sounds like a
generic problem, not an mcan problem. So document this property
centrally, implement the parsing of it centrally, etc, to encourage
other CAN drivers to use it, rather than re-invent the wheel.

	   Andrew