Subject: Re: [RFC PATCH 9/9] dt-bindings: net: dsa: Add documentation for Hellcreek switches

From: Kurt Kanzenbach <[email-masked]>
Date: Mon, 22 Jun 2020 14:02:19 +0200

On Fri Jun 19 2020, Andrew Lunn wrote:
>> > The switch is 100/100Mbps right? The MAC is only Fast ethernet. Do you
>> > need some properties in the port@0 node to tell the switch to only use
>> > 100Mbps? I would expect it to default to 1G. Not looked at the code
>> > yet...
>> 
>> No, that is not needed. That is a hardware configuration and AFAIK
>> cannot be changed at run time.
>
> I was wondering about that in general. I did not spot any code in the
> driver dealing with results from the PHY auto-neg. So you are saying
> the CPU is fixed speed, by strapping? But what about the other ports?
> Does the MAC need to know the PHY has negotiated 10Half, not 1G? Would
> that not make a difference to your TSN?

Indeed, that does make a difference. I've checked with the vendor. The
current version of the switch IP does not support configuring the speed
etc. at run time. It is hard wired to 100 Mbit/s or 1000 Mbit/s for
now. Later versions of the chip might support setting the speed etc. via
configuration registers. As a result the PHYs at the front ports should
be programmed to only advertise 100 Mbit/s or 1G depending on the
hardware setup.

Thanks,
Kurt