Re: [RFC PATCH net-next v3 4/5] net: dsa: qca8k: flag QCA8337 internal CPU PHYs for SmartSpeed

[Andrew Lunn]

> I don't know whether downshift is broken in general or only in
> switch-integrated PHYs. My only data point is the RA74 CPU link
> failure, and I have no downshift results from user ports or discrete
> at803x PHYs.

It is pretty easy to test, if you have a spare Ethernet cable you can
sacrifice.

https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair

100Mbps needs pins 1-4, 5-8 are not used. So you can either cover some
of those pins with something, maybe nail varnish, or take some of the
outer cover off the cable to expose the wires, and if it follows the
colour scheme describe cut say the brown cable.

But this is testing the wrong thing. This is testing downshift
works. What you are actually interested in is downshift happening when
it should not....

So take a closer look at the user ports. phylib should report when a
downshift occurs:

https://elixir.bootlin.com/linux/v7.2.7/source/drivers/net/phy/phy-core.c#L359

phydev->advertising should contain what the user configured, even if
the hardware has cleared the 1G advertisement. So it should trigger.

	Andrew