Re: Waiting for the PHY to complete auto-negotiation

On Wed, Dec 06, 2017 at 05:39:00PM +0100, Mason wrote:

Hello,

I've been trying to wrap my head around Ethernet auto-negotiation,
vs actual / real packets seen at the MAC layer. I found the relevant
Wikipedia article to be fairly informative:

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

The reason I care is that my Ethernet HW does not allow changing the
flow control setting once the MAC has started (more specifically, once
RX DMA has been enabled).

In nb8800_open(), the code currently works in this order:

	nb8800_start_rx(dev);
	phy_start(phydev);

The first line enables the MAC (and DMA).
The second enables the PHY and starts auto-negotiation.

This is a problem: I would like for PHY auto-negotiation to be
/complete/ before I enable the MAC.

What is the recommended way to wait for the PHY?

AFAICT, the PHY layer calls back into the eth driver through the
adjust_link() callback registered through of_phy_connect().
It seems like this might be a good place to enable the MAC?

That probably works, but you might have a few corner cases to handle.
I'm not sure changes at the PHY always transition through down. So you
could for example get a callback saying the link is up, 1Gbps, then a
second call saying it has dropped to 100Mbps, if your
cables/connectors are bad.

If your hardware has problems, it might be safest to stop everything
in the callback, make configuration changes, and they start everything
back up. A link negotiation change is not something you expect to
happen often. So making it slow but robust is O.K.

       Andrew