netdev - Re: Waiting for the PHY to complete auto-negotiation

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Message-ID: <20171206165903.GM27063@lunn.ch>
Date:   Wed, 6 Dec 2017 17:59:03 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Mason <slash.tmp@...e.fr>
Cc:     netdev <netdev@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        David Miller <davem@...emloft.net>
Subject: 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