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

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Date:   Wed, 6 Dec 2017 19:03:48 +0100
From:   Mason <slash.tmp@...e.fr>
To:     Andrew Lunn <andrew@...n.ch>
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 06/12/2017 17:59, Andrew Lunn wrote:

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

What you've described is, in fact, the existing implementation! ;-)

nb8800_pause_config() checks for netif_running() and, when true,
tries to disable everything, make the change, then re-enable
everything.

The problem with this is the following mind-boggling quirk of
the hardware: once RX DMA is enabled, there is no supported
way to disable it! Thus, I'm trying to find a clean way to set
the control flow parameter BEFORE enabling the MAC.

Regards.