netdev - Re: BCM54220: After the BCM54220 closes the auto-negotiation, the configuration forces the 1000M network port to be linked down all the time.

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Message-ID: <ae617cad-63dc-333f-c4c4-5266de88e4f8@gmail.com>
Date:   Tue, 21 Feb 2023 10:44:44 -0800
From:   Doug Berger <opendmb@...il.com>
To:     "Wang, Xiaolei" <Xiaolei.Wang@...driver.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "bcm-kernel-feedback-list@...adcom.com" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "richardcochran@...il.com" <richardcochran@...il.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: BCM54220: After the BCM54220 closes the auto-negotiation, the
 configuration forces the 1000M network port to be linked down all the time.

On 2/17/2023 12:06 AM, Wang, Xiaolei wrote:
> hi
>
>      When I use the nxp-imx7 board, eth0 is connected to the PC, eth0 is turned off the auto-negotiation mode, and the configuration is forced to 10M, 100M, 1000M. When configured to force 1000M，
>      The link status of phy status reg(0x1) is always 0, and the chip of phy is BCM54220, but I did not find the relevant datasheet on BCM official website, does anyone have any suggestions or the datasheet of BCM54220?
>
> thanks
> xiaolei
>
It is my understanding that the 1000BASE-T PHY requires peers to take on 
asymmetric roles and that establishment of these roles requires 
negotiation which occurs during auto-negotiation. Some PHYs may allow 
manual programming of these roles, but it is not standardized and tools 
like ethtool do not support manual specification of such details.

Therefore manual configuration of a 1000BASE-T link cannot be assumed to 
work. If you want to "force" a gigabit link you should keep 
auto-negotiation on and only advertise support for the speed you want.


Regards,

     Doug