netdev - Re: [PATCH net] net: phy: dp83869: Set auto mdix bit for forced 100Base-Tx mode

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Message-ID: <0a9d0ec8-09c4-4029-8449-8c8f08c6dca0@lunn.ch>
Date: Wed, 11 Feb 2026 15:12:17 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Meghana Malladi <m-malladi@...com>
Cc: pabeni@...hat.com, kuba@...nel.org, edumazet@...gle.com,
	davem@...emloft.net, linux@...linux.org.uk, hkallweit1@...il.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, srk@...com,
	Vignesh Raghavendra <vigneshr@...com>,
	Roger Quadros <rogerq@...nel.org>, danishanwar@...com
Subject: Re: [PATCH net] net: phy: dp83869: Set auto mdix bit for forced
 100Base-Tx mode

On Wed, Feb 11, 2026 at 06:40:32PM +0530, Meghana Malladi wrote:
> When using DP83869 in force 100Base-Tx mode, the PHY is required
> to have robust Auto-MDIX feature enabled from register 1Eh.
> Refer to 7.4.1.2 100BASE-TX section in the TRM [1] for more details.

What are the results of not enabling this? How would somebody know
they need this patch?

Is this a bug fix? Should this be back ported to stable?

> +static void dp83869_link_change_notify(struct phy_device *phydev)
> +{
> +	int cfg4;
> +
> +	/* When using DP83869 in force 100Base-Tx mode, the PHY is required
> +	 * to have robust Auto-MDIX feature enabled
> +	 */
> +	if (phydev->autoneg == AUTONEG_DISABLE &&
> +	    phydev->speed == SPEED_100 &&
> +	    phydev->duplex == DUPLEX_FULL) {

So forced 100Mbs half duplex does not require robust robust Auto-MDIX?

What about when you change the configuration out of forced 100 Full?
Shouldn't the configuration be returned to how the user wants
auto-MDIX configured? Should you disable robust Auto-MDIX so that
"unreliable Auto-MDIX" is used?

	    Andrew