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

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Message-ID: <b500116b-afdc-4e5b-af4b-6c1e15116111@ti.com>
Date: Wed, 11 Feb 2026 19:23:12 +0530
From: Meghana Malladi <m-malladi@...com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
CC: <pabeni@...hat.com>, <kuba@...nel.org>, <edumazet@...gle.com>,
	<davem@...emloft.net>, <hkallweit1@...il.com>, <andrew@...n.ch>,
	<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

Hi Russell,

On 2/11/26 18:56, Russell King (Oracle) wrote:
> 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.
>>
>> [1]: https://www.ti.com/lit/ds/symlink/dp83869hm.pdf
>> Signed-off-by: Meghana Malladi <m-malladi@...com>
>> ---
>>   drivers/net/phy/dp83869.c | 19 +++++++++++++++++++
>>   1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/net/phy/dp83869.c b/drivers/net/phy/dp83869.c
>> index 1f381d7b13ff..70d759c45c6a 100644
>> --- a/drivers/net/phy/dp83869.c
>> +++ b/drivers/net/phy/dp83869.c
>> @@ -110,6 +110,7 @@
>>   
>>   /* CFG4 bits */
>>   #define DP83869_INT_OE	BIT(7)
>> +#define DP83869_CFG4_ENABLE_AUTO_MDIX_100FD	BIT(9)
>>   
>>   /* OP MODE */
>>   #define DP83869_OP_MODE_MII			BIT(5)
>> @@ -900,6 +901,23 @@ static int dp83869_phy_reset(struct phy_device *phydev)
>>   	return dp83869_config_init(phydev);
>>   }
>>   
>> +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) {
>> +		cfg4 = phy_read(phydev, DP83869_CFG4);
>> +		if (cfg4 >= 0) {
>> +			cfg4 |= DP83869_CFG4_ENABLE_AUTO_MDIX_100FD;
>> +			phy_write(phydev, DP83869_CFG4, cfg4);
>> +		}
>> +	}
> 
> Shouldn't this kind of configuration be done via .config_aneg() ?
> 

AFAIK, in case of fixed mode, auto-neg might not be triggered all the 
time. In the phy_ethtool_ksettings_set() function, auto-neg is initiated 
only if the PHY hasn't started. PHY drivers are new to me, please do 
correct me if I am wrong.