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

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Message-ID: <aYyMgK73UtBd6vqc@shell.armlinux.org.uk>
Date: Wed, 11 Feb 2026 14:04:48 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Meghana Malladi <m-malladi@...com>
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

On Wed, Feb 11, 2026 at 07:23:12PM +0530, Meghana Malladi wrote:
> 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.

In phy_ethtool_ksettings_set():

        /* Restart the PHY */
        if (phy_is_started(phydev)) {
                phydev->state = PHY_UP;
                phy_trigger_machine(phydev);
        } else {
                _phy_start_aneg(phydev);
        }

If the PHY is started, then the state machine is triggered. That
will result in phy_state_machine() being called.

This calls _phy_state_machine(), which, as phydev->state is set to
PHY_UP, will set state_work = PHY_STATE_WORK_ANEG. This will then
result in:

        if (state_work == PHY_STATE_WORK_ANEG) {
                err = _phy_start_aneg(phydev);
                func = &_phy_start_aneg;
        }

which also results in _phy_start_aneg() being called, and thus the
.config_aneg() method.

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