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

[Andrew Lunn]

On Sun, Feb 15, 2026 at 08:15:23PM +0530, Malladi, Meghana wrote:
> Hi Maxime,
> 
> On 2/13/2026 12:10 AM, Maxime Chevallier wrote:
> > Hi Meghana, On 12/02/2026 07: 21, Meghana Malladi wrote: > Hi Andrew, >
> > > On 2/11/26 19: 42, Andrew Lunn wrote: >> On Wed, Feb 11, 2026 at 06: 
> > 40: 32PM +0530, Meghana Malladi wrote: >>> When using DP83869 in force
> > 100Base-Tx
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> > Hi Meghana,
> > 
> > On 12/02/2026 07:21, Meghana Malladi wrote:
> > > Hi Andrew,
> > > 
> > > On 2/11/26 19:42, Andrew Lunn 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.
> > > > 
> > > > What are the results of not enabling this? How would somebody know
> > > > they need this patch?
> > > 
> > > This is the fix for 100M full duplex when auto-neg gets disabled, it
> > > fails to have a link up and this patch fixes it. I will add more
> > > description to the commit message.
> > 
> > I have a board here with a DP83869 + imx93's dwmac, but I couldn't
> > trigger the bug locally. I forced speed, dpx and aneg settings with :
> > 
> > ethtool -s eth0 speed 100 duplex full autoneg off
> > 
> > I did the same on the link partner, but I was still able to get link-up.
> > 
> 
> Your steps so far are correct. But I didn't do the same settings on the link
> partner. With other speeds (10M, 1G) I was seeing a link up without the need
> to force speed on the link partner side. So I was expecting the same with
> 100M as well.

This is not correct. If you turn autoneg off on one link partner, the
other link partner also must be forced, because it should not be able
to perform autoneg. Some PHYs will default to 10/Half when autoneg
fails, but as far as i'm known, that it not part of 802.3.

> > I'm not saying the bug isn't real, I mostly curious about the kind of
> > circumstances that causes this prblem to show-up. Do you have any
> > clarifications on that ?

Given the testing you have done, it seems to need an invalid
configuration?

> Also when I tried the same between DP83869 PHYs of two AM64x (TI DUTs) I
> couldn't see this issue. When I tried between DP83869 PHY and an external
> switch or PC I was able to reproduce this issue.

Did you force the switch to 100Full? Or was autoneg still enabled?

	Andrew