Re: [PATCH v1] net: phy: dp83867: Disable EEE support as not implemented

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Sat, Oct 25, 2025 at 09:44:31AM +0100, Russell King (Oracle) wrote:
> On Thu, Oct 23, 2025 at 04:48:53PM +0200, Emanuele Ghidoli wrote:
> > While the DP83867 PHYs report EEE capability through their feature
> > registers, the actual hardware does not support EEE (see Links).
> > When the connected MAC enables EEE, it causes link instability and
> > communication failures.
> > 
> > The issue is reproducible with a iMX8MP and relevant stmmac ethernet port.
> > Since the introduction of phylink-managed EEE support in the stmmac driver,
> > EEE is now enabled by default, leading to issues on systems using the
> > DP83867 PHY.
> 
> Wasn't it enabled before? See commit 4218647d4556 ("net: stmmac:
> convert to phylink managed EEE support").
> 
> stmmac's mac_link_up() was:
> 
> -       if (phy && priv->dma_cap.eee) {
> -               phy_eee_rx_clock_stop(phy, !(priv->plat->flags &
> -                                            STMMAC_FLAG_RX_CLK_RUNS_IN_LPI));
> -               priv->tx_lpi_timer = phy->eee_cfg.tx_lpi_timer;
> -               stmmac_eee_init(priv, phy->enable_tx_lpi);
>                 stmmac_set_eee_pls(priv, priv->hw, true);
> -       }
> 
> So, if EEE is enabled in the core synthesis, then EEE will be
> configured depending on what phylib says.
> 
> In stmmac_init_phy(), there was this:
> 
> -               if (priv->dma_cap.eee)
> -                       phy_support_eee(phydev);
> -
>                 ret = phylink_connect_phy(priv->phylink, phydev);
> 
> So phylib was told to enable EEE support on the PHY if the dwmac
> core supports EEE.
> 
> So, from what I can see, converting to phylink managed EEE didn't
> change this. So what really did change?

I suspect it is a change in board designs. iMX8MP EVB variants are using
Realtek PHYs with the SmartEEE variant. Therefore, the MAC is not able
to control LPI behavior. Designs based on the EVB design (with the
Realtek PHY) are not affected. I mean, any bug on the MAC or software
side will stay invisible.

Some new designs with special requirements for TSN, for example
low-latency TI PHYs, are a different story. They promise "Extra low
latency TX < 90ns, RX < 290ns" and also announce EEE support. These two
promises are not compatible with each other anyway, and at the same
time, even if LPI does work, it will most probably fail with the FEC
driver. I do not know about STMMAC.

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